All living organisms must control and coordinate the activities taking place inside their bodies. Coordination means making different cells, tissues, organs and organ systems work together properly and helping the organism respond correctly to changes in its surroundings.

Why do organisms need control and coordination?

A unicellular organism has only one cell, so controlling its life processes is comparatively simple.

A multicellular organism contains many cells. These cells form tissues → organs → organ systems. Different organ systems perform different functions, but they must work together. For example, during running, the muscles need more oxygen. Therefore, breathing becomes faster and the heart pumps blood faster. This is an example of coordination between organ systems.

Control and coordination also allow an organism to detect and respond to changes in the environment, called stimuli.

Stimulus → Detection → Coordination → Response

For example:

Touching a hot object → sensory cells detect heat → nervous system processes the information → muscles move the hand away.

2. Control and Coordination in Plants

Plants do not have a nervous system like animals. They mainly coordinate their activities using chemical signals called plant hormones.

Plants also show different types of movements in response to stimuli. The textbook mentions:

Phototropic movement – response to light
Chemotactic movement – response to chemicals
Thigmotactic movement – response to touch/contact

For example, a plant shoot bends towards light, while the tendril of a climber responds to contact with a support.

3. Control and Coordination in Animals

Animals generally show a higher and faster level of coordination than plants.

In higher and more complex animals, two major systems control and coordinate body activities:

1. Nervous system — electrical control
It uses electrical nerve impulses and usually produces rapid responses.

2. Endocrine system — chemical control
It uses chemical messengers called hormones, which are carried through the body.

These two systems will become more complex as we move from simple animals such as Hydra to higher animals.

4. Nervous Coordination

Different groups of animals have different types of nervous systems. The nervous system has gradually become more organized and specialized during evolution.

The textbook begins with one of the simplest examples: Hydra.

5. Nervous System in Hydra

Hydra belongs to the phylum Cnidaria. It has a very simple and primitive nervous system called a diffuse nervous system or nerve net.

What is a nerve net?

Hydra does not have a brain.

Instead, many nerve cells or neurons are scattered throughout its body and are interconnected with one another.

The interconnected neurons form a net-like structure called the:

Nerve Net

The nerve cells are connected with one another through synapses.

The nervous system is called diffuse because the nerve cells are spread throughout the body rather than concentrated into a brain or spinal cord.

Location of the nerve nets

The textbook describes two nerve nets in Hydra:

Outer nerve net: located towards the epidermis, or outer body layer.

Inner nerve net: located towards the gastrodermis, or inner body layer.

There are also sensory cells scattered throughout the body.

Near the eyes, photosensory structures are present on the dorsal side of the brain.

6. How does the Hydra nervous system work?

Hydra has neurons with nerve fibres, but there are no separate sensory nerves and motor nerves.

Another important feature is that the nerve impulse shows no fixed polarity or direction.

This means that when a sensory cell is stimulated at a particular point, the impulse can travel through the nerve net in different directions.

Simple example

Suppose food touches one of Hydra's tentacles:

Food touches tentacle → sensory cell gets activated → nerve impulse starts → impulse spreads through nerve net → tentacles/body move → food is captured

Thus, Hydra can coordinate movements during activities such as feeding even though it does not have a brain.

7. Diffuse Nervous System :Important Point

The diffuse type of nervous system is considered the first important landmark in the evolution of nervous systems.

It is found in:

Ctenophores
Cnidarians such as Hydra
The enteric nervous system (ENS) or gut wall of higher animals, including humans, also shows a diffuse arrangement.

8. How did the Nervous System Evolve?

As animals became more complex during evolution, their nervous systems also became more organized.

There was a gradual transition:

Diffuse nerve net → Organized nervous system → Centralized nervous system

An important evolutionary development was the concentration of neurons into central structures rather than having neurons scattered throughout the entire body.

This eventually resulted in the formation of structures such as the brain and nerve cords.

Why is centralization useful?

Sense organs detect information from the environment. A centralized nervous system can collect this information, process it and coordinate an appropriate response.

For example:

Sense organ → Central nervous system → Processing → Motor response

The textbook therefore describes the evolution of the nervous system as the development of a complex networking system that efficiently transmits signals between one part or organ of the body and another.

9. Nervous System in Planaria (Flatworm)

Planaria is a flatworm belonging to the phylum Platyhelminthes.

Its nervous system is more advanced and organized than that of Hydra.

Unlike Hydra, Planaria has a central nervous system (CNS).

The CNS is located on the ventral side of the body.

10. Brain of Planaria

At the anterior or front end of Planaria, there is a mass of nervous tissue called the:

Cerebral ganglion

It is described as a U-shaped brain-like structure.

Therefore:

Cerebral ganglion = primitive brain-like structure of Planaria

A pair of eyespots is also present near the anterior end. These help Planaria detect light.

11. Ventral Nerve Cords (VNC)

From each cerebral ganglion, nine branches arise towards the outer side.

On the ventral side, a pair of longitudinal nerve cords extends along the body.

These are called:

Ventral Nerve Cords (VNC)

or simply:

Long Nerve Cords

So Planaria has two main longitudinal nerve cords running backwards through its body.

12. Transverse Nerves :Why does it look like a ladder?

The two longitudinal nerve cords are connected to one another by cross-connections called:

Transverse nerves / Commissures

Think of a ladder:

Two long nerve cords = two side rails of a ladder
Transverse nerves = steps/rungs of the ladder

Therefore, the nervous system of Planaria is called a:

Ladder-type nervous system

This is an important exam point.

13. Peripheral Nervous System (PNS) in Planaria

Planaria also has a Peripheral Nervous System (PNS).

The PNS consists of a nerve plexus arising laterally from the ventral nerve cords.

In simpler words, smaller nerves branch sideways from the main nerve cords and spread to different parts of the body.

Planaria therefore shows a much more organized arrangement than Hydra.

Hydra vs Planaria

Feature	Hydra	Planaria
Type of nervous system	Diffuse nerve net	Ladder-type
Organization	Simple	More organized
Brain	Absent	Primitive brain-like cerebral ganglion
Nerve cords	No centralized nerve cords	Pair of ventral nerve cords
Cross connections	Nerve net	Transverse nerves/commissures
Nervous system position	Distributed throughout body	Mainly ventral
Sense structures	Sensory cells scattered throughout body	Eyepots present
Evolutionary level	More primitive	More advanced

The most important idea to remember

The two animals demonstrate how the nervous system became more organized during evolution:

Hydra:
Scattered neurons → Nerve net → No brain

⬇️ Evolution toward greater organization

Planaria:
Cerebral ganglion → Two ventral nerve cords → Transverse nerves → Ladder-like nervous system

⬇️ Further evolution

Higher animals:
Brain + spinal cord/major nerve cords + peripheral nerves + specialized sense organs

So, the major evolutionary trend is:

Diffuse nervous system → Centralization → Greater specialization and coordination

This allows higher animals to receive information, process it quickly and produce more precise responses.

Neural Tissue:

The nervous system controls and coordinates the activities of our body. The special tissue that forms the nervous system is called neural tissue or nervous tissue.

Neural tissue is mainly made up of two types of cells:

Neurons (nerve cells) — receive and transmit nerve impulses.
Neuroglial cells (glial cells) — support, protect and nourish neurons.

So, remember:

Neural tissue = Neurons + Neuroglial cells

1. Nerve and Tract

A nerve is a bundle of axons located outside the Central Nervous System (CNS).

A similar bundle of axons located inside the CNS is called a tract.

Therefore:

Location	Bundle of axons called
Outside CNS	Nerve
Inside CNS	Tract

Nerves may be of three types: sensory nerves, motor nerves and mixed nerves.

A sensory nerve carries information from receptors or sense organs towards the CNS. A motor nerve carries commands from the CNS to muscles or glands. A mixed nerve contains both sensory and motor fibres.

2. How Does the Nervous System Work?

The nervous system basically works in three steps:

Input → Processing → Output

Suppose you accidentally touch something hot.

Heat stimulus → Skin receptor → Sensory nerve → CNS → Motor nerve → Muscle → Hand moves away

Step 1 — Sensory input

Receptors detect changes and send sensory impulses towards the CNS.

Step 2 — Processing

The CNS, mainly the brain and spinal cord, receives and processes the information.

Step 3 — Motor response

After processing, the CNS sends motor commands to the appropriate muscles or glands.

Thus:

Receptor → Sensory neuron → CNS → Motor neuron → Effector → Response

An effector is usually a muscle or gland that actually produces the response.

3. Neuron or Nerve Cell

A neuron is the structural and functional unit of the nervous system.

"Structural unit" means neurons form the nervous system, while "functional unit" means they perform its major function — receiving and transmitting nerve impulses.

A typical neuron has three major parts:

Dendrites → Cyton → Axon

4. Cyton / Cell Body

The cyton is the main body of the neuron.

It contains a distinct central nucleus with a nucleolus and the surrounding neuroplasm, which is the cytoplasm of the neuron.

A thin layer of cytoplasm surrounds the nucleus.

The cyton also contains important structures such as:

Neurofibrils
Nissl's granules
Other normal cell organelles

Nissl's granules

Nissl's granules are riboprotein components.

They play an important role in the synthesis of enzymes required for neurotransmitter formation.

Neurofibrils

Neurofibrils help in the transmission of nerve impulses.

5. Dendrons and Dendrites

From the cyton arise many small, conical and highly branched processes called dendrons.

Dendrons are further divided into fine branches called dendrites.

Their important function is to receive messages and carry them towards the cyton.

Direction:

Dendrites → Cyton

An easy way to remember:

Dendrites = Detect and Deliver towards the cell body.

6. Axon

The axon is usually a single, long and mostly unbranched process arising from the cyton at a region called the axon hillock.

Unlike dendrites, an axon carries the nerve impulse away from the cyton.

Direction:

Cyton → Axon → Axon terminals

Therefore, the basic direction of an impulse through a neuron is:

Dendrites → Cyton → Axon → Axon terminal

An axon contains neurofibrils, but Nissl's granules are absent from the axon.

7. Axon Branches

An axon may occasionally give out lateral or side branches called:

Collaterals

At its end, the axon divides into terminal branches called:

Telodendrons

These terminal branches may connect with a muscle, gland, skin or the dendrites of another neuron.

8. Synapse

A synapse is the functional connection between:

two neurons, OR
a neuron and a motor organ/effector.

It allows the nervous message to pass from one cell to another.

So the sequence can be understood as:

Neuron 1 → Synapse → Neuron 2

9. Nerve Fibres and Their Coverings

Small groups of nerve fibres inside a nerve are covered by connective tissue called:

Endoneurium

Several nerve fibres form bundles called nerve fasciculi.

These fasciculi are surrounded by:

Perineurium

The entire large bundle forming the nerve is covered externally by:

Epineurium

So remember the arrangement:

Individual nerve fibre → Endoneurium → Fascicle → Perineurium → Whole nerve → Epineurium

Blood is supplied to nerves to provide oxygen and nutrients.

10. Myelinated and Non-myelinated Nerves

Some axons have a fatty insulating covering called a myelin sheath.

Myelinated nerve fibre

It possesses a myelin sheath.

Non-myelinated nerve fibre

It does not possess a myelin sheath.

The textbook states that small groups of cell bodies inside the white matter of the brain are called basal nuclei.

Bundles of axons called nerve may be covered only by neurilemma in non-myelinated nerves, while in medullated/myelinated nerves they are covered by a medullary/myelin sheath and externally by neurilemma.

Why is myelin important?

Myelin acts like the insulation around an electrical wire. It helps the nerve impulse travel much faster.

According to the textbook, conduction of impulses in medullated/myelinated nerves may be about 50 times faster than in non-medullated nerves.

11. Neuroglial Cells

Neurons cannot perform all their functions alone. They are supported by another group of cells called:

Neuroglial cells / Glial cells

Neuroglial cells are far greater in number than neurons.

Most supporting cells of the nervous system are derived from the same embryonic tissue layer, the ectoderm, that produces neurons.

The term neuroglia refers to the supporting cells of both:

CNS — Central Nervous System

and

PNS — Peripheral Nervous System

12. Neuroglial Cells of the CNS

There are four important types shown in your textbook.

1. Oligodendrocytes

Oligodendrocytes have relatively few branches.

Their main function is to form the myelin sheath around axons in the CNS.

The myelin sheath contains protein and fatty substances and acts as an insulating covering.

This allows rapid transmission of electrical impulses.

Remember:

Oligodendrocytes → Myelin in CNS

2. Microglia / Brain Macrophages

Microglia are small cells with few branches.

They are derived from monocytes and behave like macrophages.

If neurons die or cell debris is produced after an injury, microglia move to that area and clean up the dead cells and debris.

They also help in the immune response of the CNS.

Think of them as the:

"Cleaning and defence cells of the brain."

3. Astrocytes

Astrocytes are star-shaped cells.

They are the most abundant glial cells of the CNS and perform several important functions.

They help in:

Repair
Secretion and absorption of neurotransmitters
Maintenance of the blood-brain barrier (BBB)
Regulation of transmission of electrical impulses in the brain

Think of astrocytes as support and maintenance workers of the CNS.

4. Ependymal Cells

Ependymal cells form a single layer of squamous or columnar cells.

They are often ciliated.

They line:

Ventricles of the brain
Central canal of the spinal cord

Their main role is associated with the production and circulation of cerebrospinal fluid (CSF).

What is CSF?

Cerebrospinal fluid is the fluid present around and within parts of the brain and spinal cord. It helps protect and support the CNS.

So remember:

Ependymal cells → CSF

13. Neuroglial Cells of the PNS

Two important types are given.

1. Schwann Cells

Schwann cells are among the most abundant glial cells of the PNS.

Their important function is to:

Produce the myelin sheath around myelinated nerves of the PNS.

Therefore, don't confuse:

Oligodendrocytes → CNS myelin

Schwann cells → PNS myelin

This difference is very important for exams.

2. Satellite Cells

Satellite cells are found in the PNS.

Their major function is to support the functions of neurons, especially around neuronal cell bodies in peripheral ganglia.

14. All Six Neuroglial Cells at a Glance

Neuroglial cell	Location	Main function
Oligodendrocytes	CNS	Form myelin sheath
Microglia	CNS	Defence; remove dead cells/debris
Astrocytes	CNS	Support, repair, BBB and regulation
Ependymal cells	CNS	Line ventricles/central canal; help with CSF
Schwann cells	PNS	Form myelin sheath
Satellite cells	PNS	Support neurons

Easy memory trick

For CNS, remember O-M-A-E:

O — Oligodendrocytes
M — Microglia
A — Astrocytes
E — Ependymal cells

For PNS, remember S-S:

S — Schwann cells
S — Satellite cells

15. Complete Chapter Concept in One Flow

The easiest way to connect everything you provided is:

Nervous system

↓

made of Neural tissue

↓

Neurons + Neuroglial cells

↓

Neuron receives and transmits nerve impulses

↓

Dendrites → Cyton → Axon → Telodendrons

↓

message can cross a Synapse

↓

Neuroglial cells support, protect, nourish and insulate neurons

And the nervous system as a whole performs:

Stimulus → Receptor → Sensory impulse → CNS → Processing → Motor impulse → Effector → Response

For example:

Hot object → Skin receptor → Sensory neuron → CNS → Motor neuron → Arm muscle → Hand withdrawn

That single sequence explains the basic purpose of neural tissue, neurons and nervous coordination.

Synapse & Transmission of Nerve Impulse :

Now we move from the structure of a neuron to an important question:

How does a nerve message travel through a neuron and then pass from one neuron to another?

There are two connected processes:

Within one neuron: nerve impulse travels electrically along the neuron.

Between two neurons: the message usually crosses a synapse using neurotransmitters.

1. What is a Synapse?

A synapse is the junction between two nerve cells (neurons) where a nerve impulse is transmitted from one cell to another.

There is usually a very tiny gap between the two cells called the:

Synaptic cleft

The message crosses this junction with the help of a neurotransmitter in a chemical synapse.

So:

Neuron 1 → Synapse → Neuron 2

The neuron carrying the impulse towards the synapse is the presynaptic neuron.

The neuron receiving the message after the synapse is the postsynaptic neuron.

The receiving cell does not always have to be another neuron. It may also be a muscle or gland cell.

2. Properties of Nerve Fibres

a. Excitability / Irritability

A nerve fibre can detect a stimulus and become active.

For example, touching something hot stimulates sensory nerve cells.

Stimulus → Neuron becomes excited

b. Conductivity

It is the ability of a nerve fibre to transmit an excitation or nerve impulse from one region to another.

Think of it as:

Excitability = ability to respond

Conductivity = ability to carry the response

c. Stimulus

A stimulus is any detectable physical, chemical, electrical or environmental change that causes excitation in a nerve, muscle or gland.

Examples include heat, light, pressure, chemicals and electrical changes.

For a stimulus to produce a normal nerve response, it must reach a minimum strength called the:

Threshold stimulus

3. Threshold, Subliminal and Supraliminal Stimuli

Threshold stimulus

The minimum strength of stimulus required to produce an effective nerve impulse.

Subliminal stimulus

A stimulus weaker than the threshold.

By itself, it normally does not produce an effective response.

Supraliminal stimulus

A stimulus stronger than the threshold.

Once threshold is reached, increasing stimulus strength does not make an individual nerve impulse "bigger" in the usual all-or-none sense.

This connects with the all-or-none law explained below.

4. Summation Effect

Suppose one weak stimulus is not strong enough to reach the threshold.

One weak stimulus → No impulse

But if several weak stimuli occur quickly one after another, their effects may add together.

Weak + Weak + Weak → Threshold reached → Impulse produced

This is called:

Summation effect

So, a single subliminal stimulus may not work, but several rapidly repeated subliminal stimuli can together produce an impulse.

5. All-or-None Law

A nerve fibre follows the:

All-or-none law

This means that once the stimulus reaches the threshold, the nerve fibre produces a complete nerve impulse.

If the stimulus is below threshold, it does not produce the impulse.

Think of a light switch:

Below threshold → OFF

Threshold reached → ON

There is no "half nerve impulse."

6. Refractory Period

Immediately after producing a nerve impulse, a nerve fibre needs a very short period before it can respond normally to another stimulus.

This recovery time is called the:

Refractory period

The textbook mentions approximately 1 millisecond.

It helps prevent a neuron from responding continuously without recovery.

7. Synaptic Delay

Transmission across a synapse is not instantaneous.

The impulse requires approximately 0.3–0.5 milliseconds to cross a synapse.

This is called:

Synaptic delay

Why is there a delay?

In a chemical synapse, time is required for the presynaptic neuron to release neurotransmitter, for it to cross the synaptic cleft, and for the postsynaptic cell to respond.

8. Synaptic Fatigue

If a synapse is stimulated repeatedly for a long time, transmission may temporarily decrease or stop.

This is called:

Synaptic fatigue

One reason is temporary exhaustion/depletion of neurotransmitter available for release.

After recovery, normal transmission can resume.

9. Velocity of Nerve Impulse

The velocity means the speed at which a nerve impulse travels.

The textbook states that transmission is generally faster in long and thick nerve fibres.

It is also faster in myelinated (medullated) fibres than in non-myelinated fibres.

In myelinated fibres, the impulse effectively jumps from one Node of Ranvier to the next. This is known as saltatory conduction.

The textbook gives approximate values of:

Voluntary fibres: 100–120 m/s

Autonomic/involuntary nerves: 10–20 m/s

10. Types of Synapses

There are two major types discussed here:

1. Electrical synapse

2. Chemical synapse

Let's understand the difference.

11. Electrical Synapse

In an electrical synapse, the gap between neighbouring cells is very narrow.

The two cells communicate through special connections called gap junctions, allowing electrical current/ions to pass directly between them.

The textbook mentions that the cells may be approximately 3.8 nm apart at the gap junction.

Important characteristics

Electrical transmission is generally:

Very fast

and may permit transmission in either direction, depending on the particular electrical synapse.

Because there is no need to release and diffuse a chemical neurotransmitter, transmission can be faster than at a chemical synapse.

Electrical synapses are particularly useful where rapid, synchronized responses are required.

12. Chemical Synapse

A chemical synapse uses a chemical messenger called a:

Neurotransmitter

The synaptic gap is larger than in an electrical synapse. Your textbook gives approximately 20–40 nm.

Chemical synapses commonly occur:

Neuron → Neuron

and also at junctions such as:

Neuron → Muscle

The connection between a motor neuron and a muscle is called a:

Neuromuscular junction

13. Three Parts of a Typical Chemical Synapse

A typical chemical synapse consists of:

1. Presynaptic terminal

Usually the axon terminal/synaptic knob of the neuron sending the message.

2. Synaptic cleft

The small gap between the two cells.

3. Postsynaptic membrane

The membrane of the receiving neuron, muscle or gland cell.

So:

Presynaptic neuron → Synaptic cleft → Postsynaptic cell

14. How Does a Chemical Synapse Work?

This is one of the most important processes in the section.

Step 1 — Nerve impulse reaches the synaptic knob

The electrical nerve impulse travels along the axon and reaches its terminal called the synaptic knob.

Step 2 — Calcium channels open

The arriving impulse causes voltage-gated Ca²⁺ channels to open.

Step 3 — Calcium enters

Ca²⁺ ions enter the presynaptic terminal from the extracellular fluid.

Step 4 — Synaptic vesicles respond

The increased Ca²⁺ concentration causes synaptic vesicles to move toward and fuse with the presynaptic membrane.

Synaptic vesicles are tiny membrane-bound sacs containing neurotransmitter molecules.

Step 5 — Neurotransmitter is released

The vesicles release neurotransmitters into the synaptic cleft by:

Exocytosis

Step 6 — Neurotransmitter crosses the cleft

The neurotransmitter diffuses across the tiny synaptic cleft.

Step 7 — Neurotransmitter binds receptors

It binds to specific receptors on the postsynaptic membrane.

This may open or close ion channels.

Step 8 — Postsynaptic response occurs

Depending on the neurotransmitter, the effect may be:

Excitatory → increases the chance of producing an impulse

Inhibitory → decreases the chance of producing an impulse

So the complete sequence is:

Impulse arrives → Ca²⁺ channels open → Ca²⁺ enters → Vesicles fuse → Neurotransmitter released → Crosses cleft → Binds receptor → Postsynaptic response

15. What Happens to the Neurotransmitter Afterwards?

The neurotransmitter should not remain in the synaptic cleft indefinitely.

It may be removed or broken down.

Your textbook gives the example of the enzyme:

Cholinesterase

It destroys/breaks down the neurotransmitter so that the synapse becomes ready to receive another impulse.

16. Why is Chemical Synaptic Transmission Usually One-Way?

Chemical synaptic transmission normally occurs in one direction:

Presynaptic neuron → Postsynaptic neuron

The presynaptic terminal contains the machinery for releasing neurotransmitter, while the postsynaptic membrane contains the appropriate receptors.

Therefore, the normal direction is:

Axon terminal of Neuron A → Synapse → Receiving region of Neuron B

17. Electrical vs Chemical Synapse

Feature	Electrical Synapse	Chemical Synapse
Method	Direct electrical/ionic flow	Neurotransmitters
Gap	Very narrow	Wider
Approx. textbook distance	~3.8 nm	~20–40 nm
Speed	Very fast	Comparatively slower
Direction	Often can be bidirectional	Usually one-way
Neurotransmitter	Not required	Required
Gap junctions	Present	Not the main mechanism
Synaptic delay	Very little	Present

18. Transmission of Nerve Impulse

Now we need to understand what happens along the neuron itself.

A nerve impulse is a wave of bioelectrical/electrochemical disturbance travelling along a neuron.

The neuron's membrane separates the:

Extracellular fluid — outside

from the

Intracellular fluid — inside

Both contain ions, especially:

Na⁺ = Sodium ions

K⁺ = Potassium ions

The distribution of these ions is extremely important for nerve impulses.

19. Resting Neuron — Polarised State

When a neuron is not transmitting an impulse, its membrane is in a:

Polarised state

There is an unequal distribution of ions.

Outside the neuron:

There is a predominance of Na⁺ and Cl⁻.

Inside the neuron:

There is a predominance of K⁺, along with large negatively charged proteins and nucleic acids.

Simplified:

Outside	Inside
More Na⁺	More K⁺
More Cl⁻	Large negative molecules

This produces a charge difference across the membrane.

Outside = relatively positive

Inside = relatively negative

20. Resting Potential

The electrical difference across the membrane of a resting neuron is called:

Resting potential

Your textbook gives it as approximately:

−50 to −100 mV

with an average of about:

−70 mV

The minus sign means the inside of the neuron is negative relative to the outside.

So remember:

Resting neuron ≈ −70 mV

21. Why is the Inside Negative?

Several factors contribute.

The resting membrane is generally more permeable to K⁺ than to Na⁺.

Therefore, K⁺ tends to diffuse outward more readily than Na⁺ diffuses inward.

In addition, large negatively charged proteins and nucleic acids remain inside the cell.

Together, these contribute to:

Outside → relatively positive

Inside → relatively negative

22. Na⁺/K⁺ Pump

Even at rest, some Na⁺ and K⁺ continuously leak across the membrane.

The neuron must restore and maintain the proper ion concentrations.

It does this using the:

Sodium-Potassium Pump / Na⁺-K⁺ Exchange Pump

The pump actively moves ions against their concentration/electrochemical gradients and therefore requires:

ATP energy

The standard pump cycle transports:

3 Na⁺ OUT

2 K⁺ IN

per ATP-driven cycle.

This helps maintain:

High Na⁺ outside + High K⁺ inside

and supports the resting membrane potential.

23. Gated Ion Channels

The membrane also contains special gated channels.

Two especially important ones are:

Na⁺ gated channels

K⁺ gated channels

These channels can open and close in response to changes in membrane voltage.

They allow the neuron to change its membrane potential and produce a nerve impulse.

24. What Happens During a Nerve Impulse?

At rest:

Inside negative | Outside positive

When an adequate stimulus reaches the neuron, ion channels open and the membrane potential changes.

In simplified form:

Resting / Polarisation

Na⁺ mainly outside, K⁺ mainly inside → about −70 mV

↓

Stimulation

Voltage-gated Na⁺ channels open

↓

Na⁺ enters

The inside becomes less negative and then relatively positive.

This is:

Depolarisation

↓

K⁺ channels open

K⁺ moves outward.

This helps restore the negative internal potential.

This is:

Repolarisation

↓

The Na⁺/K⁺ pump and ion movements help re-establish and maintain the original ionic gradients.

↓

Resting potential restored

25. How Does the Impulse Move Along an Axon?

Depolarisation at one part of the membrane causes electrical changes in the neighbouring region.

That neighbouring region then depolarises.

So the change spreads progressively:

Region 1 depolarises → Region 2 depolarises → Region 3 depolarises → ...

This travelling electrical change is the:

Nerve impulse / Action potential

In an unmyelinated fibre, conduction progresses along successive areas of membrane.

In a myelinated fibre, the impulse effectively jumps between the Nodes of Ranvier, making transmission much faster.

26. Complete Journey of a Nerve Message

Here is the entire concept from beginning to end:

Stimulus

↓

Dendrites receive stimulus

↓

Cyton

↓

Nerve impulse travels along axon

↓

Action potential reaches synaptic knob

↓

Voltage-gated Ca²⁺ channels open

↓

Ca²⁺ enters

↓

Synaptic vesicles fuse with membrane

↓

Neurotransmitter released by exocytosis

↓

Neurotransmitter crosses synaptic cleft

↓

Binds postsynaptic receptors

↓

Ion channels change

↓

Postsynaptic cell is excited or inhibited

↓

Message continues / response occurs

Five values worth remembering for exams

Concept	Value
Resting potential	about −70 mV
Overall textbook resting range	−50 to −100 mV
Refractory period	~1 ms
Synaptic delay	~0.3–0.5 ms
Chemical synaptic cleft	~20–40 nm

The central idea is simple: electrical changes carry the message along a neuron, while chemical neurotransmitters usually carry the message across a synapse.

Generation of Nerve Impulse & Human Nervous System

There are two major topics here:

1. Generation and conduction of a nerve impulse — depolarization, repolarization and saltatory conduction.
2. Human nervous system — CNS, protective membranes, CSF and blood-brain barrier.

A. Generation of a Nerve Impulse

First remember the condition of a neuron when it is not carrying an impulse.

1. Resting or Polarized State

At rest, the neuron has a potential difference of approximately:

−70 mV

The membrane is said to be polarized.

At this time:

Outside: more Na⁺, relatively positive
Inside: more K⁺ + negatively charged proteins, relatively negative

The Na⁺/K⁺ pump helps maintain this difference by pumping:

3 Na⁺ OUT and 2 K⁺ IN

using ATP.

So remember:

Resting neuron → Polarized → Inside negative → about −70 mV

2. Depolarization

Now suppose a sufficient stimulus is applied to the neuron.

The membrane changes its permeability, and voltage-gated Na⁺ channels open.

Because there is much more Na⁺ outside, Na⁺ rapidly enters the neuron.

Na⁺ → IN

As positive Na⁺ ions enter, the inside becomes less negative and eventually positive relative to the outside.

This reversal/change in polarity is called:

Depolarization

The textbook gives the action potential peak approximately as:

+30 mV to +60 mV

Therefore:

Resting state −70 mV → Na⁺ channels open → Na⁺ enters → membrane becomes positive → about +30 to +60 mV

This electrical change is the action potential.

3. Why Does the Impulse Move Forward?

The depolarized part of the membrane stimulates the neighbouring resting part.

So:

Area 1 depolarizes → Area 2 depolarizes → Area 3 depolarizes → Area 4...

The previous area meanwhile starts returning to its resting condition.

In this way, the electrical disturbance moves along the axon as a:

Nerve impulse

The process is self-propagating, meaning that once started, each affected region triggers the next region.

4. Repolarization

The neuron cannot remain depolarized. It must return to its resting condition.

After a short interval, the Na⁺ channels close and K⁺ channels open.

Now:

K⁺ → OUT

Positive K⁺ ions move out of the neuron.

As a result, the inside again becomes relatively negative.

This restoration of the membrane toward its original polarity is called:

Repolarization

So:

Depolarization: Na⁺ enters

Repolarization: K⁺ leaves

A very easy memory trick:

Na⁺ IN = Depolarization

K⁺ OUT = Repolarization

5. Restoration of the Resting State

After the impulse has passed, ion distributions need to be restored and maintained.

The Na⁺/K⁺ pump actively works using ATP:

3 Na⁺ → OUT

2 K⁺ → IN

This helps restore and maintain the original Na⁺ and K⁺ concentration gradients.

The neuron returns towards its normal:

Resting potential ≈ −70 mV

6. Complete Sequence of an Action Potential

This is extremely important:

Resting / Polarized

−70 mV

↓

Stimulus applied

↓

Na⁺ channels open

↓

Na⁺ enters

↓

DEPOLARIZATION

Inside becomes positive

Action potential ≈ +30 to +60 mV

↓

Na⁺ channels close + K⁺ channels open

↓

K⁺ moves out

↓

REPOLARIZATION

Inside becomes negative again

↓

Ion gradients restored/maintained

↓

Resting state ≈ −70 mV

7. Polarization, Depolarization and Repolarization

Stage	Na⁺/K⁺ movement	Inside of neuron
Polarized/resting	Gradients maintained	Negative
Depolarization	Na⁺ enters	Becomes positive
Repolarization	K⁺ leaves	Becomes negative again
Restoration	Na⁺/K⁺ gradients restored/maintained	Returns to resting condition

This table is worth memorizing.

8. Conduction in Non-Myelinated Nerve Fibres

In a nerve fibre without a myelin sheath, depolarization must occur continuously along successive portions of the membrane.

Think of a row of dominoes:

1 falls → 2 falls → 3 falls → 4 falls → 5 falls

Similarly:

Area 1 depolarizes → Area 2 → Area 3 → Area 4

Because every successive area participates, conduction is comparatively slower.

9. Conduction in Myelinated Nerve Fibres

A myelinated nerve fibre has an insulating myelin sheath around the axon.

However, there are small gaps in the myelin called:

Nodes of Ranvier

Voltage-gated ion channels are concentrated at these nodes.

Therefore, depolarization/action potentials are regenerated mainly at the Nodes of Ranvier.

The impulse appears to jump from one node to the next.

Node 1 →→ Node 2 →→ Node 3 →→ Node 4

This is called:

Saltatory Conduction

"Saltatory" basically means jumping.

The textbook gives a conduction rate of about:

120 m/s

for fast myelinated fibres.

This is much faster than continuous conduction in non-myelinated fibres.

10. Why is Myelinated Conduction Faster?

Because the action potential does not need to be regenerated continuously over every tiny part of the axonal membrane.

Instead:

Node of Ranvier → Jump → Node of Ranvier → Jump → Next node

The myelin sheath acts as electrical insulation, while the Nodes of Ranvier permit ion exchange required for action potentials.

11. Generation and Conduction of Nerve Impulse

Stimulus applied to resting neuron

↓

Membrane permeability changes

↓

Membrane becomes more permeable to Na⁺

↓

Na⁺ enters from extracellular fluid

↓

Positive ions increase inside

↓

Membrane potential changes from about −70 mV towards +30 mV

↓

DEPOLARIZATION / ACTION POTENTIAL

↓

At the peak, Na⁺ channels close and K⁺ channels open

↓

K⁺ moves out

↓

REPOLARIZATION

↓

Inside becomes negative again

↓

Na⁺/K⁺ distribution is restored and maintained

↓

RESTING POTENTIAL

B. Human Nervous System

The human nervous system is highly developed and complex.

It is broadly divided into:

1. CNS — Central Nervous System

2. PNS — Peripheral Nervous System

3. ANS — Autonomic Nervous System

12. Basic Classification

The chart in your textbook can be simplified as:

Human Nervous System

CNS
→ Brain
→ Spinal cord

PNS
→ Cranial nerves
→ Spinal nerves

ANS
→ Sympathetic nervous system
→ Parasympathetic nervous system

The brain itself is divided into:

Forebrain + Midbrain + Hindbrain

The forebrain includes structures such as the cerebrum and diencephalon.

The midbrain includes structures associated with the optic lobes/chiasma in the textbook chart.

The hindbrain includes:

Cerebellum + Pons Varolii + Medulla oblongata

13. Central Nervous System — CNS

The CNS consists mainly of:

Brain + Spinal Cord

The brain lies inside the cranium (skull).

The spinal cord lies inside the vertebral column.

These bony structures provide physical protection.

But the CNS has additional protective coverings called:

Meninges

14. Meninges

The brain and spinal cord are surrounded by three protective membranes called meninges.

From outside to inside:

Dura mater → Arachnoid mater → Pia mater

Easy memory:

D-A-P

Dura
Arachnoid
Pia

15. Dura Mater

The dura mater is the outermost meningeal layer.

It is:

Tough
Thick
Fibrous
Non-vascular

It is attached to the inner side of the cranium.

Its toughness provides strong mechanical protection to the CNS.

Remember:

Dura = Durable = Tough outer covering

16. Arachnoid Mater

The arachnoid mater is the middle layer.

It is:

Thin
Non-vascular
Made of connective tissue
Web-like in appearance

"Arachnoid" refers to its spider-web-like appearance.

Between the arachnoid mater and pia mater is a space called the:

Subarachnoid space

This space contains:

Cerebrospinal Fluid — CSF

17. Pia Mater

The pia mater is the innermost meningeal layer.

Unlike the dura, it is:

Very thin and delicate
Highly vascular
Closely associated with the CNS

It contains many blood vessels and closely follows the surface of the brain and spinal cord.

So:

Dura = outer

Arachnoid = middle

Pia = inner

18. Cerebrospinal Fluid — CSF

CSF (Cerebrospinal Fluid) is a clear, lymph-like fluid associated with the CNS.

It is mainly produced by specialized structures called the:

Choroid plexuses

inside the ventricles of the brain.

It is found in and around the CNS, including the brain ventricles, central canal and subarachnoid space.

Your textbook mentions approximately:

100–120 cc (mL) of CSF

and gives a specific gravity of approximately:

1.005

19. Functions of CSF

CSF performs several important functions.

1. Mechanical protection

It acts like a cushion or shock absorber around the brain and spinal cord.

If the head experiences a sudden movement or minor impact, the fluid helps protect the delicate nervous tissue.

2. Maintains pressure

CSF helps maintain a constant pressure inside the cranium.

3. Exchange of nutrients and wastes

It helps in the exchange of substances between blood and brain tissue, including nutrients and waste products.

4. Oxygen-related support

It contributes to the environment supporting the brain's metabolic needs.

5. Protects against desiccation

The textbook states that it helps protect nervous tissue from drying/desiccation.

20. CSF is Continuously Circulating

CSF does not simply remain trapped inside the brain ventricles.

It is continuously produced, circulated and drained/reabsorbed into the bloodstream.

Your textbook notes that CSF is continuously generated by ependymal-associated structures/choroid plexuses in the ventricles and circulates through the ventricular system and central canal before ultimately being returned to the blood.

An important point given in the textbook is:

Nervous tissue does not have conventional lymphatic vessels within the tissue itself.

21. Blood-Brain Barrier — BBB

The brain is extremely sensitive. Harmful substances circulating in blood should not be allowed to enter brain tissue freely.

Therefore, the CNS possesses the:

Blood-Brain Barrier (BBB)

The BBB controls the movement of substances from the blood into brain tissue.

It helps prevent the passage of many:

Large molecules
Potentially harmful substances
Unwanted ions/substances

while permitting necessary substances through controlled mechanisms.

The textbook highlights two important components:

Endothelial cells of brain capillaries + Astrocytes

These help create and maintain the blood-brain barrier.

Think of the BBB as a:

Security checkpoint for the brain

Blood arrives carrying many substances, but the BBB carefully controls what can enter brain tissue.

22. Local Anaesthesia — Connection With Nerve Impulses

Your textbook asks why a dentist injects an anaesthetic before extracting a tooth.

Pain is communicated to the CNS through nerve impulses.

A local anaesthetic temporarily prevents pain signals from being properly generated or conducted in nearby sensory nerves, commonly by blocking voltage-gated Na⁺ channels.

Without sufficient Na⁺ entry:

Na⁺ channels blocked → Depolarization cannot occur normally → Action potentials fail to propagate → Pain signal does not reach the CNS effectively

Therefore, the patient does not feel normal pain from the procedure.

A painkiller, by contrast, generally reduces pain through biochemical actions rather than simply producing the same local nerve-conduction block as a local anaesthetic.

Complete Connection of Everything You've Studied So Far

You can now connect the previous pages with these pages:

Stimulus

↓

Receptor detects stimulus

↓

Sensory neuron

↓

Na⁺ channels open

↓

Depolarization

↓

Action potential travels along axon

↓

In myelinated axons:

Saltatory conduction through Nodes of Ranvier

↓

Impulse reaches synaptic terminal

↓

Ca²⁺ channels open

↓

Neurotransmitter released

↓

Synaptic cleft

↓

Postsynaptic neuron activated

↓

Signal reaches CNS

↓

Brain/Spinal cord processes information

↓

Motor command

↓

Motor neuron

↓

Muscle/Gland

↓

Response

That is essentially the complete journey from a stimulus to a coordinated response.

Five terms you absolutely should not confuse

Term	Simple meaning
Polarization	Resting neuron; inside negative (~−70 mV)
Depolarization	Na⁺ enters; inside becomes positive
Repolarization	K⁺ leaves; inside becomes negative again
Action potential	Electrical change that constitutes the nerve impulse
Saltatory conduction	Impulse effectively jumps from one Node of Ranvier to another

The easiest three-line memory rule is:

Rest = −70 mV

Na⁺ IN = Depolarization

K⁺ OUT = Repolarization

Human Brain — Forebrain, Cerebrum & Diencephalon

The brain is the main control centre of the nervous system. The scientific study of all aspects of the brain is called encephalology.

The human brain can be divided into three major regions:

Forebrain → Midbrain → Hindbrain

In these pages, the main focus is the forebrain, especially the cerebrum and diencephalon.

1. Forebrain

The forebrain is the anterior and highly developed part of the brain.

According to your textbook, it consists of:

1. Olfactory lobes

2. Cerebrum

3. Diencephalon

The diencephalon is further divided into:

Epithalamus + Thalamus + Hypothalamus

2. Olfactory Lobes

The olfactory lobes are associated mainly with the:

Sense of smell

In humans, these lobes are highly reduced compared with many other animals.

They are covered by the cerebrum from all sides except the ventral side.

Each olfactory lobe consists of an:

Olfactory peduncle + Olfactory bulb

An easy association:

Olfactory = Smell

3. Cerebrum

The cerebrum is the largest part of the human brain.

According to the textbook, it makes up approximately:

85% of the total brain

It is responsible for many of our higher functions, including thinking, memory, intelligence, voluntary movements, sensations, speech and personality.

4. Two Cerebral Hemispheres

The cerebrum is divided into:

Right cerebral hemisphere

and

Left cerebral hemisphere

They are separated by a deep groove called the:

Longitudinal fissure

But the two hemispheres must communicate with each other.

They are connected internally by a thick band of nerve fibres called:

Corpus Callosum

Therefore:

Right hemisphere ← Corpus callosum → Left hemisphere

The corpus callosum is the largest commissure of the brain.

A commissure is a bundle of nerve fibres connecting corresponding regions on the two sides of the CNS.

5. Genu and Splenium

The corpus callosum has curved ends.

The textbook names them:

Anterior fold → Genu

Posterior fold → Splenium

So:

Front = Genu

Back = Splenium

6. Cerebral Cortex and Cerebral Medulla

The cerebrum has an outer and an inner region.

Outer region — Cerebral Cortex

The outer surface is called the:

Cerebral cortex

It is mainly composed of grey matter.

Grey matter contains large numbers of:

Neuron cell bodies
Non-myelinated fibres
Dendrons/dendrites

Inner region — Cerebral Medulla

The deeper inner region is called:

Cerebral medulla

It is mainly made up of white matter containing many myelinated nerve fibres.

So remember:

Cerebral cortex → Outside → Grey matter

Cerebral medulla → Inside → White matter

7. Why Is the Cerebrum So Folded?

Look at the surface of a human brain. It is not smooth.

It contains many folds.

Raised folds are called:

Gyri — singular: gyrus

Grooves between the folds are called:

Sulci — singular: sulcus

These folds greatly increase the surface area of the cerebral cortex.

More surface area allows a very large number of neurons and connections to be accommodated within the skull.

Think of folding a large sheet of paper so that it can fit into a smaller box.

8. Lobes of the Cerebrum

Each cerebral hemisphere is divided into major lobes:

1. Frontal lobe

2. Parietal lobe

3. Temporal lobe

4. Occipital lobe

There is also a fifth hidden lobe:

5. Insula / Insular cortex

The insula is folded deep inside the lateral sulcus, so it cannot normally be seen from the outer surface.

9. Important Sulci

Three important grooves help separate the lobes.

Central Sulcus

Separates:

Frontal lobe | Parietal lobe

Parieto-occipital Sulcus

Separates:

Parietal lobe | Occipital lobe

Lateral / Sylvian Sulcus

Helps separate the:

Temporal lobe from the frontal and parietal regions.

These boundaries are not complete everywhere, so the lobes are not totally isolated from each other.

10. Functional Areas of the Cerebrum

Different regions of the cerebral cortex perform different functions.

The cortex contains:

Sensory areas + Motor areas + Association areas

Sensory areas

Receive and interpret sensory information.

Motor areas

Control movements.

Association areas

Connect and integrate information from different regions and are involved in higher mental activities.

Now let's understand each lobe.

11. Frontal Lobe

The frontal lobe is extremely important for movement and higher mental functions.

Motor area

The frontal lobe contains the motor area, which controls voluntary motor activities and movements of muscles.

For example:

You decide to raise your hand.

Frontal motor area → Motor nerve signals → Arm muscles → Hand rises

Premotor Area

The premotor area is involved in the organization/control of movements and motor patterns.

The textbook describes it as a higher centre associated with involuntary and autonomous nervous-system-related movements.

Broca's Area

The frontal lobe also contains:

Broca's area / Motor speech area

Its major function is production of speech.

It helps translate thoughts into the muscular patterns required for speaking.

Therefore:

Broca's area = Speech production

Damage to this region can cause difficulty in producing fluent speech.

12. Higher Functions of the Frontal Lobe

The frontal lobe is also associated with:

Emotions
Intelligence
Willpower
Memory
Personality

So the frontal lobe is not simply a "movement centre." It contributes greatly to behaviour and higher mental activities.

13. Parietal Lobe

The parietal lobe mainly processes somatic sensations.

These include sensations such as:

Pain
Pressure
Temperature
Touch-related body sensations
Taste is also listed here in your textbook

So:

Parietal = Body sensations

14. Temporal Lobe

The temporal lobe contains centres associated with:

Smell

Hearing

Speech/language

Emotions

Therefore, auditory information from your ears is mainly processed in the temporal lobe.

15. Occipital Lobe

The occipital lobe is mainly responsible for:

Vision

It contains the visual area of the cerebral cortex.

Information coming from the eyes is ultimately processed here.

Easy memory:

Occipital = Optical = Vision

16. Wernicke's Area

An important language region is:

Wernicke's area

According to your textbook, it is located around the area of contact between the temporal, parietal and occipital lobes.

Its major function is:

Understanding spoken and written language

This gives us a very important comparison:

Area	Main function
Broca's area	Production of speech
Wernicke's area	Understanding language

An easy example:

Someone asks:

"What is your name?"

Wernicke's area → helps you understand the question

Then you decide what to say.

Broca's area → helps organize production of the spoken answer

17. Basal Nuclei / Basal Ganglia

Deep inside the white matter of the cerebrum are masses of grey matter called:

Basal nuclei / Basal ganglia

They receive neurotransmitter-related inputs from different regions and help the cerebral cortex in performing activities.

They are especially important in the control and coordination of movement.

The textbook gives examples such as:

Writing slowly or rapidly and typing.

18. Corpus Striatum

A large basal nuclear complex near the floor of the cerebrum is called:

Corpus striatum

It is associated with the basal nuclei and plays an important role in movement control.

19. Diencephalon

Now we move deeper into the forebrain.

The diencephalon is a part of the forebrain containing:

Epithalamus

Thalamus

Hypothalamus

It lies:

Below the corpus callosum

and

Above the midbrain

It surrounds a narrow cavity called the:

Third ventricle

20. Third Ventricle

The third ventricle is a narrow cavity inside the diencephalon.

It communicates with the two lateral ventricles through openings called:

Foramina of Monro

So:

Left lateral ventricle
↘
Foramen of Monro → Third ventricle
↗
Right lateral ventricle

21. Ventricles of the Brain

The brain contains interconnected fluid-filled cavities called:

Ventricles

The main ones shown in your diagram are:

Two lateral ventricles

↓

Foramina of Monro

↓

Third ventricle

↓

Cerebral aqueduct / Aqueduct of Sylvius

↓

Fourth ventricle

↓

Central canal of spinal cord

These spaces contain CSF — cerebrospinal fluid.

22. Epithalamus

The epithalamus forms the thin, non-nervous roof of the diencephalon.

Anteriorly, it is associated with structures such as the choroid plexus.

From its dorsal wall is connected an important endocrine gland:

Pineal gland

23. Pineal Gland

The pineal gland is a small, stalked gland.

It produces the hormone:

Melatonin

Melatonin is strongly associated with regulation of the body's:

Sleep–wake cycle / biological clock

In simple terms:

Darkness → Melatonin increases → Helps regulate sleep timing

Therefore:

Pineal gland → Melatonin → Sleep-wake rhythm

24. Thalamus

The thalamus consists mainly of grey matter and forms the lateral thick walls of the diencephalon.

The two thalami are connected by a structure called the:

Habenular commissure

(as described in your textbook)

The thalamus is especially important for sensory information.

25. Thalamus as a Sensory Relay Centre

Most sensory information travelling toward the cerebrum passes through or is relayed by the thalamus.

A major exception is:

Smell / Olfaction

So:

Sensory information → Thalamus → Appropriate area of cerebral cortex

Except:

Smell → does not follow the same thalamic relay before primary cortical processing

Therefore, the thalamus is commonly called a:

Sensory relay centre

26. Reticular Activating System — RAS

Different parts of the brain are interconnected through a network called the:

Reticular Activating System (RAS)

It is important in maintaining:

Alertness
Wakefulness
Consciousness/attention

This is why the RAS is important in keeping the brain awake and responsive.

27. Hypothalamus

The hypothalamus forms part of the floor of the diencephalon.

Although relatively small, it performs an enormous number of functions.

It is richly supplied with blood vessels.

Its major overall job is:

Maintaining Homeostasis

What is homeostasis?

Homeostasis means maintaining a stable internal environment inside the body despite changes outside or inside the body.

28. Functions of the Hypothalamus

The hypothalamus regulates:

Body temperature

If the body becomes too hot or cold, hypothalamic mechanisms help restore normal temperature.

Water and electrolyte balance

It helps regulate water and salt balance.

Hunger

It contains centres involved in hunger and feeding.

Thirst

It helps regulate when we feel thirsty.

Sleep

It participates in regulating sleep and wakefulness.

Fatigue

It contributes to regulation associated with tiredness/fatigue.

Satiety

The satiety centre helps produce the feeling:

"I have eaten enough."

29. Hypothalamus and Digestive Functions

The hypothalamus also influences:

Secretions of the stomach
Secretions of the intestine
Other autonomic activities

This shows that it helps regulate many processes we do not consciously control.

30. Hypothalamus Links Nervous and Endocrine Systems

This is one of its most important functions.

The hypothalamus acts as a link between:

Nervous System ↔ Endocrine System

It controls the endocrine system mainly through its connection with the:

Pituitary gland

This is why the hypothalamus is extremely important in hormonal regulation.

31. Hypothalamic Neurosecretory Cells

The hypothalamus contains special neurosecretory cells.

These neurons can produce hormones.

The textbook specifically mentions:

Oxytocin

and

Vasopressin (ADH)

These hormones are produced by hypothalamic neurons and are transported to/released through the posterior pituitary.

So:

Hypothalamus → Oxytocin & ADH → Posterior pituitary stores/releases them

32. Hypothalamus and Emotions

The hypothalamus is also involved in emotional and motivational behaviours.

It participates in the limbic system, along with structures including parts of the:

Amygdala
Epithalamus
Hippocampus
Other related brain regions

The limbic system is associated with:

Emotions + Motivation + Memory

33. Infundibulum

The floor of the hypothalamus continues downward as a stalk called the:

Infundibulum / Hypophyseal stalk

It connects the:

Hypothalamus ↔ Pituitary gland

This provides an important physical and functional connection between the nervous and endocrine systems.

34. Optic Chiasma

On the lower surface of the hypothalamic region lies the:

Optic chiasma

This is where fibres of the two optic nerves partially cross.

It is involved in the visual pathway.

35. Mammillary Bodies

The inferior surface also contains a pair of:

Mammillary bodies

These are especially well developed in mammals and participate in neural circuits associated with:

Memory, particularly recollective memory

Quick Revision Table

Structure	Main function
Olfactory lobes	Smell
Cerebrum	Higher functions
Corpus callosum	Connects cerebral hemispheres
Frontal lobe	Movement, planning, higher functions
Broca's area	Speech production
Parietal lobe	Body sensations
Temporal lobe	Hearing, language, emotions
Occipital lobe	Vision
Wernicke's area	Language comprehension
Basal nuclei	Movement control
Epithalamus	Contains/associates with pineal region
Pineal gland	Melatonin; sleep-wake rhythm
Thalamus	Major sensory relay
Hypothalamus	Homeostasis and endocrine/autonomic control
Pituitary connection	Hormonal regulation
Optic chiasma	Partial crossing of optic fibres
Mammillary bodies	Memory-related function

The easiest way to remember the forebrain

FOREBRAIN

→ Olfactory lobes = Smell

→ Cerebrum = Thinking, sensation, voluntary movement, memory, speech

→ Diencephalon

Epithalamus → Pineal → Melatonin → Sleep

Thalamus → Sensory relay

Hypothalamus → Homeostasis + Hunger + Thirst + Temperature + Sleep + Hormonal control

And for the four major cerebral lobes:

Frontal → Movement & thinking

Parietal → Body sensations

Temporal → Hearing & language

Occipital → Vision

Human Nervous System — Midbrain, Hindbrain, Spinal Cord, PNS, Reflexes & ANS

The overall organization is:

Human Nervous System → CNS + PNS + ANS

The CNS contains the brain and spinal cord. The PNS connects the CNS with the rest of the body. The ANS mainly controls involuntary organs.

1. Midbrain

The midbrain is situated between the diencephalon of the forebrain and the pons Varolii of the hindbrain.

It contains a narrow passage called the:

Cerebral aqueduct / Aqueduct of Sylvius

This connects:

Third ventricle → Cerebral aqueduct → Fourth ventricle

So CSF can pass through it.

2. Corpora Quadrigemina

On the dorsal surface of the midbrain are four rounded elevations collectively called:

Corpora quadrigemina

They consist of:

Two superior colliculi + Two inferior colliculi

Superior colliculi

They are mainly associated with visual reflexes.

For example, if something suddenly moves in front of your eyes, you may automatically turn your eyes/head towards it.

Inferior colliculi

They are mainly associated with auditory reflexes.

For example, if someone suddenly calls your name from behind:

Sound → Auditory pathway → Reflexly turn head toward sound

So remember:

Superior → Sight

Inferior → Sound

3. Cerebral Peduncles / Crura Cerebri

The lower surface of the midbrain contains two thick bundles of nerve fibres called:

Cerebral peduncles / Crura cerebri

They contain ascending and descending nerve fibres connecting different parts of the brain.

You can think of them as major communication highways carrying information through the midbrain.

4. Red Nucleus

The midbrain contains a mass of grey matter called the:

Red nucleus

It has an important role in:

Muscle tone

Posture

Modification/coordination of motor activities

5. Reticular Activating System — RAS

The RAS passes through regions of the brainstem and has connections with higher brain regions.

It is particularly important for:

Wakefulness, alertness and consciousness

6. Hindbrain

The posterior part of the brain is called the:

Hindbrain

It consists mainly of:

Cerebellum

Pons Varolii

Medulla oblongata

These structures are extremely important for coordination, balance and several automatic life-supporting functions.

7. Pons Varolii

The pons appears as a rounded bulge on the lower surface of the brainstem.

"Pons" essentially means bridge.

It contains nerve fibres connecting different regions including the:

Cerebrum

Cerebellum

Medulla oblongata

Spinal cord

Therefore, the pons helps in communication between different parts of the nervous system.

It also contains several nuclei and participates in important brainstem functions.

8. Cerebellum

The cerebellum is the second-largest part of the brain.

It consists of:

Two lateral cerebellar hemispheres + Central vermis

The two hemispheres are connected through the central vermis.

9. Grey and White Matter in Cerebellum

The cerebellum has:

Grey matter outside → Cortex

White matter inside

The internal white matter forms a beautiful tree-like branching pattern called:

Arbor Vitae

The term means:

"Tree of life"

10. Functions of the Cerebellum

The cerebellum is mainly responsible for coordination rather than initiating voluntary movement.

It helps maintain:

Balance

Posture

Body orientation

Muscle tone

Coordination of voluntary movements

Smooth and precise movements

Activities such as:

Walking, running, cycling, writing, playing sports

require proper cerebellar coordination.

The cerebellum helps ensure that movements are smooth, accurate and coordinated rather than shaky or poorly timed.

11. Why Is the Cerebellum Well Developed in Humans?

Humans perform extremely complex coordinated movements.

For example:

Writing → fingers + wrist + arm + eyes + posture

Cycling → legs + arms + eyes + balance + posture

All of these movements must be continuously coordinated.

Therefore, a highly developed cerebellum is important for balance and fine motor coordination.

12. Medulla Oblongata

The medulla oblongata is the posterior/lower part of the brainstem and continues downward into the:

Spinal cord

It is one of the most important areas for controlling vital involuntary functions.

It contains centres controlling:

Heartbeat

Breathing

Blood-vessel diameter/vasomotor activity

Peristalsis

It also controls several reflexes such as:

Coughing

Sneezing

Swallowing

Vomiting

Yawning

This is why severe damage to the medulla can be life-threatening.

13. Fourth Ventricle

The cavity associated with the hindbrain is the:

Fourth ventricle

It contains CSF.

The fourth ventricle communicates with surrounding CSF spaces through openings including the:

Lateral foramina of Luschka

Median foramen of Magendie

These openings allow CSF to move from the ventricular system into the surrounding subarachnoid space.

14. Brainstem

The term brainstem generally includes:

Midbrain + Pons + Medulla oblongata

It connects the higher brain with the spinal cord and contains centres essential for many basic functions.

15. Spinal Cord

The spinal cord is part of the:

Central Nervous System — CNS

It begins as a continuation of the medulla oblongata.

It lies protected inside the:

Vertebral column

and is also surrounded by:

Dura mater → Arachnoid mater → Pia mater

CSF provides additional cushioning and protection.

16. Size and Extent of the Spinal Cord

According to your textbook, the adult spinal cord is approximately:

42–45 cm long

and

2.0–2.5 cm broad

It does not extend throughout the entire length of the vertebral column.

It gradually tapers to form:

Conus medullaris

and continues as a thin fibrous strand called:

Filum terminale

17. Cervical and Lumbar Enlargements

The spinal cord has two important swellings:

Cervical swelling

Associated mainly with nerves supplying the upper limbs.

Lumbar swelling

Associated mainly with nerves supplying the lower limbs.

These regions contain more neural tissue because many nerves for the limbs originate there.

18. Cauda Equina

Below the end of the spinal cord, a bundle of spinal nerve roots continues downward.

It looks somewhat like a horse's tail and is therefore called:

Cauda Equina

"Cauda equina" literally means:

Horse's tail

19. Cross Section of the Spinal Cord

The spinal cord is somewhat flattened from front to back.

In cross-section, two important grooves can be seen:

Posterior median septum — relatively narrow posterior groove/septum

Anterior median fissure — deeper and broader anterior groove

These partially divide the spinal cord into right and left halves.

20. Grey and White Matter in the Spinal Cord

This is important because the arrangement is different from the cerebrum.

In the spinal cord:

Grey matter → Inside

White matter → Outside

The inner grey matter looks approximately like:

H / Butterfly

Compare:

Structure Grey matter White matter
Cerebrum Mainly outside Mainly inside
Spinal cord Mainly inside Mainly outside

21. Horns of Grey Matter

The spinal grey matter forms projections called horns.

Important ones include:

Dorsal horn

Receives/processes incoming sensory information.

Ventral horn

Contains motor neurons whose axons leave the spinal cord.

Lateral horn

Associated particularly with autonomic nervous-system neurons in relevant spinal levels.

22. Dorsal and Ventral Roots

Each spinal nerve connects to the spinal cord through two roots.

Dorsal Root

The dorsal/posterior root carries mainly:

Sensory information INTO the spinal cord

So:

Receptor → Sensory neuron → Dorsal root → Spinal cord

A swelling on the dorsal root is called the:

Dorsal Root Ganglion

It contains the cell bodies of sensory neurons.

Ventral Root

The ventral/anterior root carries:

Motor information OUT of the spinal cord

So:

Spinal cord → Ventral root → Motor neuron → Muscle

Easy memory:

Dorsal = sensory IN

Ventral = motor OUT

23. Ascending and Descending Tracts

White matter contains bundles of myelinated nerve fibres called tracts.

Ascending tracts

Carry mainly sensory information upward:

Spinal cord → Brain

Descending tracts

Carry mainly motor commands downward:

Brain → Spinal cord

Therefore:

Ascending = Sensory → Brain

Descending = Motor → Body

24. Functions of the Spinal Cord

The spinal cord performs two major functions.

1. Conduction

It carries information between the brain and the rest of the body.

Body → Sensory signals → Spinal cord → Brain

and

Brain → Motor commands → Spinal cord → Muscles

2. Reflex Centre

The spinal cord is an important centre for many:

Reflex Actions

25. Peripheral Nervous System — PNS

The PNS connects the CNS with receptors, muscles, glands and other parts of the body.

It includes:

Cranial nerves

connected primarily with the brain

and

Spinal nerves

connected with the spinal cord.

Functionally, peripheral nerves contain:

Afferent fibres and/or Efferent fibres.

26. Afferent and Efferent Nerves

Afferent = Sensory

Afferent fibres carry sensory information:

Receptor → CNS

Efferent = Motor

Efferent fibres carry commands:

CNS → Muscle/Gland

Easy memory:

Afferent Arrives at CNS

Efferent Exits CNS

27. Cranial Nerves

Humans have:

12 pairs of cranial nerves

They are numbered using Roman numerals:

I to XII

Some are sensory, some motor, and some mixed.

Here is the essential information from your table:

No. Cranial nerve Type Major function
I Olfactory Sensory Smell
II Optic Sensory Vision
III Oculomotor Motor Eye movements
IV Trochlear/Pathetic Motor Eye movement
V Trigeminal Mixed Facial sensation + chewing
VI Abducens Motor Eye movement
VII Facial Mixed Facial expression, taste, glands
VIII Vestibulocochlear/Auditory Sensory Hearing + equilibrium
IX Glossopharyngeal Mixed Taste, salivation, swallowing
X Vagus Mixed Major visceral functions
XI Spinal accessory Motor Neck/shoulder and related movements
XII Hypoglossal Motor Tongue movement

28. Important Facts About Cranial Nerves

The textbook highlights some useful facts.

Vagus nerve — Cranial nerve X

It has a very wide distribution and supplies many internal organs.

It is an important parasympathetic nerve.

Trigeminal nerve — Cranial nerve V

It is the largest cranial nerve.

Abducens nerve — Cranial nerve VI

The textbook identifies it as the smallest cranial nerve.

29. Spinal Nerves

Humans have:

31 pairs of spinal nerves

They are divided as follows:

Region Number
Cervical 8 pairs (C1–C8)
Thoracic 12 pairs (T1–T12)
Lumbar 5 pairs (L1–L5)
Sacral 5 pairs (S1–S5)
Coccygeal 1 pair (Co1)
Total 31 pairs

All spinal nerves are generally mixed nerves, because after the sensory and motor roots unite, the spinal nerve contains both sensory and motor fibres.

30. Formation of a Typical Spinal Nerve

This is very important.

Dorsal root = Sensory

Ventral root = Motor

↓

Mixed spinal nerve

So:

Sensory dorsal root + Motor ventral root → Mixed spinal nerve

31. Branches of a Spinal Nerve

After emerging, a typical spinal nerve divides into branches called rami.

The textbook mentions:

Ramus dorsalis

Supplies skin and muscles on the dorsal/back side.

Ramus ventralis

The largest major branch; supplies much of the lateral and anterior body wall and limbs, depending on level.

Ramus communicans

Connects the spinal nerve with the sympathetic nervous system.

32. Reflex Action

A reflex action is a:

Sudden, rapid, automatic and involuntary response to a stimulus

It does not require a conscious decision before the initial response occurs.

Example:

You accidentally touch a hot object.

Hot object → Hand immediately withdraws

You don't first consciously think:

"This object is hot. I should now remove my hand."

The withdrawal can begin before conscious awareness is fully processed.

33. Reflex Arc

The pathway followed during a reflex action is called a:

Reflex Arc

A simple reflex arc is:

Stimulus

↓

Receptor

↓

Sensory/Afferent neuron

↓

Spinal cord

↓

Motor/Efferent neuron

↓

Effector muscle

↓

Response

Example:

Hot pin → Skin receptor → Sensory neuron → Spinal cord → Motor neuron → Arm muscle → Hand withdrawn

34. Cranial and Spinal Reflexes

Reflexes can be classified according to the controlling CNS region.

Cranial Reflexes

These are mediated mainly through the brain/brainstem.

Examples include reflexes associated with:

Salivation

Certain visual responses

Other cranial reflex activities

Spinal Reflexes

These are mediated primarily through the:

Spinal cord

They are particularly useful when a rapid response is needed.

Examples:

Withdrawal of leg after stepping on something sharp/hot

35. Unconditioned Reflexes

An unconditioned reflex is natural/inborn.

It does not require previous learning or experience.

Examples from the textbook:

Sneezing

Coughing

Yawning

Hiccupping

So:

Unconditioned = Inborn

36. Conditioned Reflexes

A conditioned reflex develops through learning or previous experience.

Examples may involve learned automatic responses during:

Swimming

Dancing

Cycling

These activities initially require conscious learning, but with repeated practice, many components become increasingly automatic.

The classical concept of conditioned reflexes is strongly associated with Ivan Pavlov.

So:

Conditioned = Learned

37. Monosynaptic Reflex

A monosynaptic reflex contains essentially one synapse between the sensory and motor neurons.

Classic example:

Knee-jerk reflex

Sensory neuron → one synapse → Motor neuron

Therefore it is extremely fast.

38. Polysynaptic Reflex

A polysynaptic reflex contains one or more interneurons between the sensory and motor neurons.

So:

Sensory neuron → Interneuron(s) → Motor neuron

Examples include more complex withdrawal or coordinated reflex responses.

39. Autonomic Nervous System — ANS

The Autonomic Nervous System (ANS) mainly controls involuntary organs and activities.

These include:

Heart

Smooth muscles

Glands

Digestive organs

Blood vessels

Urinary bladder

You normally don't consciously command:

"Heart, beat 75 times per minute."

The autonomic nervous system regulates such functions automatically.

40. Autonomic Pathway

An autonomic motor pathway generally involves two neurons:

Preganglionic neuron

Runs from the CNS to an:

Autonomic ganglion

Then:

Postganglionic neuron

Runs from the ganglion to the:

Effector organ

Therefore:

CNS → Preganglionic neuron → Autonomic ganglion → Postganglionic neuron → Organ

41. Two Divisions of the ANS

The ANS is divided into:

Sympathetic Nervous System

and

Parasympathetic Nervous System

They often produce opposite effects on the same organ.

This allows very fine control.

Think of them roughly as:

Sympathetic = Fight or Flight

Parasympathetic = Rest and Digest

42. Sympathetic Nervous System

The sympathetic system is called thoracolumbar outflow because its preganglionic neurons arise mainly from thoracic and upper lumbar spinal levels.

It is particularly active during:

Fear

Danger

Stress

Exercise

Emergency

Its overall purpose is to prepare the body for action.

Fight-or-Flight Response

Imagine a dog suddenly starts chasing you.

Your sympathetic nervous system helps produce:

Heart rate ↑

Blood pressure ↑

Pupils dilate

Bronchi dilate

Blood flow is redirected toward muscles

Digestive activity ↓

Adrenal activity ↑

Your body is being prepared to fight or escape.

43. Sympathetic Ganglia

Sympathetic ganglia commonly form chains along the sides of the vertebral column.

These are often described as the:

Sympathetic chain / sympathetic trunk

In the sympathetic pathway:

Preganglionic fibres are generally shorter

while

Postganglionic fibres are generally longer

because many ganglia lie relatively close to the spinal cord.

44. Sympathetic Neurotransmitters

The textbook associates sympathetic postganglionic activity mainly with:

Noradrenaline / Norepinephrine

and discusses adrenaline-related effects as well.

Therefore, many sympathetic fibres are described as:

Adrenergic fibres

45. Parasympathetic Nervous System

The parasympathetic nervous system is often called:

Craniosacral outflow

because its fibres arise from selected cranial nerves and sacral spinal regions.

The textbook lists cranial components involving:

III, VII, IX and X

with the vagus nerve (X) being particularly important for many internal organs.

46. Parasympathetic Ganglia

Parasympathetic ganglia are generally located:

Near or within the wall of the effector organ

Therefore:

Preganglionic fibres → generally long

Postganglionic fibres → generally short

This is almost the reverse of the typical sympathetic arrangement.

47. Parasympathetic Neurotransmitter

An important neurotransmitter in the parasympathetic system is:

Acetylcholine

Therefore, fibres using acetylcholine are called:

Cholinergic fibres

48. Parasympathetic — Rest and Digest

The parasympathetic system helps return the body toward its normal resting condition after sympathetic activation.

It generally promotes:

Digestion

Salivation

Normal energy conservation

Slower heartbeat

Intestinal activity

Bladder emptying

So:

Sympathetic → Emergency/activity

Parasympathetic → Recovery/maintenance

49. Sympathetic vs Parasympathetic Effects

This table from your page is especially important:

Organ Sympathetic Parasympathetic
Heartbeat Increases Decreases
Blood vessels Generally constricts many vessels Textbook lists dilation
Blood pressure Increases Decreases
Pupil Dilates Constricts
GI movement Retards peristalsis Accelerates peristalsis
Urinary bladder Relaxes bladder wall Contracts bladder wall

The physiological effects vary by tissue and receptor type, but for your board preparation, learn the textbook comparison.

50. Sensory Receptors

The final topic on your uploaded page begins:

9.7 Sensory Receptors

A sensory receptor is a specialized structure that receives/detects a particular type of stimulus from the:

External environment

or

Internal environment

For example:

Eyes → Light

Ears → Sound

Nose → Chemicals responsible for smell

Tongue → Chemicals responsible for taste

Skin → Touch, pressure, temperature, pain

51. Adequate Stimulus

Each receptor is especially sensitive to a particular kind of stimulus.

That stimulus is sometimes called its:

Adequate stimulus

For example:

Photoreceptor → Light

When an appropriate stimulus activates a receptor, the receptor converts that stimulus into an electrical signal/action potential, directly or through associated sensory neurons.

This information is then sent to the CNS.

Stimulus → Receptor → Electrical signal → Sensory neuron → CNS → Interpretation

52. How Do We Actually "See" or "Hear"?

Your textbook asks an interesting question: how can we enjoy colours or hear a bird?

Your eye itself does not "understand" a sunset.

Instead:

For vision

Light → Photoreceptors in retina → Electrical signals → Optic pathway → Brain → Visual perception

For hearing

Sound waves → Ear receptors → Electrical signals → Auditory nerve → Brain → Sound perception

For smell

Odour molecules → Olfactory receptors → Electrical signals → Brain → Smell perception

So the sense organ detects the stimulus, but the brain processes and interprets the information.

The Entire Human Nervous System in One Map

HUMAN NERVOUS SYSTEM

CNS

→ Brain

→ Forebrain
→ Midbrain — visual & auditory reflex centres
→ Hindbrain
→ Cerebellum — balance & coordination
→ Pons — bridge/communication
→ Medulla — vital involuntary functions

→ Spinal Cord

→ Sensory conduction
→ Motor conduction
→ Reflex centre

PNS

→ 12 pairs Cranial nerves
→ 31 pairs Spinal nerves
→ Afferent = sensory → CNS
→ Efferent = motor → away from CNS

ANS

→ Sympathetic = Fight or Flight
→ Parasympathetic = Rest and Digest

And the complete information pathway is:

Stimulus → Sensory receptor → Afferent neuron → CNS → Processing → Efferent neuron → Muscle/Gland → Response

That single pathway connects sensory receptors, PNS, spinal cord, brain, reflex actions and the autonomic nervous system into one concept.
Sensory Receptors, Eye, Ear & Introduction to Endocrine System
A useful big picture is:
Stimulus → Receptor → Nerve impulse → CNS → Interpretation/Response
1. Classification of Sensory Receptors
A sensory receptor is a specialized cell or structure that detects a particular stimulus.
The textbook broadly divides receptors into:
Exteroceptors
Receive stimuli coming from outside the body.
Interoceptors
Receive stimuli coming from inside the body.
2. Exteroceptors
These tell us what is happening in our external environment.
Phonoreceptors
Location: Organ of Corti in the inner ear
Stimulus: Sound
Function: Hearing
Sound → Phonoreceptors → Nerve impulse → Brain
Statoceptors
Location: Semicircular canals/vestibular apparatus of inner ear
Function: Maintain balance and equilibrium.
They tell the brain about changes in the position and movement of the head.
Photoreceptors
Location: Retina of eye
Stimulus: Light
Function: Vision
The two major photoreceptors are:
Rods and Cones
Thermoreceptors
Location: Skin
They detect:
Heat → Caloreceptors
Cold → Frigidoreceptors
Mechanoreceptors
Location: Skin and other tissues
They respond to mechanical stimuli such as:

Touch

Pressure

Vibration

Deformation/stretch

Pain is often taught alongside skin sensations in this textbook table, though pain receptors are more specifically termed nociceptors.
Chemoreceptors
They detect chemicals.
Two important examples are:
Gustatoreceptors → Taste
Olfactory receptors → Smell
Gustatory receptors are located mainly in the taste buds of the tongue.
Olfactory receptors are located in the olfactory epithelium of the nose.
The five commonly recognized basic tastes are:
Sweet + Sour + Salty + Bitter + Umami
3. Interoceptors
Interoceptors detect changes occurring inside the body.
Examples include internal signals related to:

Hunger

Thirst

Internal pain

Osmotic changes

Chemical changes

They help the CNS monitor the body's internal condition.
4. Proprioceptors
Proprioceptors are located mainly in:
Muscles + Tendons + Joints
They tell the brain about:

Position of body parts

Muscle stretch/tension

Joint movement

Close your eyes and raise your right arm.
Even without seeing your arm, you know approximately where it is.
That awareness is possible largely because of:
Proprioception
5. Baroreceptors
Baroreceptors detect changes in:
Blood pressure
Important locations include:

Carotid sinus

Aortic arch

Walls of some major blood vessels

If blood pressure changes, these receptors send information to the CNS.
The nervous system can then alter:
Heart activity + Blood vessel diameter
to help restore normal pressure.
This is part of homeostasis.
THE HUMAN EYE
The eye is the sensory organ responsible for:
Vision
Humans have two eyes located inside protective bony cavities called:
Orbits
The eyeball is approximately spherical.
6. Protection of the Eye
The eye is protected by:

Bony orbit

Eyebrows

Upper and lower eyelids

Eyelashes

Lacrimal/tear glands

The eyeball is moved by:
Six extraocular muscles
These muscles allow the eyes to move in different directions.
7. Three Layers of the Eyeball
The wall of the eyeball consists of three major layers:
1. Sclera
2. Choroid
3. Retina
Think:
Outside → Sclera → Choroid → Retina → Inside
8. Sclera
The sclera is the tough outermost layer.
It is composed mainly of dense fibrous connective tissue containing collagen.
Functions:

Protects the eyeball

Maintains its shape

Provides attachment for eye muscles

It is the visible white part of the eye.
9. Cornea
At the front of the eye, the sclera becomes a transparent curved structure called the:
Cornea
The cornea allows light to enter the eye and provides much of the eye's initial refraction of light.
It is transparent and normally lacks blood vessels.
The exposed surface is kept moist and protected by tears and the conjunctiva.
10. Conjunctiva
The exposed anterior surface is associated with a thin transparent membrane called the:
Conjunctiva
It helps:

Protect the front of the eye

Lubricate its surface

11. Choroid / Uvea
The choroid is the middle layer.
It is:

Vascular

Pigmented

Its blood vessels provide oxygen and nutrients, particularly to nearby retinal tissues.
Its dark pigmentation helps prevent unwanted internal reflection of light inside the eyeball.
12. Parts Associated with the Choroid
The uveal tract includes:
Choroid proper
Ciliary body
Iris
13. Ciliary Body
The ciliary body contains smooth muscles called:
Ciliary muscles
The lens is connected to the ciliary body through:
Suspensory ligaments
The ciliary muscles help change the shape of the lens during:
Accommodation
The ciliary processes also participate in producing:
Aqueous humour
14. Iris
The iris is the coloured part of the eye.
At its centre is an opening called the:
Pupil
The iris contains smooth muscles that regulate pupil size.
Bright light
Pupil constricts
Dim light
Pupil dilates
Therefore, the iris controls the amount of light entering the eye.
Pigment in the iris determines eye colour.
15. Lens
The lens is:

Transparent

Elastic

Biconvex

Its major job is to help focus light onto the:
Retina
The lens is held in position by suspensory ligaments.
16. Accommodation
The lens must focus light from objects at different distances.
The ability of the lens to make fine adjustments so that light rays are properly focused on the retina is called:
Accommodation
In simple words:
Near/far object → Lens shape adjusted → Image focused on retina
17. Aqueous and Vitreous Humour
The eyeball contains fluid-filled spaces.
Aqueous Humour
A clear watery fluid in the anterior part of the eye.
Vitreous Humour
A clear jelly-like material filling the large posterior cavity.
The vitreous humour helps:

Maintain eyeball shape

Support internal structures

Keep the retina properly positioned against the back of the eye

18. Retina
The retina is the innermost, delicate and light-sensitive layer.
It contains the receptors responsible for vision:
Rods + Cones
The neural retina can be simplified into three major cellular layers:
Photoreceptor layer
Rods and cones
↓
Bipolar cell layer
↓
Ganglion cell layer
The axons of the ganglion cells come together to form the:
Optic nerve
19. Rod Cells
Rods are extremely sensitive to low levels of light.
They are mainly responsible for:
Dim-light / Night vision
This is called:
Scotopic vision
Rods do not provide colour vision.
The important photosensitive pigment in rods is:
Rhodopsin
Rhodopsin is related to vitamin A, which is why severe vitamin A deficiency can interfere with night vision.
So remember:
Rods → Rhodopsin → Dim light → Black/white vision
20. Cone Cells
Cones work best in:
Bright/daylight
This is:
Photopic vision
They are responsible for:
Colour vision + Sharp vision
Humans normally have three functional cone types with different spectral sensitivities, often simplified in school texts as responding mainly to:
Red, Green and Blue regions of light
So:
Cones → Colour + Daylight + Fine detail
21. Rods vs Cones
Feature Rods Cones
Best condition Dim light Bright light
Colour No colour vision Colour vision
Main pigment Rhodopsin Photopsins
Detail Lower Higher
Major role Night vision Day/colour vision
22. Yellow Spot / Macula Lutea
An area near the centre of the retina is called:
Yellow spot / Macula lutea
At its centre is a depression called the:
Fovea centralis
The fovea contains a very high density of cone cells.
Therefore, it provides:
Very sharp and detailed vision
When you look directly at small text, its image is ideally focused around the fovea.
23. Blind Spot
The place where the optic nerve leaves the eyeball is called the:
Blind spot / Optic disc
There are:
No rods or cones
at this point.
Therefore, light falling exactly on the blind spot cannot be detected there.
24. How is an Image Produced and Seen?
This is an important flowchart from your page.
Light enters eye
↓
Cornea → Pupil → Lens
↓
Lens focuses light onto retina
↓
Light falls on rods and cones
↓
Photosensitive pigments undergo changes
↓
Photoreceptors generate electrical signals
↓
Signal → Bipolar cells
↓
Signal → Ganglion cells
↓
Ganglion-cell axons form optic nerve
↓
Optic nerve carries impulses to brain
↓
Visual cortex of cerebrum interprets the information
↓
We perceive the image
The important concept is:
The eye detects light, but the brain produces visual perception.
THE HUMAN EAR
The human ear is described as a stato-acoustic organ because it performs two major functions:
Hearing
and
Balance / Equilibrium
The ear is divided into:
External ear
Middle ear
Inner ear
25. External Ear
The external ear consists mainly of:
Pinna/Auricle
External auditory canal
Tympanic membrane
The pinna collects sound waves and directs them into the auditory canal.
So:
Sound → Pinna → Auditory canal → Tympanic membrane
26. Tympanic Membrane / Eardrum
The tympanic membrane is the:
Eardrum
Sound waves cause it to vibrate.
These vibrations are then transferred to the middle-ear bones.
27. Middle Ear
The middle ear contains three tiny bones called the:
Ear Ossicles
They are:
Malleus → Incus → Stapes
Easy memory:
M-I-S
Common names:
Malleus = Hammer
Incus = Anvil
Stapes = Stirrup
The stapes is the smallest bone in the human body.
28. Function of Ossicles
The ossicles transmit and amplify vibrations:
Eardrum → Malleus → Incus → Stapes → Inner ear
Thus they help efficiently transfer sound energy from air in the middle ear to fluid in the inner ear.
29. Eustachian / Auditory Tube
A tube connects the middle ear with the:
Pharynx
It is called the:
Eustachian tube / Auditory tube
Its important function is to:
Equalize air pressure on both sides of the eardrum
This is why your ears may "pop" during rapid changes in altitude.
30. Inner Ear
The inner ear contains the:
Labyrinth
It has:
Bony labyrinth
and
Membranous labyrinth
The bony labyrinth contains perilymph, while the membranous labyrinth contains endolymph.
Major components include:
Vestibule
Semicircular ducts
Cochlea
31. Cochlea
The cochlea is a coiled structure associated with:
Hearing
It contains fluid-filled chambers.
The textbook describes:
Scala vestibuli
Scala media
Scala tympani
The scala media contains endolymph.
Scala vestibuli and scala tympani contain perilymph.
32. Organ of Corti
Inside the cochlea is the:
Organ of Corti
It is the main sensory organ for hearing.
It lies on the:
Basilar membrane
It contains sensory:
Hair cells
Above the hair cells lies the:
Tectorial membrane
Movement of cochlear structures bends the hair-cell stereocilia, converting mechanical vibration into electrical signals.
Therefore:
Organ of Corti = Sound transducer
A transducer converts one form of energy into another.
Here:
Mechanical sound vibration → Electrical nerve signal
33. Mechanism of Hearing
This entire process should be remembered in sequence:
Sound waves
↓
Pinna collects sound
↓
Auditory canal
↓
Tympanic membrane vibrates
↓
Malleus → Incus → Stapes
↓
Vibrations enter inner ear
↓
Fluid movement in cochlea
↓
Basilar membrane moves
↓
Hair cells of Organ of Corti bend against/relative to tectorial membrane
↓
Electrical nerve impulses generated
↓
Cochlear/auditory nerve
↓
Auditory cortex of brain
↓
Sound is perceived
Again:
Ear receives sound; brain interprets it.
34. Inner Ear and Balance
The inner ear also contains the:
Vestibular apparatus
This includes:

Three semicircular ducts

Utricle

Saccule

These structures maintain:
Balance and equilibrium
35. Semicircular Ducts
There are three semicircular ducts arranged approximately at right angles to one another.
This allows movement of the head in different planes to be detected.
Each duct has an enlarged region called the:
Ampulla
Inside it is a sensory region called the:
Crista
The crista contains sensory hair cells.
These are particularly important for detecting:
Rotational/angular movements of the head — dynamic equilibrium
36. Utricle and Saccule
The utricle and saccule contain sensory areas called:
Maculae
They contain hair cells.
Above the hair cells is a gelatinous layer containing tiny particles called:
Otoliths / Otoconia
These particles contain calcium carbonate.
When the head changes position or the body accelerates linearly, the otolithic layer shifts and bends the hair cells.
This provides information about:
Head position, gravity and linear acceleration
Thus:
Cristae → mainly rotational/dynamic movements
Maculae → head position, gravity and linear acceleration
37. Nervous-System Disorders
Psychological disorders
These are disorders that may affect:

Mood

Thinking

Behaviour

Daily functioning

The textbook lists examples such as:

Intellectual disability

Autism spectrum disorder

Bipolar disorder

Depression

Anxiety disorders

ADHD

Stress-related disorders

These are distinct conditions with different causes, features and treatments; they should not be treated as one single type of illness.
38. Parkinson's Disease
Parkinson's disease is a neurological disorder strongly associated with degeneration of dopamine-producing neurons, particularly in motor-control pathways of the brain.
Common symptoms include:

Tremor

Muscle rigidity/stiffness

Slowness or difficulty in movement

Problems with balance and coordination

Symptoms generally develop gradually.
39. Alzheimer's Disease
Alzheimer's disease is a common cause of dementia.
It causes progressive problems with:

Memory

Thinking

Reasoning

Behaviour

Ability to perform everyday activities

The disease involves progressive changes and loss of neurons and abnormal accumulation of certain proteins in the brain.
There is currently no simple cure, but treatments and supportive care may help manage symptoms and quality of life.
CHEMICAL COORDINATION
So far we have mainly studied:
Nervous coordination
The body also uses:
Chemical Coordination
Cells communicate by releasing chemical substances.
Your textbook describes four broad forms.
40. Autocrine Signalling
Cell → Same cell
A cell releases a chemical signal that acts back on itself.
Think:
"I send a message to myself."
41. Paracrine Signalling
Cell → Nearby cells
A cell releases a chemical that affects neighbouring cells.
Think:
"I send a message to my neighbour."
42. Endocrine Signalling
Endocrine cell → Blood → Distant target cell
A hormone is released into the bloodstream and travels to a target organ/cell.
Example:
Endocrine gland → Hormone → Blood → Target organ
43. Pheromones
Pheromones are chemical signals released by an organism that can influence other members of the same species.
Therefore:
Pheromone = Chemical communication between individuals
ENDOCRINE SYSTEM
44. What Is the Endocrine System?
The endocrine system controls and coordinates body activities using chemical messengers called:
Hormones
Hormones are secreted by ductless endocrine glands.
They are released:
Directly into the blood
The blood carries them throughout the body.
But hormones do not affect every cell equally.
They mainly affect:
Target cells/organs containing the correct receptors
Think of:
Hormone = Key
Receptor = Lock
Only the correct key fits the correct lock.
45. Functions of Hormones
Hormones regulate processes such as:

Growth

Development

Metabolism

Reproduction

Water balance

Stress responses

Blood glucose

Many aspects of homeostasis

46. Properties of Hormones
Hormones are chemical messengers.
They:

Work in very small quantities

Can be effective at very low concentrations

Regulate or modify specific body processes

Act only on cells possessing suitable receptors

Are eventually metabolized/inactivated after performing their function

Too much hormone secretion is:
Hypersecretion
Too little is:
Hyposecretion
Either may produce disorders.
Hormonal secretion is commonly regulated through:
Positive or negative feedback mechanisms
47. Chemical Nature of Hormones
Your textbook divides hormones chemically into several groups.
1. Amines
Derived from amino acids.
Examples include:

Adrenaline/epinephrine

Noradrenaline/norepinephrine

Melatonin

Thyroid hormones are also amino-acid derivatives, though their behaviour differs from typical water-soluble amines.
2. Peptide Hormones
Made from chains of amino acids.
Examples:

Oxytocin

ADH/vasopressin

GnRH

3. Protein Hormones
Larger peptide/protein hormones.
Examples:

Insulin

Glucagon

TSH

FSH

LH

Growth hormone

Relaxin

4. Fatty-Acid Derivatives
Example:
Prostaglandins
5. Steroid Hormones
Derived from:
Cholesterol
Examples:

Estrogen

Testosterone

Aldosterone

Steroid hormones are lipid-soluble and often produce relatively long-lasting changes in gene expression.
6. Gaseous Messenger
The textbook includes:
Nitric Oxide — NO
as a gaseous signalling molecule.
48. Mechanism of Hormone Action
Hormones act by binding to:
Receptors
The receptor may be:
On the cell membrane
or
Inside the cell
This gives two major mechanisms.
49. Hormones Acting Through Membrane Receptors
Water-soluble hormones generally cannot easily cross the lipid cell membrane.
Examples include many:

Peptide hormones

Protein hormones

Catecholamines

Therefore:
Hormone stays outside the cell
It binds to a:
Receptor on the cell membrane
This activates signalling inside the cell.
50. Second Messenger
The hormone outside the cell is often considered the:
First messenger
Its receptor activation can generate intracellular signalling molecules called:
Second messengers
Your textbook mentions:

cAMP

Ca²⁺

cGMP

IP₃

These second messengers produce biochemical changes inside the target cell.
Simplified:
Hormone
↓
Membrane receptor
↓
Second messenger
↓
Biochemical reactions
↓
Physiological response
51. Example — cAMP Pathway
A simplified sequence is:
Hormone binds receptor
↓
Membrane signalling system activated
↓
Adenylyl cyclase activated
↓
ATP converted to cAMP
↓
cAMP acts as second messenger
↓
Enzymes/cellular processes activated
↓
Target-cell response
The hormone itself does not necessarily need to enter the cell.
52. Hormones Acting Through Intracellular Receptors
Lipid-soluble hormones can pass through the plasma membrane.
Important examples include:
Steroid hormones
and
Thyroid hormones
Their mechanism is different.
Step 1
Hormone crosses the cell membrane.
↓
Step 2
Hormone binds an intracellular receptor.
↓
Step 3
A hormone-receptor complex forms.
↓
Step 4
The complex influences specific regions of DNA in the nucleus.
↓
Step 5
Gene transcription changes and mRNA is produced/altered.
↓
Step 6
mRNA directs protein synthesis.
↓
Step 7
New proteins/enzymes change cell activity.
↓
Physiological response
53. Membrane vs Intracellular Hormone Action
Membrane receptor Intracellular receptor
Mainly water-soluble hormones Mainly lipid-soluble hormones
Hormone generally remains outside cell Hormone enters cell
Receptor on membrane Receptor inside cell
Often uses second messengers Often directly alters gene expression
Often faster Often slower
Effects may be shorter Effects often longer-lasting
Example: many peptide hormones Steroid & thyroid hormones
54. Hypothalamus — Beginning of Major Endocrine Glands
Hypothalamus
The hypothalamus is located in the floor of the diencephalon.
It is one of the most important links between:
Nervous System ↔ Endocrine System
Its major overall role includes maintaining:
Homeostasis
It controls much of the pituitary gland's activity through:
Releasing hormones (RH)
and
Inhibiting hormones (IH)
Special neurons in the hypothalamus called neurosecretory cells produce neurohormones.
The page specifically mentions:
ADH/Vasopressin
and
Oxytocin
Complete Connection of These Pages
You can connect everything you've learned like this:
External/Internal change
↓
Stimulus
↓
Sensory receptor
↓
Examples: photoreceptor / phonoreceptor / thermoreceptor / chemoreceptor
↓
Electrical nerve impulse
↓
Sensory neuron
↓
CNS
↓
Brain interprets information
↓
Response
For the eye:
Light → Rods/Cones → Bipolar cells → Ganglion cells → Optic nerve → Visual cortex → Vision
For the ear:
Sound → Pinna → Eardrum → Malleus → Incus → Stapes → Cochlea → Organ of Corti → Auditory nerve → Brain → Hearing
For hormonal coordination:
Endocrine gland → Hormone → Blood → Specific receptor on/in target cell → Cellular response
The 10 most important facts to memorize

Topic Key fact
Rods Dim/night vision
Cones Colour and sharp daylight vision
Fovea Sharpest vision; cone-rich
Blind spot No rods or cones
Optic nerve Carries visual impulses to brain
Organ of Corti Hearing receptor organ
Malleus → Incus → Stapes Three ear ossicles
Semicircular ducts Detect rotational head movements
Maculae of utricle/saccule Gravity/linear acceleration & head position
Hormones Chemical messengers acting on cells with appropriate receptors

Structure	Grey matter	White matter
Cerebrum	Mainly outside	Mainly inside
Spinal cord	Mainly inside	Mainly outside

No.	Cranial nerve	Type	Major function
I	Olfactory	Sensory	Smell
II	Optic	Sensory	Vision
III	Oculomotor	Motor	Eye movements
IV	Trochlear/Pathetic	Motor	Eye movement
V	Trigeminal	Mixed	Facial sensation + chewing
VI	Abducens	Motor	Eye movement
VII	Facial	Mixed	Facial expression, taste, glands
VIII	Vestibulocochlear/Auditory	Sensory	Hearing + equilibrium
IX	Glossopharyngeal	Mixed	Taste, salivation, swallowing
X	Vagus	Mixed	Major visceral functions
XI	Spinal accessory	Motor	Neck/shoulder and related movements
XII	Hypoglossal	Motor	Tongue movement

Region	Number
Cervical	8 pairs (C1–C8)
Thoracic	12 pairs (T1–T12)
Lumbar	5 pairs (L1–L5)
Sacral	5 pairs (S1–S5)
Coccygeal	1 pair (Co1)
Total	31 pairs

Organ	Sympathetic	Parasympathetic
Heartbeat	Increases	Decreases
Blood vessels	Generally constricts many vessels	Textbook lists dilation
Blood pressure	Increases	Decreases
Pupil	Dilates	Constricts
GI movement	Retards peristalsis	Accelerates peristalsis
Urinary bladder	Relaxes bladder wall	Contracts bladder wall

Feature	Rods	Cones
Best condition	Dim light	Bright light
Colour	No colour vision	Colour vision
Main pigment	Rhodopsin	Photopsins
Detail	Lower	Higher
Major role	Night vision	Day/colour vision

Membrane receptor	Intracellular receptor
Mainly water-soluble hormones	Mainly lipid-soluble hormones
Hormone generally remains outside cell	Hormone enters cell
Receptor on membrane	Receptor inside cell
Often uses second messengers	Often directly alters gene expression
Often faster	Often slower
Effects may be shorter	Effects often longer-lasting
Example: many peptide hormones	Steroid & thyroid hormones

Topic	Key fact
Rods	Dim/night vision
Cones	Colour and sharp daylight vision
Fovea	Sharpest vision; cone-rich
Blind spot	No rods or cones
Optic nerve	Carries visual impulses to brain
Organ of Corti	Hearing receptor organ
Malleus → Incus → Stapes	Three ear ossicles
Semicircular ducts	Detect rotational head movements
Maculae of utricle/saccule	Gravity/linear acceleration & head position
Hormones	Chemical messengers acting on cells with appropriate receptors

Endocrine System — Hypothalamus, Pituitary, Pineal, Thyroid, Parathyroid, Thymus, Adrenal, Pancreas & Gonads

Easy Class 10 explanation covering the topics in your uploaded pages

The endocrine system controls many body activities using chemical messengers called hormones. Unlike nerves, which carry rapid electrical impulses, endocrine glands release hormones into the blood, which carries them to specific target cells or organs.

A useful starting sequence is:

Endocrine gland → Hormone → Blood → Target organ/cell → Specific receptor → Response

1. Hypothalamus — Controller Linking Nervous and Endocrine Systems

The hypothalamus is located in the floor of the diencephalon of the brain.

It is extremely important because it forms a functional connection between:

Nervous System ↔ Endocrine System

It controls much of the activity of the pituitary gland.

The hypothalamus contains special neurons called:

Neurosecretory cells

These cells produce chemical messengers called neurohormones.

2. Hypothalamic Releasing and Inhibiting Hormones

The hypothalamus controls the anterior pituitary by producing hormones that either stimulate or inhibit it.

Important examples from your textbook include:

ACTH-Releasing Hormone

Stimulates the anterior pituitary to release ACTH.

Thyrotropin-Releasing Hormone

Stimulates release of TSH.

Gonadotropin-Releasing Hormone — GnRH

Stimulates release of FSH and LH.

Prolactin-Inhibiting Hormone

Inhibits release of prolactin.

Somatostatin

Inhibits release of growth hormone.

Somatocrinin/Growth-Hormone-Releasing Hormone

Stimulates release of growth hormone.

The page also mentions gastrin-releasing peptide (GRP) and gastric inhibitory peptide (GIP) in the broader hormonal context.

3. Hypophyseal Portal System

How do hypothalamic hormones reach the anterior pituitary?

Through a special blood-vessel system called the:

Hypophyseal Portal System

The portal vessels carry hypothalamic regulatory hormones directly to the anterior pituitary.

Therefore:

Hypothalamus → Releasing/Inhibiting hormones → Hypophyseal portal blood → Anterior pituitary → Pituitary hormones

This allows the hypothalamus to control pituitary secretion efficiently.

4. Pituitary Gland / Hypophysis

The pituitary gland, also called the hypophysis, is a small endocrine gland situated below the hypothalamus.

It is connected to the hypothalamus by a stalk called the:

Infundibulum / Hypophyseal stalk

It lies in a bony depression called the:

Sella turcica

Because the pituitary controls several other endocrine glands, it has traditionally been called the:

Master Endocrine Gland

However, remember that the hypothalamus itself controls much of the pituitary's activity.

5. Two Major Parts of Pituitary

The pituitary consists mainly of:

Anterior pituitary — Adenohypophysis

and

Posterior pituitary — Neurohypophysis

They have different embryological origins and different functions.

6. Adenohypophysis — Anterior Pituitary

The adenohypophysis is the larger, glandular part.

It contains hormone-secreting cells and is divided into regions including:

Pars distalis
Pars intermedia
Pars tuberalis

The anterior pituitary produces several extremely important hormones.

7. Growth Hormone — GH/STH

Also called:

Somatotropic Hormone / Somatotropin / STH

Its major function is:

Growth and development of tissues

It promotes:

Protein synthesis
Cell division
Growth of bones and other tissues
Repair and replacement of tissues

GH secretion is especially high during periods of growth.

8. GH Deficiency — Pituitary Dwarfism

Too little GH during childhood causes:

Pituitary dwarfism

The person shows reduced/stunted physical growth.

9. GH Excess — Gigantism

Too much GH during childhood, before the long-bone growth plates close, causes:

Gigantism

The individual becomes abnormally tall because of excessive skeletal growth.

So:

Child + ↓ GH → Dwarfism

Child + ↑ GH → Gigantism

10. Acromegaly

If excessive GH occurs in an adult, after normal long-bone lengthening has largely stopped, it produces:

Acromegaly

It causes enlargement/thickening of structures such as:

Hands
Feet
Jaw
Facial bones
Nose

Therefore:

Adult + Excess GH → Acromegaly

11. Thyroid-Stimulating Hormone — TSH

Also called:

Thyrotropin

TSH acts on the:

Thyroid gland

and stimulates it to produce and release thyroid hormones.

So:

Hypothalamus → TRH → Pituitary → TSH → Thyroid → T₃/T₄

12. ACTH

Adrenocorticotropic Hormone

ACTH acts mainly on the:

Adrenal cortex

and stimulates secretion of adrenal cortical hormones, especially glucocorticoids.

So:

Pituitary → ACTH → Adrenal cortex

13. Prolactin

Also called:

Luteotropin / Mammotropin

Prolactin acts mainly on the:

Mammary glands

It promotes development/function of mammary tissue and especially:

Milk production after childbirth

So:

Prolactin = Production of milk

Do not confuse it with oxytocin, which mainly helps eject/release already-produced milk.

14. Gonadotropins

The anterior pituitary secretes important hormones acting on the reproductive organs.

These include:

FSH

and

LH

In males, LH activity is also traditionally referred to in the textbook as:

ICSH

15. FSH — Follicle-Stimulating Hormone

In females

FSH stimulates:

Growth and development of ovarian follicles

and supports estrogen production.

In males

FSH helps support:

Spermatogenesis

through actions on the seminiferous tubules/Sertoli-cell system.

16. LH — Luteinizing Hormone

In females, LH is important for:

Final maturation of ovarian follicle
Ovulation
Formation of the corpus luteum
Supporting progesterone secretion

Therefore:

LH surge → Ovulation

17. ICSH in Males

In males, LH/ICSH stimulates:

Leydig cells of testes

to produce:

Testosterone

So:

LH/ICSH → Leydig cells → Testosterone

18. Posterior Pituitary — Neurohypophysis

The posterior pituitary is closely connected with the hypothalamus by nerve fibres.

An important point:

The posterior pituitary mainly stores and releases hormones made in the hypothalamus.

The two important hormones are:

Oxytocin

ADH / Vasopressin

19. Oxytocin

Oxytocin has two major textbook functions associated with reproduction.

During childbirth

It stimulates:

Contraction of the uterus

During breastfeeding

It stimulates contraction of myoepithelial cells around mammary glands, causing:

Milk ejection / Let-down

Therefore:

Prolactin → Milk production

Oxytocin → Milk ejection

This difference is very important.

20. ADH / Vasopressin

ADH stands for:

Antidiuretic Hormone

Its major function is to:

Increase water reabsorption by the kidneys

It acts particularly on the distal nephron/collecting ducts.

Therefore:

More ADH → More water reabsorbed → Less urine

It can also cause vasoconstriction, which is why it is called vasopressin.

21. Diabetes Insipidus

If ADH is deficient or its action is impaired, the kidneys cannot conserve water properly.

This may cause:

Large amount of dilute urine
Excessive thirst
Risk of dehydration

This condition is called:

Diabetes Insipidus

Important:

Diabetes insipidus ≠ Diabetes mellitus

Diabetes insipidus is primarily related to ADH/water balance.

Diabetes mellitus is related to blood glucose/insulin.

22. Pars Intermedia and MSH

The textbook mentions that pars intermedia is poorly developed in humans.

It produces MSH — Melanocyte-Stimulating Hormone more prominently in some other vertebrates.

MSH influences:

Melanin pigmentation

23. Pineal Gland

The pineal gland arises from the roof of the diencephalon and lies between the cerebral hemispheres.

Its major hormone is:

Melatonin

Melatonin is sometimes called the:

Sleep hormone

It is synthesized from tryptophan.

24. Functions of Melatonin

Melatonin helps regulate the body's:

Biological Clock / Circadian Rhythm

This is approximately a 24-hour rhythm.

It helps regulate:

Sleep-wake cycle
Body temperature rhythms
Metabolic rhythms
Reproductive rhythms

The pineal gland is sensitive to signals related to the light-dark cycle.

25. Thyroid Gland

The thyroid gland is a large endocrine gland located in the neck, just below the larynx and around the upper tracheal region.

It consists of:

Right lobe + Left lobe

connected by a narrow band called the: Isthmus

26. Thyroid Follicles

The thyroid contains many rounded structures called:

Thyroid follicles

Each follicle is lined by:

Follicular cells

The central cavity contains a jelly-like material called:

Colloid

The thyroid is unusual because significant quantities of its hormone precursor are stored extracellularly in this colloid.

27. Thyroid Hormones

The follicular cells produce:

T₄ — Thyroxine / Tetraiodothyronine

and

T₃ — Triiodothyronine

The numbers indicate iodine atoms:

T₄ = 4 iodine atoms

T₃ = 3 iodine atoms

Iodine is therefore essential for normal thyroid hormone production.

T₃ is generally the more biologically active form.

28. Functions of Thyroid Hormones

Thyroid hormones help regulate:

Basal Metabolic Rate — BMR

They influence:

Metabolism
Protein synthesis
Growth and development
Heat production/thermoregulation
Nervous-system activity
Water and electrolyte balance
Reproductive functions

In simple words:

Thyroid hormones regulate how actively the body's cells use energy.

29. Calcitonin

The thyroid also contains:

Parafollicular / C cells

These produce:

Calcitonin

Calcitonin helps lower elevated blood calcium, including by reducing bone resorption and favoring calcium deposition in bone.

So, simplified:

Calcitonin → Blood Ca²⁺ ↓

30. Hyperthyroidism

Hyperthyroidism means excessive production/action of thyroid hormones.

This increases metabolism.

Possible features include:

Increased BMR
Heat intolerance
Sweating
Rapid heartbeat
Nervousness
Tremor
Weight loss despite appetite changes

31. Graves' Disease / Exophthalmic Goitre

Graves' disease is a common autoimmune cause of hyperthyroidism.

Features can include:

Increased thyroid hormone activity
Increased BMR
Weight loss
Rapid heartbeat
Nervousness
Tremors
Goitre
Characteristic eye changes/protrusion in some patients

32. Hypothyroidism

Hypothyroidism means insufficient thyroid hormone.

The effects differ depending on age.

In infants/children

Severe untreated congenital hypothyroidism can cause major problems with:

Physical growth
Brain development
Intellectual development

Your textbook uses the older term cretinism for this condition; modern medical terminology generally prefers congenital hypothyroidism.

33. Myxoedema

Severe hypothyroidism in adults has traditionally been associated with:

Myxoedema

Features may include:

Low metabolic rate
Tiredness
Cold intolerance
Slower heart rate
Weight gain
Puffy/thickened skin
Reduced activity

34. Simple Goitre

The body requires iodine to produce thyroid hormones.

If iodine intake is insufficient:

↓ Iodine → ↓ Thyroid hormone production → ↑ TSH stimulation → Thyroid enlargement

This enlargement is called:

Simple/Iodine-deficiency Goitre

Using iodized salt is an important public-health measure to prevent iodine-deficiency disorders.

35. Why Does TSH Increase in Hypothyroidism?

This is a useful concept.

Normally:

Thyroid hormones ↑ → Feedback to pituitary/hypothalamus → TSH stimulation ↓

But if thyroid hormone levels become too low:

T₃/T₄ ↓ → Negative feedback decreases → TSH ↑

The pituitary is essentially telling the thyroid:

"Produce more thyroid hormone."

This is an example of:

Negative Feedback

36. Parathyroid Glands

The parathyroid glands are small endocrine glands usually located on the posterior surface of the thyroid gland.

They secrete:

Parathyroid Hormone — PTH / Parathormone

37. Function of PTH

PTH is extremely important for maintaining:

Calcium and phosphate balance

Its major overall effect is to:

Increase blood Ca²⁺

It does this through coordinated effects on:

Bones
Kidneys
Intestines indirectly through vitamin D activation

Simplified for school:

PTH → Blood Ca²⁺ ↑

Calcitonin → Blood Ca²⁺ ↓

They therefore have broadly opposing effects on blood calcium regulation.

38. Hyposecretion of PTH

Too little parathyroid hormone can cause blood calcium to fall.

Low blood calcium increases nerve and muscle excitability.

This can produce:

Tetany

Tetany involves involuntary muscle spasms/contractions.

So:

↓ PTH → ↓ Blood Ca²⁺ → Increased neuromuscular excitability → Tetany

39. Hypersecretion of PTH

Too much PTH can lead to excessive calcium mobilization from bones.

Over time, this can weaken bones and contribute to:

Bone demineralization
Softening/fragility
Increased fracture risk

The textbook connects excessive loss of calcium from bones with osteoporosis.

40. Thymus Gland

The thymus is located in the upper chest behind the sternum.

It is relatively prominent in children and generally becomes smaller/involutes with age.

Therefore, the textbook calls it a:

Temporary gland

Its important hormone/signalling factors include:

Thymosin

41. Function of Thymus

The thymus plays a major role in development of the:

Immune system

It is particularly important for maturation of:

T-lymphocytes / T cells

Therefore:

Thymus → T-cell development → Immunity

42. Adrenal / Suprarenal Glands

There are two adrenal glands, one located on top of each kidney.

Each gland has two major regions:

Outer Adrenal Cortex

Inner Adrenal Medulla

These two regions produce very different hormones.

43. Adrenal Cortex

The adrenal cortex has three major zones.

From outside to inside:

Zona glomerulosa

Zona fasciculata

Zona reticularis

Easy memory:

G-F-R

44. Zona Glomerulosa

Produces mainly:

Mineralocorticoids

The most important example is:

Aldosterone

Aldosterone helps regulate:

Sodium
Potassium
Water balance
Blood volume
Blood pressure

A simplified major effect:

Aldosterone → Na⁺ reabsorption ↑ → Water retention ↑

and promotes K⁺ excretion.

45. Zona Fasciculata

Produces mainly:

Glucocorticoids

The major example is:

Cortisol

Cortisol influences metabolism of:

Carbohydrates
Proteins
Fats

It helps maintain blood glucose during stress and has important anti-inflammatory/immunomodulatory effects.

Cortisol is commonly associated with the:

Stress response

46. Zona Reticularis

Produces mainly:

Adrenal androgens

These contribute to sex-hormone activity and secondary sexual characteristics.

47. Addison's Disease

Reduced production of adrenal cortical hormones causes:

Adrenal insufficiency / Addison's disease

when due to primary adrenal failure.

Possible features include:

Weakness
Weight loss
Low blood pressure
Electrolyte disturbances
Vomiting/diarrhoea or gastrointestinal symptoms
Low cortisol, often with low aldosterone in primary disease

48. Cushing's Syndrome

Excessive glucocorticoid activity produces:

Cushing's syndrome

Features may include:

High blood glucose
Increased central/trunk fat
Rounded face
Muscle wasting/weakness
High blood pressure
Skin changes

So:

Too little adrenal cortex hormones → Addison's

Too much cortisol → Cushing's

49. Adrenal Medulla

The inner part of the adrenal gland is the:

Adrenal Medulla

It produces mainly:

Adrenaline / Epinephrine

and

Noradrenaline / Norepinephrine

These hormones are strongly associated with the:

Fight-or-Flight Response

50. Adrenaline — Emergency Hormone

Adrenaline prepares the body to deal with danger or stress.

It can produce:

Heart rate ↑

Blood flow to skeletal muscles ↑

Airways dilate

Energy availability ↑

Alertness ↑

This is why adrenaline is commonly called an:

Emergency hormone

Noradrenaline also contributes strongly to regulation of vascular tone and blood pressure.

51. Pancreas — Both Exocrine and Endocrine

The pancreas is unusual because it functions as both:

Exocrine gland

and

Endocrine gland

Its exocrine part releases digestive enzymes into ducts.

Its endocrine part consists of clusters called:

Islets of Langerhans

52. Cells of Islets of Langerhans

Your textbook describes four major endocrine cell types.

Alpha (α) cells

Approximately 20%

Produce:

Glucagon

Glucagon raises blood glucose, especially by promoting release of glucose from liver stores.

Glucagon → Blood glucose ↑

Beta (β) cells

Approximately 70%

Produce:

Insulin

Insulin promotes uptake and storage/use of glucose and therefore:

Insulin → Blood glucose ↓

This is one of the most important hormone pairs:

Insulin ↓ glucose

Glucagon ↑ glucose

Delta (δ) cells

Produce:

Somatostatin

It inhibits secretion of both insulin and glucagon and also influences gastrointestinal activity.

PP/F cells

Produce:

Pancreatic Polypeptide — PP

It influences pancreatic and gastrointestinal functions.

53. Diabetes Mellitus

Diabetes mellitus is characterized by persistently elevated blood glucose.

Common symptoms can include:

Increased urination
Excessive thirst
Dehydration
Glucose in urine when blood glucose exceeds the renal threshold
Metabolic disturbances

54. Type 1 Diabetes Mellitus

In Type 1 diabetes, pancreatic beta cells are destroyed, usually through an autoimmune process.

Therefore:

Insulin production becomes severely deficient

So:

β-cell destruction → Insulin ↓↓↓ → Blood glucose ↑

People with Type 1 diabetes require insulin replacement.

55. Type 2 Diabetes Mellitus

In Type 2 diabetes, body tissues become less responsive to insulin:

Insulin Resistance

Over time, insulin secretion may also become inadequate.

So:

Insulin resistance ± declining insulin production → Blood glucose ↑

56. Gonads

The reproductive organs or gonads are:

Ovaries in females

Testes in males

They produce reproductive cells and also function as endocrine glands.

57. Ovarian Hormones

The ovaries produce several hormones.

Estrogen

Produced mainly by developing ovarian follicles.

Functions include:

Development of female secondary sexual characteristics
Reproductive tract development/function
Regulation of menstrual/reproductive cycle

58. Progesterone

After ovulation, the ruptured follicle forms the:

Corpus luteum

which produces large amounts of:

Progesterone

Progesterone helps:

Prepare and maintain the uterine endometrium
Support implantation
Support pregnancy
Influence mammary-gland development

59. Relaxin

Relaxin is associated particularly with pregnancy.

It helps prepare the body for childbirth by promoting changes such as:

Relaxation/softening of pelvic ligaments
Cervical and reproductive-tract adaptations

60. Inhibin

Inhibin provides feedback regulation of the pituitary.

Its major effect is:

Inhibition of FSH secretion

So:

Inhibin → FSH ↓

61. Testes and Testosterone

The testes produce male sex hormones called:

Androgens

The principal androgen is:

Testosterone

It is produced mainly by:

Leydig / Interstitial cells

under stimulation by:

LH / ICSH

So:

Pituitary LH/ICSH → Leydig cells → Testosterone

62. Functions of Testosterone

Testosterone contributes to:

Development of male reproductive organs
Male secondary sexual characteristics
Facial and body hair
Deeper voice
Muscle and bone development
Sexual/reproductive function
Support of spermatogenesis together with FSH

63. Placenta as an Endocrine Organ

During pregnancy, the:

Placenta

acts as a temporary endocrine organ.

It produces several hormones including:

Estrogens
Progesterone
hCG — Human Chorionic Gonadotropin
Human placental lactogen and others

These hormones help support and maintain pregnancy.

64. hCG and Pregnancy Test

A very important textbook point:

Presence of hCG in urine can indicate pregnancy.

Pregnancy tests detect hCG or its components after implantation has led to sufficient hormone production.

65. Gastrointestinal Hormones

The digestive tract also contains endocrine cells.

These produce hormones that regulate digestion.

Gastrin

Stimulates gastric glands and promotes:

Gastric secretion

Secretin

Secretin stimulates mainly the pancreas and biliary system to produce secretions important for neutralizing acidic intestinal contents.

Cholecystokinin — CCK

CCK stimulates:

Pancreatic digestive enzyme secretion
Gallbladder contraction and bile release

So:

CCK → Pancreatic enzymes + Gallbladder contraction

Gastric Inhibitory Peptide / GIP

GIP participates in gastrointestinal and metabolic regulation; the textbook emphasizes inhibition/modification of gastric activity.

66. Other Organs That Produce Hormones

Not every hormone comes from a classical endocrine gland.

Kidney

Produces:

Renin — involved in blood-pressure regulation

Erythropoietin/EPO — stimulates red blood-cell production

and participates in producing active vitamin D:

Calcitriol

Heart

Produces:

ANP — Atrial Natriuretic Peptide

ANP promotes sodium loss by the kidneys and tends to reduce:

Blood volume and blood pressure

A useful contrast is:

Aldosterone → Na⁺ retention

ANP → Na⁺ excretion

67. Hormone Therapy

Your textbook finally introduces:

Hormone Therapy — HT

Hormones or hormone-modifying treatments may be used medically in situations such as:

Certain menopausal symptoms
Specific hormone deficiencies
Some fertility/reproductive conditions
Some cancers
Selected metabolic/endocrine disorders

The exact hormone, dose, benefit and risk depend strongly on the medical condition, so hormone therapy is not one single treatment.

Master Table — Major Endocrine Glands

Gland	Important hormone(s)	Main idea
Hypothalamus	RH/IH, ADH, oxytocin	Controls pituitary; homeostasis
Anterior pituitary	GH, TSH, ACTH, FSH, LH, prolactin	Controls growth & several glands
Posterior pituitary	Releases ADH & oxytocin	Water balance, childbirth/milk ejection
Pineal	Melatonin	Biological clock
Thyroid	T₃, T₄, calcitonin	Metabolism & calcium regulation
Parathyroid	PTH	Raises blood Ca²⁺
Thymus	Thymic hormones	T-cell development
Adrenal cortex	Aldosterone, cortisol, androgens	Salt balance, stress, metabolism
Adrenal medulla	Adrenaline, noradrenaline	Fight or flight
Pancreas	Insulin, glucagon	Blood glucose
Ovary	Estrogen, progesterone, inhibin, relaxin	Female reproduction
Testis	Testosterone, inhibin	Male reproduction
Placenta	hCG, estrogen, progesterone, etc.	Supports pregnancy

The easiest hormone pairs to memorize

Insulin ↓ blood glucose ↔ Glucagon ↑ blood glucose

Calcitonin ↓ blood Ca²⁺ ↔ PTH ↑ blood Ca²⁺

Prolactin = produces milk ↔ Oxytocin = ejects milk

ADH = conserves water ↔ ANP = promotes Na⁺/water loss

Sympathetic/adrenaline = Fight or Flight ↔ Parasympathetic = Rest and Digest

And the most important endocrine control pathway is:

Hypothalamus

↓ releasing hormone

Anterior Pituitary

↓ stimulating/tropic hormone

Target Endocrine Gland

↓ final hormone

Target Tissues

↓

Physiological Response

↖︎ Negative feedback

That feedback system is how the body prevents most hormones from becoming continuously too high or too low.

Control and Coordination Notes

Why do organisms need control and coordination?

2. Control and Coordination in Plants

3. Control and Coordination in Animals

4. Nervous Coordination

5. Nervous System in Hydra

What is a nerve net?

Nerve Net

Location of the nerve nets

6. How does the Hydra nervous system work?

Simple example

7. Diffuse Nervous System :Important Point

8. How did the Nervous System Evolve?

Why is centralization useful?

9. Nervous System in Planaria (Flatworm)

10. Brain of Planaria

Cerebral ganglion

11. Ventral Nerve Cords (VNC)

Ventral Nerve Cords (VNC)

Long Nerve Cords

12. Transverse Nerves :Why does it look like a ladder?

Transverse nerves / Commissures

Ladder-type nervous system

13. Peripheral Nervous System (PNS) in Planaria

Hydra vs Planaria

The most important idea to remember

Diffuse nervous system → Centralization → Greater specialization and coordination

Neural Tissue:

1. Nerve and Tract

2. How Does the Nervous System Work?

Input → Processing → Output

Step 1 — Sensory input

Step 2 — Processing

Step 3 — Motor response

3. Neuron or Nerve Cell

Dendrites → Cyton → Axon

4. Cyton / Cell Body

Nissl's granules

Neurofibrils

5. Dendrons and Dendrites

Direction:

6. Axon

Direction:

Dendrites → Cyton → Axon → Axon terminal

7. Axon Branches

Collaterals

Telodendrons

8. Synapse

9. Nerve Fibres and Their Coverings

Endoneurium

Perineurium

Epineurium

10. Myelinated and Non-myelinated Nerves

Myelinated nerve fibre

Non-myelinated nerve fibre

Why is myelin important?

11. Neuroglial Cells

Neuroglial cells / Glial cells

12. Neuroglial Cells of the CNS

1. Oligodendrocytes

Remember:

2. Microglia / Brain Macrophages

"Cleaning and defence cells of the brain."

3. Astrocytes

4. Ependymal Cells

What is CSF?

13. Neuroglial Cells of the PNS

1. Schwann Cells

Produce the myelin sheath around myelinated nerves of the PNS.

2. Satellite Cells

14. All Six Neuroglial Cells at a Glance

Easy memory trick

15. Complete Chapter Concept in One Flow

Stimulus → Receptor → Sensory impulse → CNS → Processing → Motor impulse → Effector → Response

Synapse & Transmission of Nerve Impulse :

1. What is a Synapse?

Synaptic cleft

2. Properties of Nerve Fibres

a. Excitability / Irritability

b. Conductivity