Electrical Shift During Depolarization of Ovum during fertilization

The egg's membrane is like a battery.

Before a sperm enters, the egg membrane has a voltage difference across it:

Membrane Potential = V_inside − V_outside ≈ −70 mV

This means:

• The inside of the egg is more negative than the outside.
• There are more positive ions outside the cell and relatively more negative charge inside.
• This condition is called the resting membrane potential.

What Happens When the First Sperm Fuses?

The sperm's contact opens ion channels in the egg membrane.

Large numbers of sodium ions (Na⁺) rush into the egg because:

1. Sodium concentration is higher outside the cell.
2. Sodium ions carry a positive charge.
3. They naturally move into the cell through the opened channels.

Why Does the Voltage Change?

Imagine:

• Inside charge = −70 units
• Outside charge = 0 units

When many positively charged sodium ions enter:

• The inside becomes less negative.
• Then it becomes neutral.
• Then it becomes positive.

The membrane potential changes approximately like this:

• Resting state: −70 mV
• Sodium enters: −50 mV
• More sodium enters: −20 mV
• Reaches 0 mV
• Continues to about +10 to +20 mV

This rapid change from negative to positive is called depolarization.

Why Does a Positive Membrane Prevent Other Sperm?

The proteins involved in sperm–egg fusion work properly only when the egg membrane is at its normal negative voltage.

When the membrane suddenly becomes positive:

• The membrane's electrical environment changes.
• Additional sperm can no longer fuse effectively with the egg membrane.
• This creates the fast block to polyspermy.

Simple Analogy

Imagine a nightclub with a green "OPEN" sign.

• Before fertilization, the sign is green (negative membrane potential), so sperm can enter.
• The first sperm enters and instantly switches the sign to red (positive membrane potential).
• Other sperm reach the door but cannot get in.

Summary: The electrical shift is the egg changing its membrane voltage from about −70 mV to +10–20 mV because positively charged sodium ions rush inside. This rapid voltage change temporarily blocks other sperm from fusing with the egg.