C2.2.8 (HL)—Depolarization and repolarization

Depolarization is the rising phase of an action potential as sodium conductance increases; repolarization returns the membrane toward negative values as potassium conductance rises.

Voltage-gated channels have distinct activation and inactivation timing. The overlap creates a refractory period and ensures the impulse moves forward rather than immediately backward. The changing voltage is evidence of ion movement across the membrane, not a flow of the whole neuron along its axon.

Read the voltage trace with channel states:

• Na⁺ activation: rising phase
• Na⁺ inactivation: peak
• K⁺ activation: falling phase
• delayed K⁺ closing: undershoot

A sodium-channel blocker removes the rapid rising phase, while a potassium-channel blocker prolongs repolarization.

The membrane does not become permanently positive; ion gradients and channel timing restore the resting state.