Action Potential Simulations

Introduction

An action potential begins when a stimulus depolarizes the neuron’s cell membrane from the resting potential of -70 mV to the threshold of approximately −55 mV, triggering a rapid and predictable sequence of electrical events that can be divided into three distinct phases: depolarization, repolarization, and hyperpolarization.

Two types of voltage-gated channel proteins in the axon membrane regulate these changes in membrane potential: Na⁺ voltage-gated channels and K⁺ voltage-gated channels. As their names suggest, these channels are sensitive to changes in membrane potential and open once the threshold is reached.

The Na⁺ voltage-gated channel has two gates: an activation gate and an inactivation gate. The K⁺ voltage-gated channel has only one gate: an activation gate.

Resting: Channel States

At rest, most voltage-gated Na⁺ and K⁺ activation gates are closed, but the channels are available to respond to a threshold-level stimulus. In voltage-gated Na⁺ channels, the inactivation gate is open/non-blocking. This means the Na⁺ channel is closed at rest, but ready to open when a threshold stimulus is reached.

Depolarization Phase: Channel States

A threshold-level stimulus affects both types of voltage-gated channels, but Na⁺ channels respond first. Their activation gates open rapidly, allowing Na⁺ ions to enter the cell down their electrochemical gradient. As positive charges enter the cell, the membrane depolarizes and overshoots 0 mV to approximately +30 to +40 mV.

Repolarization Phase: Channel States

Depolarization ends when the Na⁺ inactivation gates move into a blocking position. This time-dependent response to the earlier threshold stimulus prevents continued Na⁺ entry through most voltage-gated Na⁺ channels, even though the membrane remains depolarized. As more Na⁺ channels become inactivated, Na⁺ influx decreases, and further depolarization stops.

By this stage, the slower voltage-gated K⁺ channels have begun to open, allowing K⁺ ions to efflux (move outward) down their electrochemical gradient. Repolarization begins when outward K⁺ movement exceeds inward Na⁺ movement, causing the membrane potential to move back toward more negative values.

Hyperpolarization Phase

Because voltage-gated K⁺ channels close slowly, K⁺ continues to leave the cell after repolarization. This continued K⁺ efflux briefly makes the inside of the membrane more negative than its resting level, producing hyperpolarization, or an undershoot, to about −80 to 90 mV. During repolarization and hyperpolarization, the return toward a more negative membrane potential causes Na⁺ activation gates to close and Na⁺ inactivation gates to move back into their open, non-blocking position.

Return To Resting State

As voltage-gated K⁺ channels close, the extra outward movement of K⁺ decreases, so the membrane is no longer driven further into hyperpolarization. The same mechanisms that establish and maintain the resting membrane potential, especially K⁺ leak channels and the Na⁺/K⁺ pump, return the membrane to its resting state and maintain the Na⁺ and K⁺ gradients over time.