Action Potential

When a neuron responds to a stimulus, a sudden change in the voltage (stimulated by the stimulus) across the dendrites and the cell body causes specialized channels called voltage-gated ion channels to open on the surface of axons, triggering an action potential.

An action potential is defined as a rapid membrane depolarization that changes the normal resting negative potential to a positive potential follow by a repolarization back to the normal negative membrane potential.

Involved Membrane Channels

Ungated potassium channel: always open; maintains K⁺ efflux

Voltage-gated sodium channel: closed under resting conditions, quickly opens and closes when detecting nearby membrane depolarization; once closes, will not respond to a second stimulus until the cell almost completely repolarizes. This channel is required for the depolarization phase (influx of Na⁺) of an action potential, and preventing the opening of these channels, which halts depolarization, will prevent the development of an action potential.

Voltage-gated potassium channel: As is the case for the voltage-gated sodium channel, membrane depolarization is the signal that causes it to open. However, it opens more slowly than the sodium channel, and thus its opening peaks later during the action potential. It provides a rapid repolarization phase, so preventing its opening slows repolarization.

Threshold and Subthreshold

When the neuron is depolarized to a level called the threshold, it fires an action potential. Subthreshold potentials of all types are referred to as electrotonic potentials (graded potentials).

Subthreshold potential v. Action potential:

Proportional to stimulus strength (graded) │   independent of stimulus strength (all or none)

Not propagated but decremental with distance │   propagated unchanged in magnitude

Exhibits summation │   summation not possible

Depolarization phase

Initial depolarization: voltage-gated sodium channels open (opens fast, close fast). Membrane conductance to sodium increases, rapid Na⁺ influx, depolarizing the membrane close to the sodium equilibrium potential (+65 mV).

Sodium channels are opening throughout depolarization, and peak sodium conductance is not reached until just before the peak of the action potential. Even though peak sodium conductance represents a situation with a large number of open sodium channels, influx is minimal because the membrane potential is close to the sodium ion equilibrium potential (low electric force; mentioned in Resting Potential).

Repolarization phase

Early repolarization: the voltage-gated sodium channels rapidly close, eliminating Na⁺ influx. Meanwhile, the voltage-gated potassium channels are still opening (they are slower, remember?), increasing potassium conductance beyond the value under resting conditions. This leads to rapid potassium ion efflux that repolarizes the cell.

Peak potassium conductance does not occur until about mid-repolarization. At this point, even though the force on the potassium ions is less than at the beginning of repolarization, there is greater efflux because of the much greater conductance. If the voltage-gated potassium channels do not open during repolarization, the cell will still repolarize through the ungated potassium channels. However, the process will be slower.

The original gradients are reestablished via the Na/K-ATPase pump.

Refractory periods

Absolute refractory period: during this period, no matter how strong the stimulus is, a second action potential cannot be induced. Therefore, its length determines the maximum frequency of action potentials.

Relative refractory period: during this period, a greater than normal stimulus is required to induce a second action potential.

Breakdown of an action potential.

Axon Action Potential and Changes in Conductance

Reference:

Campbell, et al. Biology: A Global Approach. 11th ed., Pearson, 2017.

Robert B. Dunn. 2002. USMLE Step 1: Physiology Notes.