Action Potentials

Initiation, Depolarization, Repolarization, and Recovery Phases

Introduction

In neurons, action potentials are sudden and brief disruptions of the resting membrane potential along small portions of the axon. As it occurs, each action potential stimulates the adjacent section of the membrane and initiates another action potential. As a result, action potentials propagate along the entire length of an axon, forming an electrochemical signal, or impulse, that allows neurons to communicate with other cells, including other neurons.

Action Potential Animation

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Axons at Rest

Before an action potential occurs, the axon membrane is at rest. During this state, the membrane is slightly polarized (-60 mV to -70 mV), meaning the outside is relatively positive compared to the inside. Several types of ion channels and pump proteins embedded in the axon membrane help establish and maintain the resting potential.

Membrane Proteins

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Protein pumps constantly exchange Na⁺ and K⁺ ions across the membrane. For every 3 Na⁺ ions moved into the extracellular fluid (ECF), 2 K⁺ ions are transported into the intracellular fluid (ICF). This pumping process also establishes chemical gradients for both ions. K⁺ leak channels allow many K⁺ ions to return to the ECF along the gradient generated by the pump proteins. Voltage-gated channels remain closed, preventing the movement of Na⁺ and K⁺ ions through these potential membrane pathways.

Action Potential Phases

Initiation

An action potential begins when a stimulus depolarizes the axon membrane to approximately -55mV. This is the threshold potential for opening the activation gates of many nearby voltage-gated Na⁺ channels. As a result, positively charged Na⁺ ions begin to move along their concentration gradient and enter an axon. Most voltage-gated K+ channels remain closed during the initiation phase, thereby blocking K⁺ ion exit.

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Depolarization

The initial sodium current triggers additional voltage-gated sodium channels to open, creating positive feedback. These events occur so rapidly that it appears as if the threshold potential causes all the nearby Na⁺ channels to open simultaneously. As a result, the membrane enters a period of rapid depolarization as positive ions enter the axon.

As the membrane potential rises, an increasing number of nearby voltage-gated K⁺ channels convert to an open state (gates open), allowing positively charged K⁺ ions to exit the cell driven by their concentration gradient. However, the inflow of Na⁺ ions remains far greater than the outflow of K⁺ ions, and the membrane continues to depolarize.

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Depolarization Peak

An action potential typically peaks at approximately +30 mV to +40 mV for two reasons. (a) First, the voltage-gated Na⁺ channel inactivation gates close in a timed response to the original stimulus. The change in Na⁺ channel conformation (shape) blocks positively charged Na⁺ ions from entering the cell. (b) Secondly, most voltage-gated K⁺ channels are in the open state, and the outflow of positively charged K⁺ ions offsets the decrease in the inflow of positively charged Na⁺ ions.

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Repolarization

Repolarization occurs while the voltage-gated K⁺ channels remain open and the voltage-gated Na⁺ channel inactivation gates remain closed. The resulting rapid efflux of positively charged K⁺ ions through the open channels causes the intracellular side of the membrane to become increasingly negative relative to the extracellular side.

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Hyperpolarization

A brief period of hyperpolarization or undershoot occurs because many of the voltage-gated K⁺ channels are slow to close as the axon membrane approaches its resting potential (-60 mv to -70 mv). The hyperpolarization phase ends when most of the voltage-gated K+ channels enter a closed state (gates closed) and the efflux of positively charged K+ ions stops. Additionally, during this time, the voltage-gated Na⁺ channels begin to reset to a closed state during this time period (their activation gates close, and their inactivation gates open).

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Return to Resting Potential

While the voltage-gated K⁺ channels remain closed and voltage-gated Na⁺ channels continue to reset, K⁺ leak channels and Na⁺/K⁺ pump proteins return the axon membrane to its resting potential.

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Phase Durations

Surprisingly, the entire action potential occurs within just a few milliseconds. Once a stimulus reaches the threshold, the membrane potential rapidly depolarizes, reaching a peak of approximately +30 to +40 mV within 1 ms. It then repolarizes over the next 1 to 2 ms, briefly dropping below the resting potential (hyperpolarization). Finally, the cell’s equilibrium is restored, and the resting potential of -70 mV is reestablished after approximately 4-5 ms.

All-or-None Principle

Action potentials are described as “all-or-none.” Subthreshold stimuli, whether applied individually or summed over time, do not trigger an action potential. Threshold and suprathreshold stimuli produce action potentials of approximately the same size and duration.

Action potentials result from the activity of voltage-gated Na⁺ and K⁺ channels embedded in the axon membrane. Once initiated, the activity of these channels is synchronously timed and is not significantly affected by stimulus strength.

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Refractory Periods

The refractory period in neurons is the amount of time after an action potential begins when another threshold stimulus cannot produce a second action potential. This interval, which consists of an absolute refractory period and a relative refractory period, determines how rapidly action potentials can occur in the same region of the axon.

Refractory Periods

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The absolute refractory period corresponds to the interval when the membrane is depolarizing and repolarizing. The Na⁺ channels are open during depolarization and inactivated during repolarization. A second stimulus, no matter what the strength, cannot affect Na⁺ channels that are already open or inactivated.

The relative refractory period corresponds to the segment when the membrane is in hyperpolarization. Some of the Na⁺ channels have transitioned back to their resting state, making it possible to start a new action potential. However, a suprathreshold stimulus would be required to do so. The membrane is in hyperpolarization, and many of the potassium channels are still open during this time. These conditions make it difficult for the membrane potential to depolarize to a threshold level needed to re-open the Na⁺ channels.

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Reference and Attributions

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