The Action Potential

(Sample Lesson)

Introduction

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

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Axons at Rest

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

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Protein pumps continuously exchange Na⁺ and K⁺ ions across the membrane. For every 3 Na⁺ moved into the extracellular fluid (ECF), 2 K⁺ are moved into the intracellular fluid (ICF). The pumping process also creates chemical gradients for both ions. K⁺ leak channels allow many of the K⁺ ions to move back into the ECF along the gradient created by the pump proteins. Voltage-gated channels remain closed, blocking the movement of Na⁺ and K⁺ ions through these potential membrane passageways.

Action Potential Phases

1. Initiation Phase

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

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2. Depolarization Phase

The initial sodium current triggers even more sodium channel activation gates to open, creating a positive feedback effect. 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 due to positive ions entering 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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3. Peak of Depolarization Phase

An action potential typically peaks around +30 mV to +40 mV because of two factors. (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 of the voltage-gated K⁺ channels have converted to an open state, and the outflow of positively charged K⁺ ions offsets the decreasing inflow of positively charged Na⁺ ions.

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4. Repolarization Phase

Repolarization occurs while the voltage-gated K⁺ channels remain in an open state 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 interior of the membrane to become increasingly negative relative to the exterior.

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5. Hyperpolarization Phase

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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6. 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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Duration of Phases

Amazingly, all the steps involved in an action potential take just a few thousandths of a (milliseconds). After the stimulus is applied, the membrane potential rises to a peak potential within about 1 ms, and it then drops just as quickly and briefly hyperpolarizes. Finally, the resting potential is re-established after approximately 4-5 ms.

All-or-None Principle

Action potentials are characterized as being “all-or-none”.

• Subthreshold stimuli cannot produce an action potential.
• Threshold and suprathreshold stimuli produce action potentials of the same magnitude and duration.

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

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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.

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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.

Page Attributions

OpenStax, Anatomy and Physiology

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Reference: “The Action Potential“

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Reference: “The Action Potential“