Action Potential
An action potential is a rapid, temporary, and self-propagating change in the membrane potential of an excitable cell (such as a neuron or muscle fiber). Often referred to as a nerve impulse or "spike," it serves as the primary mechanism for long-distance electrical communication within the nervous and muscular systems.
Unlike graded potentials, which fade over distance, action potentials operate on an all-or-none principle—if a stimulus depolarizes the membrane to a specific threshold, a full-scale action potential is generated; if the threshold is not reached, no action potential occurs.
Phases of the Action Potential
The sequential changes in membrane voltage during an action potential are driven by the opening and closing of voltage-gated ion channels:
1. Resting State: The cell resides at its baseline resting membrane potential, typically around -70 mV for a large neuron. At this stage, voltage-gated sodium and potassium channels are closed, while non-gated potassium leak channels maintain the negative internal charge.
2. Depolarization to Threshold: A stimulus causes a local influx of positive ions, shifting the membrane potential in a positive direction. If this local voltage reaches the threshold potential (typically between -55 mV and -50 mV), it triggers the explosive opening of voltage-gated sodium channels.
3. Rising Phase (Depolarization): Once threshold is breached, voltage-gated sodium channels open rapidly, drastically increasing the membrane's permeability to sodium. Driven by both its chemical concentration gradient and the negative electrical charge inside, sodium rushes into the cell. This massive influx causes the internal potential to skyrocket toward the sodium equilibrium potential, peaking around +35 mV to +40 mV.
4. Falling Phase (Repolarization): As the membrane voltage peaks, two simultaneous events occur: the voltage-gated sodium channels automatically shut via an inactivation gate, and voltage-gated potassium channels open wide. Sodium influx stops completely, while potassium rushes out of the cell, carrying positive charges away and driving the membrane potential back down toward its baseline negative value.
5. Undershoot Phase (Hyperpolarization): Because voltage-gated potassium channels are slow to close, potassium continues to exit the cell even after the baseline resting potential is reached. This causes the internal voltage to temporarily drop below the standard resting level, approaching the potassium equilibrium potential of -90 mV.
6. Return to Rest: The voltage-gated potassium channels eventually close. The continuous background activity of the non-gated leak channels, supported by the steady work of the sodium-potassium ATPase pump, restores the original resting ion balances and returns the membrane to -70 mV.
The Refractory Periods
To ensure that action potentials travel in only one direction and do not blend together, excitable membranes exhibit two distinct refractory periods:
Absolute Refractory Period: The timeframe during which a second action potential cannot be generated under any circumstances, no matter how strong the stimulus. This occurs from the opening of the sodium channels until they reset to their baseline closed state during repolarization. This absolute limit sets a ceiling on the maximum firing frequency of a neuron.
Relative Refractory Period: The timeframe following the absolute period where a second action potential can be fired, but it requires a significantly stronger-than-normal stimulus. This occurs during the hyperpolarization phase. Because the baseline voltage is deeper than normal and potassium channels are still closing, a larger positive influx is needed to drag the membrane up to threshold.
Factors Influencing Conduction Velocity
The speed at which an action potential propagates down an axon depends on its anatomical characteristics:
Axon Diameter: Larger axons offer less internal resistance to the flow of ions, allowing local currents to spread faster and depolarize adjacent sections of the membrane more quickly.
Myelination and Saltatory Conduction: In myelinated axons, the insulating myelin sheath prevents ion leakage across the membrane. Ion exchange is restricted entirely to the uninsulated gaps called Nodes of Ranvier. Instead of crawling continuously down the membrane, the action potential leaps from node to node. This process, known as saltatory conduction, significantly increases velocity while conserving energy, as the sodium-potassium pump only needs to clear ions at the nodes.
Action Potential References
Hodgkin, A. L., & Huxley, A. F. (1952). A quantitative description of membrane current and its application to conduction and excitation in nerve. The Journal of Physiology, 117(4), 500-544.
Kandel, E. R., Schwartz, J. H., & Jessell, T. M. (2013). Principles of Neural Science (5th ed.). New York: McGraw-Hill.
Hall, J. E., & Hall, M. E. (2020). Guyton and Hall Textbook of Medical Physiology (14th ed.). Philadelphia: Elsevier.
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