An action potential is a rapid, transient change in the electrical potential across the membrane of an excitable cell, such as a neuron or muscle cell. This electrical impulse serves as the fundamental unit of communication within the nervous system, allowing signals to be transmitted quickly. The process operates on an “all-or-nothing” principle: once a threshold is reached, the action potential fires with consistent magnitude, regardless of stimulus strength. If the stimulus does not meet this threshold, no action potential will occur.
The Neuron’s Resting State
Before an action potential, a neuron maintains a resting membrane potential, an electrical charge difference across its cell membrane. The inside of the neuron is typically negatively charged compared to the outside, often around -70 millivolts (mV). This negative charge is primarily established and maintained by the unequal distribution of ions, particularly sodium (Na+) and potassium (K+), across the membrane.
The cell membrane is selectively permeable, with ion channels allowing certain ions to pass. At rest, there are more open potassium “leak” channels than sodium leak channels, allowing potassium ions to slowly diffuse out of the cell, contributing to the negative internal charge.
The sodium-potassium pump, an active transport protein, maintains this gradient by expelling three sodium ions from the cell for every two potassium ions it brings in, using ATP. This ensures a high concentration of sodium outside and potassium inside, setting the stage for rapid changes during an action potential.
The Rising Phase: Depolarization
An action potential begins when a stimulus causes the neuron’s membrane potential to depolarize, becoming less negative. If this depolarization reaches a specific threshold, typically around -55 mV, it triggers the rapid opening of voltage-gated sodium channels.
These channels are sensitive to voltage changes, opening quickly to allow a large influx of positively charged sodium ions into the neuron. The rapid entry of sodium ions causes the inside of the cell to become progressively more positive, quickly reversing the membrane potential from negative to positive.
This swift change in polarity constitutes the rising phase of the action potential. The membrane potential can briefly reach a peak positive value, often around +30 mV, during this phase. This influx is a self-amplifying process, as initial depolarization opens more sodium channels.
The Falling Phase: Repolarization and Undershoot
Following depolarization’s peak, the neuron rapidly repolarizes, returning to a negative membrane potential. This phase begins as the voltage-gated sodium channels inactivate, effectively stopping the influx of sodium ions.
Almost simultaneously, voltage-gated potassium channels, which open more slowly in response to the depolarization, become fully active. The opening of these potassium channels allows positively charged potassium ions to flow out of the cell, driven by their concentration gradient and the now positive internal charge. This efflux of positive charge causes the membrane potential to swiftly become negative again.
Potassium channels are slow to close, leading to a brief undershoot (hyperpolarization) where the membrane potential becomes even more negative than rest. This transient hyperpolarization ensures that the neuron is temporarily less excitable.
Recovery and Refractory Periods
After the undershoot, the neuron gradually returns to its resting membrane potential. The slow closure of voltage-gated potassium channels eventually allows the membrane potential to normalize. Throughout this recovery, the sodium-potassium pump continuously restores ion concentrations by moving sodium ions out and potassium ions back into the cell, re-establishing gradients for future action potentials.
During and immediately after an action potential, the neuron experiences refractory periods, making it less capable of firing another. The absolute refractory period occurs during the depolarization and initial repolarization phases, when voltage-gated sodium channels are either already open or inactivated and cannot be reopened.
This prevents another action potential from being generated, regardless of stimulus strength, and ensures that the impulse travels in one direction. The relative refractory period follows, during which a new action potential can be triggered, but only if the stimulus is stronger than usual, because some potassium channels are still open and a greater depolarization is required to reach the threshold.