An action potential is a rapid, temporary change in the electrical potential across a neuron’s membrane, representing the signal nerve cells use to communicate. These electrical impulses carry instructions from the brain to muscles and relay sensory information from the body back to the brain. This process enables everything from simple reflexes to complex thoughts.
Setting the Stage: The Neuron’s Resting State
Before a neuron sends a signal, it exists in a baseline condition known as the resting membrane potential, where the inside of the cell is about 70 millivolts more negative than the outside. This electrical gradient is maintained by the cell membrane, which separates the internal and external environments.
The gradient is created by different concentrations of ions. The fluid outside the neuron has a high concentration of positively charged sodium ions (Na+), while the fluid inside has a high concentration of positively charged potassium ions (K+) and negatively charged proteins.
The membrane is selectively permeable through proteins called ion channels. At rest, potassium channels are more open than sodium channels, allowing positive potassium ions to leak out of the cell. To counteract this and maintain ion concentrations, the sodium-potassium pump actively transports three sodium ions out for every two potassium ions it brings in, preserving the negative resting state.
The Spark: From Stimulus to Threshold
An action potential requires a trigger, usually a chemical signal called a neurotransmitter from a neighboring neuron. When neurotransmitters bind to a neuron’s receptors, they cause small, localized changes in the membrane potential. These minor fluctuations are known as graded potentials.
Graded potentials are variable in strength and diminish as they travel from the point of stimulation. Some stimuli cause depolarization, making the cell’s interior less negative, while others cause hyperpolarization, making it more negative. A neuron integrates these incoming signals, as a single graded potential is not enough to cause it to fire.
For an action potential to begin, the cumulative effect of these graded potentials must depolarize the membrane at the axon hillock to the threshold potential, which is around -55 millivolts. If the depolarization is too weak and fails to reach this threshold, nothing significant happens and the membrane returns to its resting state. If the threshold is reached, an irreversible cascade of events is set in motion.
The Main Event: Phases of the Action Potential
When the threshold is reached, voltage-gated ion channels open and close in a predictable sequence. The first phase is rapid depolarization, where voltage-gated sodium channels open, allowing a flood of positive sodium ions into the neuron. This influx of positive charge causes the inside of the membrane to become positive, reaching a peak of about +40 millivolts.
This peak begins the repolarization phase. The voltage-gated sodium channels inactivate, and the slower voltage-gated potassium channels fully open. Positively charged potassium ions then rush out of the cell, causing the membrane potential to swing back toward a negative value.
The process often overshoots the resting potential, leading to a brief period of hyperpolarization where the membrane becomes more negative than its resting state. This happens because the potassium channels are slow to close. Once these channels close, the membrane potential stabilizes back at its resting state.
Signal on the Move: Propagation and Properties
An action potential travels down the length of the axon as a propagating wave. The depolarization of one segment of the membrane triggers the depolarization of the adjacent segment. This process ensures the signal regenerates itself along the way, arriving at the end of the axon with the same strength it started with.
This process follows the “all-or-none” principle. If a stimulus is strong enough to reach the threshold, a full-sized action potential is generated. If the stimulus is weaker, no action potential occurs, meaning the neuron fires at its maximum capacity or not at all.
Following an action potential, the neuron enters a refractory period, a brief time when it is difficult or impossible to generate another one. This period ensures that the signal travels in only one direction, from the cell body toward the axon terminal, and prevents signals from overlapping.
The speed of the signal is influenced by two main factors. A larger axon diameter results in faster travel. The presence of a fatty insulating layer called myelin also significantly speeds up communication, as the signal jumps between gaps in the myelin in a process called saltatory conduction.