An action potential represents a rapid, temporary shift in the electrical potential across a neuron’s membrane. This transient change from a negative to a positive charge is caused by the sudden movement of ions into and out of the neuron. Action potentials serve as the primary means of communication within the nervous system, allowing neurons to transmit information throughout the body.
The Neuron’s Baseline Electrical State
Before an action potential occurs, a neuron maintains a resting membrane potential of around -70 millivolts (mV). This negative charge is established and maintained by the unequal distribution of ions across the cell membrane, primarily sodium (Na+) and potassium (K+) ions. The membrane is more permeable to potassium ions than sodium ions due to a higher number of potassium leak channels.
Potassium ions are highly concentrated inside the cell, while sodium ions are highly concentrated outside. As potassium ions leak out of the cell down their concentration gradient, they carry positive charges with them, contributing to the negative charge inside. The sodium-potassium pump, also known as Na+/K+-ATPase, maintains these ion gradients by actively transporting three sodium ions out of the cell for every two potassium ions it brings in, using energy from ATP.
How the Signal is Generated
The generation of an action potential begins when a stimulus causes the neuron’s membrane potential to become less negative, a process known as depolarization. If this depolarization reaches a specific level, called the threshold potential, an action potential will fire. The threshold potential is around -50 to -55 mV, where voltage-gated sodium channels open rapidly.
Once the threshold is reached, a rapid influx of positively charged sodium ions into the cell occurs through these newly opened voltage-gated sodium channels. This causes the membrane potential to quickly become positive, reaching a peak of approximately +30 to +55 mV. This phase is known as the rising phase.
Immediately following the peak, voltage-gated sodium channels inactivate, preventing further sodium influx, while voltage-gated potassium channels open. Potassium ions, which are more concentrated inside the cell, then flow out, carrying positive charges with them. This outward movement of potassium ions causes the membrane potential to return to a negative state, a process called repolarization.
As potassium channels close slowly, the membrane potential briefly becomes even more negative than the resting potential, a phase known as hyperpolarization or undershoot. The “all-or-none” principle states that if the threshold is reached, a full action potential of a consistent size will fire, regardless of the stimulus strength beyond the threshold.
How the Signal Travels Along the Neuron
Once generated, the action potential propagates along the axon, the neuron’s long extension. The depolarization in one segment of the axon triggers the opening of voltage-gated sodium channels in the adjacent segment, creating a wave of depolarization that moves down the axon. This sequential activation ensures the signal travels.
The action potential travels in one direction, from the axon hillock towards the axon terminals, due to the refractory period. The absolute refractory period is a brief time immediately after an action potential fires when voltage-gated sodium channels are inactivated and cannot open again, preventing the signal from moving backward. Following this, a relative refractory period occurs where a stronger stimulus is needed to trigger another action potential.
Myelin sheaths, fatty layers that insulate some axons, increase the speed of action potential conduction. Instead of continuous propagation, the action potential “jumps” between unmyelinated gaps in the myelin called Nodes of Ranvier, a process known as saltatory conduction. At these nodes, voltage-gated sodium channels are concentrated, allowing the action potential to be regenerated. Larger axon diameters also contribute to faster conduction velocities by reducing resistance to ion flow.
Why Action Potentials Matter
Action potentials are important to the function of the nervous system. They enable rapid and long-distance communication between neurons. This electrical signaling is the basis for sensory perception, motor control, thought processes, and all other complex neural activities. Without the precise generation and propagation of action potentials, the nervous system would be unable to coordinate and execute its vast array of functions.