Action potential

A brief overview of action potentials in neuroscience

Action Potential

An action potential is a rapid, temporary change in a cell's membrane potential, which is a fundamental mechanism by which neurons communicate. Action potentials are essential for the functioning of the nervous system, allowing for the transmission of signals over long distances within the body.

File:Neuron action potential.svg

Diagram of an action potential in a neuron.

Overview

Action potentials occur in the axons of neurons and are initiated when a neuron receives a sufficient stimulus to depolarize its membrane to a critical threshold. This depolarization is followed by a rapid rise in membrane potential, a peak, and then a return to the resting potential.

Phases of an Action Potential

Resting Potential

The resting potential is the baseline state of a neuron, typically around -70 mV. This potential is maintained by the sodium-potassium pump and the differential permeability of the membrane to sodium and potassium ions.

Depolarization

When a neuron is stimulated, voltage-gated sodium channels open, allowing sodium ions to rush into the cell. This influx of positive ions causes the membrane potential to become less negative, leading to depolarization.

Repolarization

Following depolarization, voltage-gated potassium channels open, allowing potassium ions to flow out of the cell. This efflux of positive ions helps to return the membrane potential to a more negative value.

Hyperpolarization

The membrane potential temporarily becomes more negative than the resting potential due to the continued efflux of potassium ions. This phase is known as hyperpolarization.

Return to Resting Potential

The sodium-potassium pump and other ion channels restore the resting potential, preparing the neuron for the next action potential.

Propagation of Action Potentials

Action potentials propagate along the axon of a neuron. In myelinated axons, this propagation occurs via saltatory conduction, where the action potential "jumps" from one node of Ranvier to the next, increasing the speed of transmission.

File:Saltatory conduction.svg

Illustration of saltatory conduction in a myelinated axon.

Refractory Periods

The refractory period is a time during which a neuron is unable to fire another action potential. It consists of two phases:

• Absolute refractory period: No new action potential can be initiated, regardless of the strength of the stimulus.
• Relative refractory period: A stronger-than-normal stimulus is required to initiate an action potential.

Significance in the Nervous System

Action potentials are crucial for the functioning of the central nervous system and the peripheral nervous system. They enable the rapid transmission of signals necessary for muscle contraction, sensory perception, and cognitive processes.

Related Pages

• Neuron
• Synapse
• Neurotransmitter
• Electrophysiology