An electrical nerve impulse is a propagating change in the electrical potential across the membrane of a neuron, usually referring to an action potential travelling along an axon. It is produced by the controlled movement of charged ions through voltage gated ion channels in the neuronal membrane. At rest, the membrane maintains an electrical potential difference because ions are distributed unevenly between the inside and outside of the cell and because the membrane has selective permeability.
When a sufficient depolarising stimulus reaches threshold at an excitable region, voltage gated sodium channels open rapidly, sodium enters the neuron and the membrane potential becomes less negative and can become positive. Sodium channels then inactivate while voltage gated potassium channels open, allowing potassium to leave the cell and helping restore the membrane potential.
An action potential is described as all or none because, once threshold is reached under normal conditions, its amplitude is relatively stereotyped for a particular axon. Increasing stimulus strength does not normally produce a proportionally larger action potential. Instead, stronger stimulation can increase the frequency of action potentials or recruit additional neurons. This distinction is important because neural information is often encoded through firing rate, timing and patterns of activity rather than through continuously variable action potential amplitude.
The threshold depends on the electrical state and properties of the membrane. Following an action potential, the neuron enters refractory periods during which another action potential is difficult or impossible to initiate. These periods help maintain one way propagation and limit the maximum firing frequency.
The action potential has several phases. During depolarisation, voltage gated sodium channels open and sodium conductance rises sharply. The membrane potential moves towards the sodium equilibrium potential, although it normally does not reach it fully. Sodium channels then enter an inactivated state, while potassium conductance increases. Potassium exits the cell, producing repolarisation and often a temporary hyperpolarisation called the afterhyperpolarisation. Potassium channels subsequently close and other membrane transport mechanisms help restore the longer term ionic distributions. The sodium potassium pump is important for maintaining concentration gradients over time, although it is not responsible for the rapid rising phase of each individual action potential. The precise dynamics vary among neuron types because different cells express different ion channels.
Action potentials propagate because depolarisation at one region of the axon changes the electrical state of neighbouring membrane and brings it towards threshold. In unmyelinated axons, propagation involves sequential activation of adjacent sections of membrane. In myelinated axons, the insulating myelin sheath restricts ion movement across much of the membrane, while high concentrations of voltage gated channels occur at nodes of Ranvier. The action potential effectively jumps from node to node in saltatory conduction.
This greatly increases conduction velocity and improves energy efficiency. Axon diameter also affects conduction because larger axons generally offer less internal resistance to current flow. These properties explain why different neural pathways transmit signals at different speeds, with some requiring rapid conduction for immediate sensory or motor responses.
The electrical nerve impulse should be distinguished from an electrical current in an ordinary metal wire. In a neuron, the signal depends on biological membranes, ion gradients, selective ion channels and active cellular maintenance of those gradients. The moving ions do not simply travel from one end of the axon to the other as electrons do through a conventional conductor. Instead, local changes in membrane potential propagate along the membrane.
The action potential itself is regenerative because each newly activated region produces the next part of the signal. This mechanism permits reliable long distance transmission without the signal gradually fading in the way a passive voltage change would. The neuron can therefore communicate over substantial distances while preserving the essential timing and form of its action potential.
Electrical nerve impulses are fundamental to sensory and motor physiology. Sensory receptors can convert physical stimuli such as pressure, temperature, light or chemical concentration into changes in membrane potential that influence action potential generation. Motor neurons use action potentials to carry commands towards muscles, where neurotransmitter release at neuromuscular junctions triggers muscle fibre activity. Autonomic neurons use related mechanisms to regulate organs and glands. Reflexes depend on rapid action potential transmission through sensory, central and motor pathways. The speed of these processes depends on axon diameter, myelination, temperature and other physiological factors. Demyelinating disorders, nerve compression, toxic substances and metabolic disturbances can impair conduction and therefore produce weakness, numbness, altered reflexes or other neurological symptoms.
The action potential also illustrates how neural systems encode information. Because action potential amplitude is relatively constant, the nervous system can represent stimulus intensity through the frequency of impulses, the timing of individual impulses, the number of active neurons and the spatial pattern of activity across populations. Some neurons fire regularly, some in bursts and others in response to particular patterns of input. A single impulse therefore rarely represents a complete message by itself.
The information conveyed depends on the context in which it occurs. Neural circuits interpret trains and populations of impulses, while synapses transform those patterns as signals move through the nervous system. This provides the basis for complex processing despite the apparent simplicity of the all or none action potential.
A common misconception is that an electrical nerve impulse is identical to a thought or that it travels at the speed of electricity through a household cable. Neural conduction is much slower than electromagnetic propagation in free space, although it can still be extremely rapid biologically. Another misconception is that action potentials are generated by the brain alone. Any excitable neuron capable of producing them can generate action potentials in response to appropriate conditions.
Their importance lies in providing reliable long distance signalling within biological tissue. The action potential is one component of a larger communication system that includes sensory transduction, synaptic transmission, receptor activation, intracellular signalling and network processing. Its discovery and experimental study were central to modern neurophysiology and provided a mechanistic explanation for how nervous systems transmit information.
