What is an action potential?

It is a brief, self-regenerating reversal of the membrane voltage that travels along an axon at constant amplitude, generated by the sequential opening of voltage-gated sodium and potassium channels.

Why do myelinated axons conduct faster?

Myelin insulates the internodal membrane so that regenerative currents are concentrated at the nodes of Ranvier, letting the impulse jump from node to node (saltatory conduction) instead of propagating continuously.

Axonal Physiology: Action Potentials and Impulse Conduction

Axonal physiology is the study of how axons generate and propagate the electrical signals that carry information through the nervous system. Its central object is the action potential, a brief, self-regenerating reversal of membrane voltage that travels along the axon without losing amplitude. This area gathers the mechanisms that make excitability possible: the ionic currents through voltage-gated channels, the threshold and refractory behaviour that shape firing, the myelination that accelerates conduction, and the passive cable properties that set how signals spread.

Definition

Axonal physiology concerns the biophysical generation, regulation, and propagation of action potentials along axons, including the ionic currents, channel gating, excitability thresholds, and passive electrical properties that govern impulse conduction.

Scope

This area orients the reader across the physiology of the axon as a signalling cable. It links the molecular machinery of voltage-gated ion channels to the macroscopic action potential, and the action potential in turn to its conduction along unmyelinated and myelinated fibres. It covers the quantitative Hodgkin-Huxley framework, the all-or-none and refractory properties, saltatory conduction, and cable theory, treating them as foundational reference knowledge rather than clinical instruction.

Sub-topics

Core questions

How does an axon convert a graded depolarisation into an all-or-none action potential?
Which ionic currents underlie the rising and falling phases of the action potential, and how are they gated by voltage?
Why and how does myelination increase conduction velocity?
How do the passive cable properties of an axon determine the spread and speed of electrical signals?

Key concepts

Action potential
Voltage-gated ion channels
Threshold and all-or-none firing
Refractory periods
Saltatory conduction
Myelination
Cable properties and length constant
Conduction velocity

Key theories

Hodgkin-Huxley theory of the action potential: A quantitative model in which the action potential arises from voltage- and time-dependent sodium and potassium conductances, formalised as a set of differential equations that reproduce the measured nerve impulse and its conduction.
Cable theory of axonal conduction: A treatment of the axon as a leaky electrical cable in which membrane resistance and capacitance together with axial (longitudinal) resistance determine how passive potentials decay with distance and how impulse velocity scales with fibre size.

Mechanisms

An action potential begins when depolarisation reaches threshold and opens voltage-gated sodium channels, producing a regenerative sodium influx that drives the membrane toward the sodium equilibrium potential; sodium-channel inactivation and the delayed opening of voltage-gated potassium channels then repolarise the membrane. Hodgkin and Huxley captured this interplay as voltage- and time-dependent conductances. The depolarisation at one point spreads passively to adjacent membrane according to the axon's cable properties, bringing the next region to threshold and so propagating the impulse. In myelinated fibres the insulating sheath restricts current entry to the nodes of Ranvier, so the impulse appears to jump from node to node (saltatory conduction), greatly increasing speed and efficiency, while fibre diameter and internal resistance further set conduction velocity.

Clinical relevance

The physiology of axonal conduction underlies clinical electrophysiology, including nerve conduction studies, and provides the conceptual basis for understanding demyelinating and channel-related disorders. This area describes normal mechanisms and the principles behind such tests; it is reference and educational material and is not a basis for individual diagnosis or treatment.

Evidence & guidelines

The core mechanisms in this area rest on classic quantitative electrophysiology, above all the Hodgkin-Huxley series on the squid giant axon, with later reviews extending the framework to mammalian central neurons. These are descriptions of physiological mechanism rather than clinical guidelines.

History

The modern understanding of axonal signalling was built in the mid-twentieth century on the squid giant axon, whose large size allowed direct measurement of membrane currents. Hodgkin and Huxley's 1952 synthesis turned voltage-clamp recordings into a predictive mathematical model of the action potential, for which they later shared a Nobel Prize. In parallel, Rushton's cable analysis explained how fibre size governs conduction, and subsequent work tied these biophysical principles to the molecular structure of ion channels and to conduction in myelinated mammalian nerve.

Key figures

Alan Hodgkin
Andrew Huxley
Bernard Katz
William Rushton
Bertil Hille

Seminal works

hodgkin-huxley-1952
rushton-1951
bean-2007

Frequently asked questions

What is an action potential?: It is a brief, self-regenerating reversal of the membrane voltage that travels along an axon at constant amplitude, generated by the sequential opening of voltage-gated sodium and potassium channels.
Why do myelinated axons conduct faster?: Myelin insulates the internodal membrane so that regenerative currents are concentrated at the nodes of Ranvier, letting the impulse jump from node to node (saltatory conduction) instead of propagating continuously.