Threshold, All-or-None Principle, and Refractory Periods

Three linked properties govern when and how often an axon fires. Threshold is the membrane voltage at which depolarisation becomes self-sustaining; the all-or-none principle means that any suprathreshold stimulus produces a full action potential of fixed amplitude; and refractory periods are the intervals after a spike during which the membrane cannot, or only with difficulty, fire again. Together they set the reliability and maximum frequency of neuronal signalling.

Definition

Threshold is the critical membrane potential at which inward sodium current first exceeds outward current and triggers a regenerative action potential; the all-or-none principle states that a suprathreshold stimulus produces a stereotyped, fixed-amplitude spike; and the refractory periods are the absolute interval during which no second spike can be elicited and the relative interval during which a stronger-than-normal stimulus is required.

Scope

This topic explains threshold, the all-or-none response, and the absolute and relative refractory periods, and how each follows from the kinetics of voltage-gated channels. It also notes where in the neuron action potentials are initiated. It is reference physiology, not clinical guidance.

Core questions

• What determines the threshold for firing an action potential?
• Why is the action potential all-or-none rather than graded with stimulus strength?
• What molecular state of voltage-gated channels produces the absolute and relative refractory periods?
• Where along the neuron is the action potential normally initiated?

Key concepts

• Threshold potential
• All-or-none principle
• Absolute refractory period
• Relative refractory period
• Sodium-channel inactivation and recovery
• Axon initial segment as the site of initiation
• Firing-rate limitation

Mechanisms

Threshold is the voltage at which the regenerative inward sodium current just exceeds the outward potassium and leak currents, so that further depolarisation becomes self-amplifying; below it, depolarisations decay, while at or above it a full spike follows. Because the upstroke is regenerative, its size does not scale with stimulus strength, giving the all-or-none response. After the upstroke, sodium channels are inactivated and cannot reopen until the membrane repolarises, producing the absolute refractory period; as channels recover and while potassium conductance remains elevated, a larger-than-normal stimulus is needed to reach threshold, the relative refractory period. In many neurons the threshold is lowest at the axon initial segment, owing to its high density of sodium channels, which is therefore the usual site of action-potential initiation.

Clinical relevance

Threshold and refractoriness limit firing frequency and explain why conduction can fail when channels are altered or blocked; they underpin the interpretation of excitability testing. This entry is descriptive reference material and is not a basis for individual diagnosis or treatment.

Evidence & guidelines

These properties are derived from the Hodgkin-Huxley channel kinetics and from later studies of action-potential initiation at the axon initial segment in mammalian neurons; they are mechanistic findings rather than clinical guidelines.

History

The all-or-none character of the nerve impulse was recognised in early twentieth-century electrophysiology and was given a mechanistic explanation by the Hodgkin-Huxley model, in which threshold and refractoriness emerge directly from sodium-channel activation, inactivation, and recovery. Later work localised action-potential initiation to the axon initial segment and attributed it to the distribution of specific sodium-channel subtypes.

Key figures

• Alan Hodgkin
• Andrew Huxley
• Bruce Bean
• Greg Stuart

Seminal works

• hodgkin-huxley-1952
• kole-stuart-2012
• hu-2009

Frequently asked questions

Why is the action potential all-or-none?

Once threshold is crossed, sodium influx is self-amplifying, so the spike reaches a full, fixed amplitude regardless of how far above threshold the stimulus was; subthreshold stimuli produce no spike at all.

What causes the absolute refractory period?

During and just after the spike, voltage-gated sodium channels are inactivated and cannot reopen until the membrane repolarises, so no second action potential can be triggered no matter how strong the stimulus.