Refractory period (neurophysiology)

The interval after a neuronal action potential when the neuron cannot, or is less likely to, fire again. It consists of absolute and relative phases caused chiefly by sodium-channel inactivation and potassium conductance.

Absolute refractory period: The interval after an action potential when no stimulus can trigger another action potential. In this phase, inactivated voltage-gated sodium channels prevent a second spike.
Saltatory conduction: Propagation of action potentials along myelinated axons by regeneration at gaps called nodes of Ranvier. Refractory membrane behind each advancing spike helps keep propagation directed forward.
Accommodation (neurophysiology): A decline in neuronal excitability during slowly increasing or sustained depolarization. Accommodation can prevent firing without being the recovery interval after a spike.
Electrophysiology: The study of the electrical properties and activity of biological cells and tissues. Measurements of spike timing and recovery reveal refractory behavior in neurons.
Relative refractory period: The interval when a stronger-than-usual stimulus can trigger another action potential. It follows the absolute phase while potassium conductance remains elevated and some sodium channels have recovered.
Node of Ranvier: A gap in the myelin sheath where an axon membrane can regenerate action potentials. Recovery at successive nodes affects how rapidly impulses can be regenerated along myelinated axons.
Threshold potential: The membrane potential at which regenerative ion-channel activation initiates an action potential. Refractoriness changes how difficult it is to reach threshold; threshold itself is not a recovery phase.
Hodgkin–Huxley model: A mathematical model of action potentials based on voltage-dependent sodium and potassium conductances. Its channel dynamics reproduce the ionic recovery that produces refractory phases.
Voltage-gated sodium channel: A membrane protein that opens in response to depolarization, allowing sodium ions to enter a cell. Its transition into an inactivated state creates the absolute limit on firing.
Maximum firing rate: The highest frequency at which a neuron can generate action potentials under specified conditions. The time needed to recover from each spike constrains how closely spikes can be spaced.

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