Refractory period

(neuroscience) A brief period of time following the stimulation of a nerve during which the nerve will not respond to a second stimulus.

Signals are transmitted around the nervous system, along the fibres (axons) of nerve cells, in the form of electrical impulses called action potentials. After an action potential has swept along a single nerve fibre, a second nerve impulse cannot be initiated immediately. Instead a finite time, known as the refractory period, must elapse before another action potential can be generated in response to a further stimulus (such as an electric shock to the nerve). Neurophysiologists sometimes divide this interval into the absolute refractory period, during which a second action potential cannot be elicited, no matter how strong the stimulus, and the relative refractory period, during which a second action potential can be evoked, but only if the stimulus strength is increased.

The refractory period sets a limit on the frequency at which action potentials can be conducted along single nerve fibres. In mammals, the absolute refractory period is about 1 millisecond and the maximum firing frequency is around 1000 impulses per second (although it is rare for fibres to fire naturally at rates above a few hundred per second). Some animals manage faster rates: the Gymnotid electric fish of South America, for example, can transmit impulses at rates of up to 1600 per second.

The refractory period is a consequence of the molecular processes that underlie the action potential. Action potentials are elicited when tiny pores in the nerve cell membrane, known as sodium channels, open up in response to a stimulus. The sodium channels can exist in three different states: closed, open, and inactivated. At rest, the sodium channels are closed. In response to electrical stimulation, the sodium channels open, but they then pass into the inactivated state, in which the pore is closed but the channel is unable to open in response to a further stimulus. It takes some time for the sodium channels to recover from inactivation and return to the closed state, even after the action potential is over and the nerve cell membrane has returned to its resting state. During this time, the nerve is refractory to stimulation. The refractory period thus reflects the time it takes for the sodium channels to recover.

— Frances M. Ashcroft

Bibliography

• Hodgkin, A. L. (1963). The conduction of the nervous impulse. Liverpool University Press

See also action potential.

If absolute, a period of total inexcitability of a nerve or muscle cell immediately following stimulation. After this absolute refractory period, there is a relative refractory period during which only a stronger than normal stimulus can excite the nerve or muscle.

The term "refractory period" may refer to:

• Refractory period (physiology), the amount of time it takes for an excitable membrane to be ready for a second stimulus once it returns to its resting state following excitation in the areas of biology, physiology, and cardiology.
• Refractory period (sex), the recovery phase after orgasm during which it is physiologically impossible for an individual to have additional orgasms.
• Psychological refractory period, the delay in response to the second of two closely spaced psychological stimuli.
• The period of weeks or even months following a series of seizures during which seizures cannot be induced when referring to epilepsy, known as the Postictal state.

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