refractory period 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.

refractory period The period after the transmission of an impulse in a nerve or muscle in which the membrane of the axon or muscle fibre regains its ability to transmit impulses (see action potential). This period lasts approximately 3 milliseconds and is divided into an absolute refractory period, during which a second impulse may not be generated; and a relative refractory period, during which it is possible to generate an impulse only if there is an abnormally strong stimulus.

refractory period n. (in neurology) the time of recovery needed for a nerve cell that has just transmitted a nerve impulse or for a muscle fibre that has just contracted. During the refractory period a normal stimulus will not bring about excitation of the cell.