Refractory period

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In biology and medicine, the refractory period is the period after the triggering of an action potential in which the triggering nerve cell or the aggregate cannot temporarily react again to a stimulus .

biology

An action potential is triggered by the opening of voltage-dependent sodium channels in the membrane of the axon of the nerve cells . After triggering, the affected channels close automatically and are then not immediately ready to open again. Due to the outflow of potassium ions out of the cell, the membrane has to be repolarized below -50 mV so that the sodium channels can be reactivated. The cell can therefore not react again to a stimulus for a certain period of time, which is referred to as the refractory period or also the refractory phase.

A distinction is made between an absolute and a relative refractory period:

  • Absolute refractory period: During this time, no action potential can be triggered, regardless of the stimulus strength, since the voltage-dependent sodium channels are in an inactivated, closed state . (approx. 2 ms for skeletal muscles or classic nerve cells, 300 ms for myocardial cells)
  • Relative refractory period: In this phase, due to the progressive repolarization, several, but not all, sodium channels are again in an activatable, but still closed state . Action potentials can be triggered, but this requires a higher stimulus strength and the amplitude of the action potential and the steepness of the depolarization are reduced. (approx. 1.5 ms additionally)

The refractory period limits the maximum action potential frequency of a neuron to around 500 Hertz and prevents retrograde conduction of excitation.

cardiology

So that a pacemaker does not stimulate itself and allows the heart's own rhythm as far as possible, the signal recognition (sensing) of the individual channels of the pacemaker is deactivated for a certain period of time. A distinction is made between the ventricular refractory period and the atrial refractory period .

In cardiac muscle cells, the refractory period is significantly longer due to the plateau phase of the action potential than in neurons (about 300 milliseconds). It protects the heart from a rapid sequence of uncoordinated contractions as a result of circular excitation ( ventricular fibrillation ) with cardiac arrest .

See also

Individual evidence

  1. ↑ membrane and action potential
  2. ↑ Klinke, Pape, Kurz, Silbernagl: Physiologie . 6th edition. Thieme, 2010, p. 148f.