Refractory period (physiology)

As a refractory period refers to the period after triggering an action potential in which the triggering nerve cell, or the unit can not react to a stimulus again.

Biology

An action potential is triggered by the opening of voltage-dependent sodium channels in the membrane of nerve cells. After the fires, the affected channels close automatically and are then not immediately prepared to open. The membrane must be repolarized at -50 mV by efflux of potassium ion out of the cell, so that the sodium channels can be activated again. The cell may be so for a period of time referred to as the refractory period as a refractory or not respond to a stimulus again.

A distinction is an absolute and a relative refractory period:

  • Absolute Refractory Period: During this time, no action potential can be initiated regardless of the stimulus intensity as the voltage-dependent sodium channels are present in an inactivated closed state. (about 2 ms)
  • Relative refractory period: In this phase already the progressive repolarization several, but not all, sodium channels again in the activated, but not the closed state. It can be triggered action potentials, however, a higher stimulus intensity, this requires and the amplitude of the action potential and the slope of the depolarization are reduced. (about 1.5 ms in addition )

The refractory period limits the maximum action potential frequency of a neuron and prevents retrograde spread of excitation.

Cardiology

Thus, a pacemaker is not itself stimulated and as far as possible allow the remaining intrinsic rhythm of the heart, the signal detection ( sensing) is disabled for each channel of the pacemaker for a certain period of time. A distinction is made between ventricular refractory period and atrial refractory period.

The refractory period in cardiac muscle cells by the plateau phase of the AP is substantially longer than in neurons ( about 300 milliseconds). It protects the heart against too rapid succession of uncoordinated contractions due circling excitation ( ventricular fibrillation) with circulatory arrest.

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