Refractory Period

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Refractory periods are a short phase in time following an action potential where another action potential cannot be generated. There are two types of refractory periods:

  1. The absolute refractory period is a period where it is completely impossible for another action potential to be activated, regardless of the size of the trigger (stimulus). This is because the sodium channels are inactivated and remain that way until hyperpolarisation occurs. In the cardiovascular mechanism, this refractory period is sometimes called effective refractory period (ERP)[1].
  2. The relative refractory period is the period that occurs during the undershoot phase; where an action potential can be activated but only if the trigger (stimulus) is large enough. This is because some of the sodium channels have been reactivated and have recovered but it is a difficult process due to the counter-acting potassium flow as some potassium ion channels are still open[2].

A fibre first enters the absolute refractory period directly after an action potential has been fired, then the relative refractory period. The absolute refractory period of a human muscle fibre is typically between 2.2 and 4.6 ms[3].

The refractory period causes 3 things to occur:


  1. CV Pharmacology. Effective Refractory Period. 2017 [cited 1 December 2017]. Available from:
  2. Wayne M. Becker, Lewis J. Kleinsmith, Jeff Hardin. (2006) The World of the Cell, 6th edition, San Francisco: Benjamin Cummings
  3. Brian R. MacIntosh, Phillip F. Gardiner, Alan J. McComas, Skeletal Muscle: Form and Function, Second Edition, Champaign, Ill. ; Leeds : Human Kinetics, 2006, December 2017
  4. Bruce, A, Et al. Molecular biology of the cell. (6th ed.). New York: Garland Science, Taylor and Francis Group, LLC, an informa business; 2015. P622-623
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