Muscle contractions occur when the myocyte is depolarized enough for an action potential to occur. Depolarization is caused by acetylcholine released from the adjacent motor neuron, which activates nicotinic acetylcholine receptors and opens the sodium/potassium channel. The fast influx of sodium and slow efflux of potassion trigger the action potential. This action potential activates L-type voltage-dependent calcium channels on the membrane and ryanodine receptors on the sarcoplasmic reticulum, both which cause calcium ions to be released into the cytosol. In smooth muscle, ionic calcium induces muscle contraction by binding to and activating myosin light chain kinase, while in striated muscle contraction results from ionic calcium binding to and activating troponin C.
References
Muscle/Heart Contraction Xuan References
Horowitz A, Menice CB, Laporte R, Morgan KG: Mechanisms of smooth muscle contraction. Physiol Rev. 1996 Oct;76(4):967-1003. doi: 10.1152/physrev.1996.76.4.967.
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