The current physiology teaching is that extracellular calcium is not required for skeletal muscle contraction. Depolarization of the t-tubule stimulates the dihydropyridine receptors (DHPR) and this results in calcium release from the sarcoplasmic reticulum (SR). This process is called voltage induced calcium release or VICR. This is in contrast to the cardiac muscle where calcium entry through the t-tubules during depolarization causes calcium release from the SR (calcium induced calcium release or CICR). Now it has been reported that calcium enters the skeletal muscle during depolarization. This calcium current is called action potential-activated calcium current or APACC. The physiological role of this calcium entry during depolarization is yet to be understood.
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