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Answered 2009-10-22 16:30:52
Cardiac Muscle and Tetany

In cardiac muscle, the mechanism of contraction is essentially the same as in skeletal, but the excitation-contraction coupling mechanism differs slightly. T-tubules invaginate at the level of the Z-lines. The SR is relatively poorly developed (cisternae are small or absent - in most EM sections 'diads' and not 'triads' are seen), and provides insufficient Ca2+ to fully activate the contractile apparatus. Unlike those in skeletal muscle the ryanodine channels in cardiac muscle are activated by Ca2+ in the cytosol (calcium activated calcium release) and Ca2+ entry through the dihydropyridine channels is an important trigger of Ca2+ release. Significant amounts of Ca2+ enter the fibre from the ECF during the AP, which consequently has a long plateau phase caused by slowly inactivating Ca2+ channels in the sarcolemma, prolonging AP (c. 200 ms). Ca2+ is also released from sub-sarcolemmal binding sites during the AP. Because the AP lasts almost as long as the twitch, heart muscle cannot be tetanized.

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