Muscular System

Why can't a cardiac muscle be completely tetanized?

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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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