A single cardiac muscle contraction lasts 10-15 times longer than skeletal muscle, due to a prolonged release of Ca ++ into the sarcoplasm from SR and Ca++ from interstitial fluid that bathes the fibres. Also, channels that allow inflow of Ca++ from the interstitial fluid stay open for a longer period of time. We know that skeletal muscle contracts in response to the release of ACh from a nerve impulse. However, stimulation of cardiac muscle contraction occurs by autorhythmic stimulation. Under normal resting state, cardiac muscle tissue contracts and relaxes about 75x per minute. The mitochondria of cardiac muscle tissue are large and numerous suggesting that cardiac muscle depends largely on aerobic cellular respiration to generate ATP and thus requires a constant supply of oxygen. Cardiac muscle fibres are also able to use lactic acid produced by skeletal muscle fibres to make ATP – a benefit during exercise.
Smooth Muscle Tissue
Smooth muscle tissue differs from skeletal muscle tissue in that it is involuntary and non-striated, or smooth, in appearance. It is found lining the hollow structures of the body, like the GI tract and blood vessels, as well as within most internal organs. The anatomy of a smooth muscle fibre is also different from skeletal muscle fibres. The sarcoplasm of smooth muscle contains both thick and thin filaments in ratios of 1:10 and 1:15, but they are not arranged in orderly sarcomeres like skeletal muscle. Smooth muscle also contains an Intermediate Filament. Because there is no regular pattern of overlap, smooth muscle fibres do not exhibit striations but exhibit a smooth appearance. Smooth muscle lacks T- tubules and have only a small amount of SR for Ca++ storage. There are small pouch-like invaginations of the plasma membrane called Caveolae that contain extracellular Ca++ that can be used for muscle contraction.
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