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Myosin filaments

Figure 1-6

Excitation-contraction coupling in the muscle, showing (1) an action potential that causes release of calcium ions from the sarcoplas-mic reticulum and then (2) re-uptake of the calcium ions by a calcium pump.

concentration, a 500-fold increase, which is about 10 times the level required to cause maximum muscle contraction. Immediately thereafter, the calcium pump depletes the calcium ions again. The total duration of this calcium "pulse" in the usual skeletal muscle fiber lasts about 1/20 of a second, although it may last several times as long in some fibers and several times less in others. (In heart muscle, the calcium pulse lasts about 1/3 of a second because of the long duration of the cardiac action potential.)

During this calcium pulse, muscle contraction occurs. If the contraction is to continue without interruption for long intervals, a series of calcium pulses must be initiated by a continuous series of repetitive action potentials, as discussed in Chapter 6.

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