The Sarcoplasmic Reticulum

Where does the calcium come from to trigger the interaction of actin and myosin underlying the sliding of the filaments? Recall from Chapter 8 that a rise in internal calcium is also responsible for the release of chemical transmitter during synaptic transmission, and that in that case the calcium enters the cell from the ECF through voltage-sensitive calcium channels in the plasma membrane. In the case of skeletal muscle, however, the calcium does not come from outside the cell; rather, the calcium is injected into the intracellular fluid from a separate intracellular compartment called the sarcoplasmic reticulum. The sarcoplasmic reticulum is an intracellular sack that surrounds the myofibrils of a muscle cell. This sack forms a separate intracellular compartment, bounded by its own membrane that is not continuous with the plasma membrane of the muscle cell.

The concentration of calcium ions inside the sarcoplasmic reticulum is much higher than it is in the rest of the space inside the cell; in fact, it is probably higher than the concentration of calcium in the ECF. This accumulation of calcium inside the sarcoplasmic reticulum is accomplished by a calcium pump in the membrane of the sarcoplasmic reticulum. Like the sodium pump of the plasma membrane, this calcium pump uses metabolic energy in the form of ATP to transport calcium ions across the membrane against a large concentration gradient; in this case, the pump moves calcium ions into the sarcoplasmic reticulum. To initiate a contraction, a puff of calcium ions is released from the sarcoplasmic reticulum, which is strategically located surrounding the contractile apparatus of the myofibrils. The action of the released calcium is terminated as the ions are pumped back into the sarcoplasmic reticulum by the calcium pump. Here, then, is a third role for ATP in the contraction process: ATP, as the fuel for the calcium pump, is responsible for terminating a contraction as well as for energizing myosin and breaking the bond between actin and myosin.

Calcium is released from the sarcoplasmic reticulum via calcium-selective ion channels, which are located in the sarcoplasmic reticulum membrane. These calcium channels are quite different from the voltage-dependent calcium channels we have encountered previously in our discussion of synaptic transmission. Rather than being activated by depolarization, as are the voltage-dependent calcium channels, these calcium channels in the sarcoplasmic reticulum are activated by an increase in cytoplasmic calcium concentration. For this reason, they are referred to as calcium-induced calcium release channels. If calcium is released from the sarcoplasmic reticulum via channels that are themselves activated by an increase in calcium, then the calcium release process exhibits positive feedback reminiscent of the rising phase of the action potential (where depolarization opens sodium channels, which in turn produce further depolarization). This positive feedback ensures that the calcium release is large and rapid, producing fast and complete activation of the contraction mechanism.

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