The Transverse Tubule System

How does an action potential in the plasma membrane of a muscle cell trigger release of calcium from the sarcoplasmic reticulum, whose membrane is separate from the plasma membrane? The crucial aspect of the action potential in triggering contraction is depolarization of the plasma membrane. However, to affect the sarcoplasmic reticulum surrounding myofibrils deep within the muscle cell, the depolarization produced by the action potential at the outer surface of the cell must somehow be transmitted to the interior of the muscle cell. To accomplish this task, the plasma membrane of the muscle cell forms periodic infoldings, called transverse tubules, that extend into the depths of the muscle fiber (Figure 10-10). The long fingers of plasma membrane projecting into the cell provide a path for depolarization resulting from an action potential in the surface membrane to influence events in the interior of the cell.

In most species, the transverse tubules are located at the boundary between the A band and the I band. This location represents the edge of overlap between the thick and thin filaments in a resting muscle fiber, and it makes sense that calcium release should be triggered first at this position at the leading edge of filament sliding. Where the transverse tubules encounter the sarcoplasmic reticulum, the membranes come into close apposition to form a structure called a triad. This is presumably the point of interaction between the depolarizing signal and the membrane of the sarcoplasmic reticulum. Note, however, that although the membranes are close together at a triad, they do not touch. Because the membranes are not in continuity, the depolarization produced during the action potential cannot spread directly to the sarcoplasmic reticulum. Therefore, some indirect signal is required to link depolarization of the transverse tubule to calcium release by the sarcoplasmic reticulum.

Because calcium is released from the sarcoplasmic reticulum through calcium-induced calcium release channels, it is natural to suppose that the link between transverse tubules and sarcoplasmic reticulum is mediated by an

Sarcoplasmic reticulum Transverse tubule

Sarcoplasmic Transverse Sarcoplasmic reticulum tubule reticulum

Sarcoplasmic Transverse Sarcoplasmic reticulum tubule reticulum

I band A band

Figure 10-10 The sarcoplasmic reticulum and transverse tubules. (a) The transverse tubules are invaginations of the plasma membrane of the muscle cell. Depolarization during an action potential can spread along the transverse tubules to the interior of the fiber. The sarcoplasmic reticulum is an intracellular compartment surrounding each myofibril in the muscle cell. Calcium ions that trigger contraction are released from the sarcoplasmic reticulum. The membranes of the transverse tubules and the sarcoplasmic reticulum come close together at the triad. Depolarization of the membrane of the tubules triggers calcium release from the sarcoplasmic reticulum. (b) The triad near a single myofibril, viewed through the electron microscope. (Electron micrograph provided by B. Walcott of the State University of New York at Stony Brook.)

I band A band

Figure 10-10 The sarcoplasmic reticulum and transverse tubules. (a) The transverse tubules are invaginations of the plasma membrane of the muscle cell. Depolarization during an action potential can spread along the transverse tubules to the interior of the fiber. The sarcoplasmic reticulum is an intracellular compartment surrounding each myofibril in the muscle cell. Calcium ions that trigger contraction are released from the sarcoplasmic reticulum. The membranes of the transverse tubules and the sarcoplasmic reticulum come close together at the triad. Depolarization of the membrane of the tubules triggers calcium release from the sarcoplasmic reticulum. (b) The triad near a single myofibril, viewed through the electron microscope. (Electron micrograph provided by B. Walcott of the State University of New York at Stony Brook.)

influx of calcium ions from the extracellular space. Indeed, the membrane of the transverse tubules contain voltage-dependent calcium channels that are opened by depolarization, and these calcium channels are required for the initiation of contraction. In cardiac muscle, influx of calcium from the extracellular fluid via these depolarization-activated calcium channels is in fact required to r-

initiate the muscle contraction, so that calcium ions through the tranverse tubule calcium channels do indeed trigger the calcium release from the sar-coplasmic reticulum in cardiac muscle cells. However, in skeletal muscle, calcium influx via the calcium channels of the transverse tubules is not required to trigger contraction, and some other linkage mechanism is required. The mechanism is not yet understood in detail, but it is thought that the calcium channels of the transverse tubule act as voltage sensors to detect the depolarization produced by the action potential and that there is a direct physical link extending through the intracellular space connecting single calcium-induced calcium-release channels in the sarcoplasmic reticulum membrane to single voltage-dependent calcium channels in the immediately adjacent membrane of the transverse tubule. Through this physical link, a conformation change in the transverse-tubule calcium channel upon depolarization is thought to induce a conformation change in the calcium-induced calcium-release channels. The sarcoplasmic reticulum channel then opens, locally releasing calcium and initiating the explosive calcium-induced release of calcium from the sarcoplasmic reticulum as a whole.

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