Structure of Skeletal Muscle

Figure 10-1 shows the structure of a typical mammalian skeletal muscle at progressively greater magnification. To the naked eye, an intact muscle appears to be vaguely striped longitudinally, as in Figure 10-1a. Upon closer inspection, the muscle is made up of bundles of individual cells: the muscle cells or muscle fibers (Figure 10-1b). In mammalian muscle, the individual cells are about 50 ^m in diameter and are typically as long as the whole muscle. Thus, muscle cells are long, thin fibers similar in shape to neuronal axons. The end-plate region, where synaptic input from the motor neuron is located, is only a few microns in length. Therefore, a rapidly propagating action potential like that of a nerve cell is required in skeletal muscle cells to transmit the depolarization initiated at the end-plate along the entire length of the muscle fiber.

Individual muscle cells consist of bundles of still smaller fibers called myofibrils. The plasma membrane of a single muscle cell encloses many myofibrils. At the level of the myofibrils, the structural basis of the crosswise striations of skeletal muscle cells becomes apparent. As shown in Figure 10-1c, myofibrils exhibit a repeating pattern of crosswise light and dark stripes: the A band, I band, and Z line. The I band is a predominantly light region with the dark Z line at its center, while the A band is a darker region separating two I bands of the repeating pattern. At still higher magnification, the A band can be seen to have its own internal structure (Figure 10-1d); two darker areas at the outer edges of the A band are separated by a lighter region with a faint dark line, called the M line, at the center. The basic unit of the repeating striation pattern ofa myofibril is called a sarcomere, which is defined as extending from one Z line to the next that is, from the center of one I band to the center of the next I band.

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