How Does Skeletal Muscle Work

Like neurons, skeletal muscle fibers are also excitable. In fact, the excitability process of muscle cells is very similar to that of neurons, while the end result is different. Excitability in muscle fibers leads to the contraction of the muscle cell while neurons merely carry the electrical nerve impulse to another neuron or to skeletal muscle or to other tissue and organs.

The inside of skeletal muscle fibers appears very different from other cells because of the contractile apparatus it contains. Each muscle fiber contains a tremendous amount of small fibrous units called myofibrils, as shown in Figures 2.6 and 2.7. The prefix myo refers to muscle and fibril means little fiber. Each myofibril is a stalk-like collection of proteins. The predominant proteins are actin and myosin, which are referred to as the thin and thick filaments, respectively. They are organized into a series of tiny contraction regions called a sarcomere (Figure 2.7). Myofibrils are composed of thousands of sarcomeres situated side by side.

When skeletal muscle fibers become excited, calcium (Ca2+) channels open and calcium floods in and around the myofibrils and bathes the sarcomeres. Calcium then interacts with specific proteins associated with

(thin filament)

Figure 2.7 Inside a skeletal muscle cell are proteins involved in contraction.

These are myosin (thick filaments) and actin (thin filaments). Mitochondria are the site of aerobic energy (ATP) formation.

(thin filament)

Figure 2.7 Inside a skeletal muscle cell are proteins involved in contraction.

These are myosin (thick filaments) and actin (thin filaments). Mitochondria are the site of aerobic energy (ATP) formation.

actin and induces sarcomere contraction. The contraction of one muscle fiber is really the net result of the shortening of all the tiny sarcomeres in each myofibril within that cell. Further, the contraction of the muscle itself is the net result of contraction and shortening of muscle fibers that make up that muscle.

Skeletal muscle cells have another unique characteristic. They contain an organelle called the sarcoplasmic reticulum which is actually a modified version of the endoplasmic reticulum found in other cells. This organelle stores large quantities of calcium. In fact, when a skeletal muscle cell is stimulated, most of the calcium that bathes the sarcomeres actually comes from the sarcoplasmic reticulum.

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