Structure of skeletal muscle

A whole muscle is composed of muscle cells, or muscle fibers. Muscle fibers are elongated, cylindrical cells. Due to fusion of many smaller fibers during embryonic development, muscle fibers are the largest cells in the body, with several nuclei near their surface. Muscle fibers lie parallel to each other and extend along the entire length of the muscle. These fibers may be a few millimeters in length (muscles of the eyes) or up to 2 or more feet in length (muscles of the legs).

Muscle fibers are incapable of mitosis. In fact, the number of muscle fibers per muscle is likely determined by the second trimester of fetal development. Therefore, enlargement of a whole muscle is not due to an increase in the number of fibers in the muscle, but rather to the hypertrophy of existing fibers. Because muscle fibers have no gap junctions between them, electrical activity cannot spread from one cell to the next. Therefore, each muscle fiber is innervated by a branch of an alpha motor neuron. A motor unit is defined as an alpha motor neuron and all of the muscle fibers that it innervates.

Internally, muscle fibers are highly organized. Each fiber contains numerous myofibrils — cylindrical structures that also lie parallel to the long axis of the muscle. The myofibrils are composed of thick filaments and thin filaments. It is the arrangement of these filaments that creates alternating light and dark bands observed microscopically along the muscle fiber. Thus, skeletal muscle is also referred to as striated muscle.

Sarcomeres. The thick and thin filaments are organized into repeating segments referred to as sarcomeres, which are the functional units of skeletal muscle. (In other words, the sarcomere is the smallest contractile unit within skeletal muscle.) A myofibril is composed of hundreds or thousands of sarcomeres in series along its length. When a muscle is stimulated, each of these sarcomeres contracts and becomes shorter. As a result, the entire muscle contracts and becomes shorter. Therefore, the function of the sarcomere determines whole muscle function. A sarcomere is the area between two Z lines (see Figure 11.1, panel c). The function of the Z line is to anchor the thin filaments in place at either end of the sarcomere. The thick filaments are found in the central region of the sarcomere. Ultimately, interaction between the thick and thin filaments causes shortening of the sarcomere.

Thick filaments. Each thick filament contains 200 to 300 myosin molecules. Each myosin molecule is made up of two identical subunits shaped like golf clubs: two long shafts wound together with a myosin head, or cross-bridge, on the end of each. These molecules are arranged so that the shafts are bundled together and oriented toward the center of the thick filament. The myosin heads project outward from either end of the thick filament (see Figure 11.1, panel a).

Thin filaments. The thin filaments are composed of three proteins:

• Tropomyosin

Actin Tropomyosin Troponin

Thin Filament

Thin Filament

Z line

Thin Filament

Thick Filament

Sarcomere

Z line

Sarcomere

Figure 11.1 Components of the sarcomere. (a) The thick filament is composed of myosin molecules shaped like golf clubs and consisting of a long shaft with a globular portion at one end. The myosin is arranged so that the shafts are in the center of the thick filament and the globular portions, or myosin crossbridges, protrude from each end of the thick filament. The myosin crossbridges bind to the actin of the thin filament. (b) The thin filament consists of three proteins. Globular actin molecules join together to form two strands of fibrous actin that twist around each other. Tropomyosin is a filamentous protein found on the surface of the actin, physically covering the binding sites for the myosin cross-bridges. Troponin molecules stabilize the tropomyosin filaments in position on the actin. (c) Sarcomere — the thick and thin filaments are highly organized. They are arranged to form the sarcomere that is the functional unit of skeletal muscle. The sarcomere is the region between two Z lines.

The predominant protein, actin, consists of spherical subunits (globular actin) arranged into two chains twisted around each other (fibrous actin). Tropomyosin is a long, thread-like protein found on the outer surface of the actin chain. Each tropomyosin molecule is associated with six to seven actin subunits. The function of tropomyosin is to cover binding sites for myosin on the actin subunits when the muscle is in the resting state. This prevents the interaction between actin and myosin that causes muscle contraction. Troponin is a smaller protein consisting of three subunits. One subunit binds to actin, another binds to tropomyosin, and the third binds with calcium. When the muscle is relaxed, troponin holds the tropomyosin in its blocking position on the surface of the actin (see Figure 11.1, panel b).

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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