Skeletal muscle is composed of numerous muscle cells (muscle fibers) that are typically 10 to 80 µm in diameter and up to 25 cm in length. The appearance of striations in skeletal muscle is due to the highly organized arrangement of thick and thin filaments in the myofibrils of skeletal muscle fibers. The sarcomere is a contractile unit in skeletal muscle. Each sarcomere is approximately 2 µm in length at rest and is bounded by two Z lines. Sarcomeres are arranged in series along the length of the myofibril. Thin filaments, containing actin, extend from the Z line toward the center of the sarcomere. Thick filaments, containing myosin, are positioned in the center of the sarcomere and overlap the actin thin filaments. Muscle contraction results from the Ca++-dependent interaction of myosin and actin, in which myosin pulls the thin filaments toward the center of the sarcomere.
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Contraction of skeletal muscle is under control of the central nervous system (i.e., voluntary). Motor centers in the brain control the activity of α motor neurons in the ventral horns of the spinal cord. These α motor neurons, in turn, synapse on skeletal muscle fibers. Whereas each skeletal muscle fiber is innervated by only one motor neuron, a motor neuron innervates several muscle fibers within the muscle. A motor unit refers to all the muscle fibers innervated by a single motor neuron.
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The motor neuron initiates contraction of skeletal muscle by producing an action potential in the muscle fiber. As the action potential passes down the T tubules of the muscle fiber, dihydropyridine receptors (DHPRs) in the T tubules undergo conformational changes that result in the opening of neighboring SR Ca++ channels called ryanodine receptors (RYRs), which then release Ca++ to the myoplasm from the SR. The increase in myoplasmic Ca++ promotes muscle contraction by exposing myosin-binding sites on the actin thin filaments (a process that involves binding of Ca++ to troponin C, followed by movement of tropomyosin toward the groove in the thin filament). Myosin cross-bridges then appear to undergo a ratchet action, with the thin filaments pulled toward the center of the sarcomere and contracting the skeletal muscle fiber. Relaxation of the muscle follows as myoplasmic Ca++ is resequestered by Ca++-ATPase (SERCA) in the SR.
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The force of contraction can be increased by the activation of more motor neurons (i.e., recruiting of more muscle fibers) or by an increase in the frequency of action potentials in the muscle fiber, which produces tetany. The increase in force during tetanic contractions is due to prolonged elevation of intracellular [Ca++].
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The two basic types of skeletal muscle fibers are distinguished on the basis of their speed of contraction (i.e., fast-twitch versus slow-twitch). The difference in speed of contraction is attributed to the expression of different myosin isoforms that differ in myosin ATPase activity. In addition to the difference in myosin ATPase activity, fast-and slow-twitch muscles also differ in metabolic activity, fiber diameter, motor unit size, sensitivity to tetany, and recruitment pattern.
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Typically, slow-twitch muscles are recruited before fast-twitch muscle fibers because of the greater excitability of motor neurons innervating slow-twitch muscles. The high oxidative capacity of slow-twitch muscle fibers supports sustained contractile activity. Fast-twitch muscle fibers, in contrast, tend to be large and typically have low oxidative capacity and high glycolytic capacity. The fast-twitch motor units are thus best suited for short periods of activity when high levels of force are required.
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