Calcium is stored in the sarcoplasmic reticulum SR away from the thin and thick

Calcium is stored in the sarcoplasmic reticulum sr

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Calcium is stored in the sarcoplasmic reticulum (SR), away from the thin and thick filaments. The calcium exposes a site on the actin that is necessary for the binding of the myosin heads, the formation of cross-bridges, and sarcomere shortening (muscle contraction). When calcium is pumped back into the SR, the cross-bridges are broken, and the muscle relaxes. The energy released by ATP is necessary for cross-bridges to both form and disengage. Skeletal Muscle and Nerves Somatic Motor Neuron Skeletal muscle contraction can take place only when the muscle is first stimulated by a nerve. The type of nerve that supplies the skeletal muscle is a somatic motor nerve. A motor nerve, composed of many nerve cells called motor neurons, emerges from the spinal cord and travels to the skeletal muscle. Muscle contraction occurs when the myosin heads bind to actin, thereby causing the thin and thick filaments to slide past one another. The sliding occurs only in the presence of calcium. What role does calcium play in muscle contraction and relaxation? The thin filament is composed of two proteins: actin and the troponin–tropomyosin complex. (The cup-shaped troponin sits on the tropomyosin.) The actin protein contains sites to which myosin heads bind. In the relaxed state the myosinbinding sites on actin are blocked by the troponin–tropomyosin complex. Thus, cross-bridge formation and muscle contraction cannot occur.
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What the “Stiffness of Death” Is? Both the formation of cross-bridges (muscle contraction) and the detachment of cross-bridges (muscle relaxation) depend on ATP. When a person dies, calcium leaks out of the SR, causing the muscles to contract. In addition, the production of ATP ceases. The deficiency of ATP prevents the detachment of the crossbridges, so muscles remain contracted and become stiff. This change is called rigor mortis, or “stiffness of death.” As a motor neuron approaches the skeletal muscle, it forms many branches. Each branch innervates a single muscle fiber. The motor unit. Consists of a single motor neuron and the muscle fibers that are supplied by the motor neuron. Each muscle is innervated by many motor neurons, thereby forming many motor units . The strength of muscle contraction can vary, depending on the number of motor units that are stimulated. The consequence of activation of additional motor units is called recruitment. The Neuromuscular Junction The area where the motor neuron meets the muscle is called the neuromuscular junction (NMJ) . The NMJ includes the membrane at the end of the neuron, the space between the neuron and the muscle, and the receptors on the muscle membrane. What happens at the NMJ? The stimulated neuron causes the release of a chemical substance that diffuses across the NMJ and stimulates the muscle membrane. Four steps are involved in the transfer of the information from neuron to muscle at the NMJ: Step 1 . Stimulation of the neuron causes an electrical signal, or nerve impulse, to move along the neuron toward the ending of the neuron.
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  • Fall '19
  • muscle weakness, muscle atrophy, functions of muscles

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