muscleandendocrinesummaries - Skeletal Muscle there are...

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Skeletal Muscle - there are three types of muscles – skeletal, smooth, and cardiac - skeletal – muscle attached to bones and moves and supports the skeleton - smooth – surrounds hollow cavities and tubes - cardiac – muscle of the heart structure - skeletal muscles, composed of cylindrical muscle fibers (cells), are linked to bones by tendons at each end of the muscle - skeletal muscle fibers have a repeating, striated pattern of light and dark bands due to the arrangement of the thick and thin filaments within the myofibrils - actin-containing thin filaments are anchored to the Z lines at each end of a sarcomere. Their free ends partially overlap the myosin-containing thick filaments in the A band at the end of the sarcomere molecular mechanisms of skeletal muscle contraction - when a skeletal muscle fiber actively shortens, the thin filaments are propelled toward the center of their sarcomere by movements of the myosin cross-bridges that bind to actin - the two globular heads of each cross-bridge contain a binding site for actin and an enzymatic site that splits ATP - FIGURE 9-8 - TABLE 9-1 - in a resting muscle, tropomyosin molecules that are in contact with the actin subunits of the thin filaments block the attachment of cross-bridges to actin - contraction is initiated by an increase in cytosolic Ca2+ concentration. the calcium ions bind to troponin, producing a change in its shape that is transmitted via tropomyosin to uncover the binding sites on actin, allowing the cross-bridges to bind to the thin filaments - the rise in cytosolic Ca2+ conc is triggered by an action potential in the plasma membrane. The action potential is propagated into the interior of the fiber along the transverse tubules to the region of the SR, where DHP receptors sense the voltage change and open ryanodine receptors, releasing calcium ions from the SR - relaxation of a contracting muscle fiber occurs as a result of the active transport of cytosolic calcium ions back into the SR - branches of a motor neuron axon form neuromuscular junctions with the muscle fibers in its motor unit. Each muscle fiber is innervated by a branch from only one motor neuron - ACh released by an action potential in a motor neuron binds to receptors on the motor end plate of the muscle membrane, opening ion channels that allow the passage of Na+ and + ions, which depolarize the end-plate membrane - a single action potential in a motor neuron is sufficient to produce an action potential in a skeletal muscle fiber - FIGURE 9-15 - TABLE 9-2 mechanics of single-fiber contractions - contraction refers to the turning on of the cross-bridge cycle. Whether there is an accompanying change in muscle length depends upon the external forces actin on the muscle - three types of contractions can occur following activation of a muscle fiber - isometric contraction – muscle generates tension but does not change length - concentric – isotonic contraction in which the muscle shortens moving a load - eccentric – isotonic contraction in which the external load on the muscle causes the muscle to lengthen during the period of contractile activity
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This note was uploaded on 03/27/2012 for the course PHYSIOLOGY 335 taught by Professor Strang during the Spring '11 term at University of Wisconsin.

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muscleandendocrinesummaries - Skeletal Muscle there are...

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