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chapter 21.docx

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Jon Houseman

EXCITATION-CONTRACTION COUPLING Muscle contraction is initiated by the release of ACh from the axon terminals of alpha motor neurons. ACh produces a large EPSP in the postsynaptic membrane due to the activation of ACh receptors which is sufficient to evoke an action potential in the muscle fiber.  By the process of excitation-contraction coupling, this action potential (excitation) triggers the release of Ca2+from an organelle inside the muscle fiber, which leads to contraction of the fiber. 2+  Relaxation occurs when the Ca levels are lowered by reuptake into the organelle. Muscle Fiber Structure – See Fig. 13.10  Fusion of myoblasts in early development leads to the formation of multinucleated muscle fibers.  Muscle fibers are enclosed by an excitable cell membrane called the sarcolemma.  Within the muscle fiber are myofibrils which contract in response to an action potential sweeping down the sarcolemma. 2+ o Myofibrils are surrounded by the sarcoplasmic reticulum (SR) (stores Ca ). o Action potentials sweeping along the sarcolemma gain access to the SR by way of tunnels called T tubules. o When the T tubule comes in close apposition to the SR, there is a specialized coupling of proteins in the two membranes. o A voltage-sensitive cluster of four calcium channels (tetrad) in the T tubule membrane is linked to a calcium release channel in the SR (see Fig. 13.11). o The arrival of an action potential in the T tubule membrane causes a conformational change in the tetrad, which opens the calcium release channel in the SR membrane.  This leads to an increase in free Ca2+within the cytosol which causes the myofibrils to contract The Molecular Basis of Muscle Contraction – See Fig. 13.12 The myofibril is divided into segments called sarcomeres by disks called Z lines.  On each side of the Z lines is a series of bristles called thin filaments. Thin filaments form adjacent Z lines face one another but do not come in contact.  Between the two sets of thin filaments are thick filaments Muscle contraction occurs when the thin filaments slide along the thick filaments, bringing adjacent Z lines toward one another. Thus, the sarcom
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