Skeletal Muscle

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Skeletal Muscle

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Lecture Note
University of Southern California
Bisc 307l - General Physiology
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BISC 307L 1st Edition Lecture 11 Current Lecture  Excitation-Contraction Coupling o The neuromuscular junction o Picture: Given that there is an AP in the muscle fiber spreading down the sarcolemma (plasma membrane in the muscle). Mechanism of the action potential is the same as an unmyelinated axon. What happens is  1.T-tubules (invaginations of the plasma membrane) which are transversally oriented and spaced one per sarcomere. Extends in a ring around the myofibril  2. Smooth ER (blue) flat sac of membrane arranged in a belt along each myofibril . Flanking the SR is this sac o The AP spreads through the T-tubule. (How much depolarizes?- a lot because there are high density of Na+ channels so whole T tubule system gets depolarized. o Connections between the T-tubule and the SR:  Blue membrane = t tubule membrane  Yellow membrane = SR membrane  The membrane potential exists across the T tubules membrane for restin condition  2 complexes of proteins  1. Dihydropyridine receptor  t tubule  voltage sensing protein  not a channel but the membrane it sits in is depolarized  2. Ca2+ channel pump (Ryanodine receptor)  takes Ca2+ out of the cytoplasm and pumped into the SR lumen  lowers concentration of Ca2+ inside the cell so that there is no contractions at resting  Ca2+ released from SR when the Ca2+ channel opens which will trigger contractions  After t tubule membrane is depolarizedcauses conformational change in dihydropyridine receptor which is connected to the Ca2+ channels (dotted lines) and when these links pullcalcium channel opens (mechanically gated but voltage dependent)Contractioncontinues until Ca2+ sequestered again into the SR by the pumps

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