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Excitation - potentials 2 ACh bound to the receptors...

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Excitation-Contraction Coupling As the action potential spreads across the muscle membrane, it enters the T-tubules . The SR membrane is very close to the T-tubule system and has special voltage-gated calcium channels that open in response to the AP on the surface of the muscle cell. Calcium diffuses out of the SR into the cytoplasm through these open channels and binds to troponin on the actin myofilament. The troponin-tropomyosin complex changes shape and exposes the active sites on actin, allowing the myosin heads to bind to actin. Relaxation The relaxation of muscle cells is the result of several actions. 1) The neuron no longer releases ACh that causes muscle cell depolarization and the muscle is no longer excited to produce action
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Unformatted text preview: potentials. 2) ACh bound to the receptors dissociates and no longer activates the receptor. 3) Acetylcholinesterase in the synaptic cleft removes any remaining ACh so the ACh receptors are no longer activated. 4) Calcium released from the SR during contraction is actively pumped back into the SR which results in the troponin-tropomyosin complex recovering the active sites on actin. The muscle fiber returns to its resting length due to elasticity (and/or contraction of antagonistic muscles that cause the contracted muscle to be stretched)....
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