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Chapter 5. Excitable Tissue: Muscle

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A 22-year-old man is going in for surgery and requires intubation. He is put under a general anesthetic and prior to intubation given succinylcholine to temporarily “paralyze” muscles and allow for easier insertion of the endotracheal tube. Succinylcholine competes with the natural signal at the neuromuscular junction (acetylcholine) and causes an extended depolarization that prevents muscles from initiating action potentials and contracting. The action potential of an unparalyzed skeletal muscle

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A. has a prolonged plateau phase.

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B. spreads inward to all parts of the muscle via the T tubules.

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C. causes the immediate uptake of Ca2+ into the lateral sacs of the sarcoplasmic reticulum.

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D. is longer than the action potential of cardiac muscle.

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E. is not essential for contraction.

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The correct answer is B. The initial signaling for muscle depolarization is quickly spread across the muscle plasma membrane via the T tubules. The repolarization phase of skeletal muscle varies by fiber type, however, in each case there is a limited plateau phase, especially when compared to a cardiomyocyte, ruling out A and D. Depolarization at the plasma membrane is followed by a quick release of Ca2+ from intracellular stores and not an uptake of Ca2+ as stated in C. Skeletal muscle cells are excitable cells and require electrical signaling (ie, action potential) for contraction, ruling out E.

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A physical therapy student recently joined a team at a hospital unit that specializes in evaluation, diagnosis and treatment plans for infants and children who have arthrogryposis. During his studies, a pre-adolescent patient with limited range of motion in the hands and feet is treated to improve range of motion. The physical therapist suspects that the patient could have Sheldon–Hall syndrome (arthrogryposis type 2B) and suggests genetic testing. Testing reveals a mutation in TPM2, a gene coding for tropomyosin, consistent with the Sheldon–Hall syndrome diagnosis. The patient’s symptoms are attributable to the loss of which function of tropomyosin in skeletal muscle?

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A. Sliding on actin to produce shortening

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B. Releasing Ca2+ after initiation of contraction

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C. Binding to myosin during contraction

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D. Acting as a “relaxing protein” at rest by covering up the sites where myosin binds to actin

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E. Generating ATP, which it passes to the contractile mechanism

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The correct answer is D. Tropomyosin is a muscle protein found in thin filaments. At low [Ca2+], tropomyosin prevents myosin/actin binding. ...

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