Anatomy and Physiology Muscle Histology MCQs Answers 2023/24

PHYS 506 Anatomy and Physiology Muscle Histology MCQs Answers 2023/24 Muscle Histology 1. Skeletal and cardiac muscle, which are both striated, at resting length contain an A band in each sarcomere. This A band contains: (A)Essentially all the contractile protein myosin, but no actin (B) Essentially all the contractile protein actin, but no myosin (C) Essentially all the myosin, plus some actin (D)Essentially all the actin, plus some myosin (E) Troponin and tropomyosin, but no actin Answer. C. The A band is the region of the thick (myosin) filaments. Since at resting length there will be some overlap between the myosin and actin filaments, the A band will also contain some actin. 2. When skeletal muscle shortens in response to stimulation, there is... (A)A decrease in the width of the I band (B) A decrease in the width of the A band (C) An increase in the width of the H zone (D)All of the above (E) A and B (F) None of the above Answer: A. A band: the length of thick filament, remain unchanged during contraction. The width of both I band and H zone decrease during contraction due to cross-bridge interaction between thin and thick filaments. 3. Which of the following decreases in length during the contraction of a skeletal muscle fiber? (A) A band of the sarcomere (B) I band of the sarcomere (C) Thick filaments (D) Thin filaments (E) Z discs of the sarcomere Answer: B. The physical lengths of the actin and myosin filaments do not change during contraction. Therefore, the A band, which is composed of myosin filaments, does not change either. The distance between Z discs decreases, but the Z discs themselves do not change. Only the I band decreases in length as the muscle contracts. 4. A person lifting up their physiology textbook contracts their bicep muscle isotonically. Which one of the following does not change its length in this process when compared to when the muscle is at rest? (A) I band (B) A band (C) H zone (D) Sarcomere (E) Biceps tendon Answer: B. The A band represents the area of the sarcomere that is made up of the thick filaments. The length of the thick filaments do not change during any type of contraction. Therefore the A band does not change length during an isotonic contraction of the biceps muscle. 5. A cross-sectional view of a skeletal muscle fiber through the H zone would reveal the presence of what? (A) Actin and titin (B) Actin, but no myosin (C) Actin, myosin, and titin (D) Myosin and actin (E) Myosin, but no actin Answer: E. The H zone is the region in the center of the sarcomere composed of the lighter bands on either side of and including the M line. In this region, the myosin filaments are centered on the M line, and there are no overlapping actin filaments. Therefore, a cross-section through this region would reveal only myosin. 6. Which of the following binds Ca2+ in order to uncover the active site on F-actin? (A) Actin monomer (B) Myosin (C) Tropomyosin (D) Troponin (E) Crossbridge Answer: D. Troponin (Tn) is a complex made up of three subunits; Tn T bound to the tropomyosin protein, Tn I that is believed to inhibit the active site and Tn C that binds to calcium. Depolarization of the muscle membrane leads to an elevation in intracellular calcium levels (via different mechanisms in skeletal and cardiac muscle). This increases the number of Tn C bound to calcium, which in turn increases the number of active sites on the thin filament available for binding to the myosin heads of the thick filament. 7. A 64-year-old man was admitted to the hospital with edema and congestive heart failure. He was found to have diastolic dysfunction characterized by inadequate filling of the heart during diastole. The decrease in ventricular filling is due to a decrease in ventricular muscle compliance. Which of the following proteins determines the normal stiffness of ventricular muscle? (A) Calmodulin (B) Troponin (C) Tropomyosin (D) Titin (E) Myosin light chain kinase Answer: D. Titin is a large protein that connects the Z lines to the M lines, thereby providing a scaffold for the sarcomere. Titin contains two types of folded domains that provide muscle with its elasticity. The resistance to stretch increases throughout a contraction, which protects the structure of the sarcomere and prevents excess stretch. Excitation-Contraction Coupling 8. In skeletal muscle, which of the following events occurs before depolarization of the T tubules in the mechanism of excitation-contraction coupling? (A)Depolarization of the sarcrolemmal membrane (B) Opening of Ca2+ release channels on the sarcoplasmic reticulum (C) Uptake of Ca2+ into the sarcoplasmic reticulum by Ca2+ -ATPase (D)Binding of Ca2+ to troponin C (E) Binding of actin and myosin Answer: A. In the mechanism of excitation-contraction coupling, excitation always precedes contraction. Excitation refers to the electrical activation of the muscle cell, which begins with an action potential in the sarcrolemmal membrane that spreads to the T tubules. Depolarization of the T tubules then leads to the release of Ca2+ from the nearby sarcoplasmic reticulum, followed by an increase in intracellular Ca2+ concentration, binding of Ca2+ to troponin C, and then contraction. 9. Which of the following best describes the action potential of skeletal muscle upon stimulation? (A)It spreads inward to all parts of the muscle via the T-tubules (B) It has a prolonged plateau phase (C) It causes the immediate uptake of Ca2+ into the lateral sacs of SR (D)It is longer than the action potential of cardiac muscle (E) It is not essential for contraction Answer: A. Depolarization of the muscle fiber is essential for initiating muscle contraction. The action potential transmitted to all of the fibrils along T-tubules, triggering the release of Ca2+ from the lateral sacs of the SR next to T-system. It is shorter than the action potential of cardiac muscle and doesn't have a prolonged plateau phase. 10. The action potential of skeletal muscle: (A)Has a prolonged plateau phase (B) Spreads inward to all parts of the muscle via T-tubules (C) Causes the immediate uptake of calcium into the lateral sacs of sarcoplasmic reticulum (D)Is longer than the action potential of cardiac muscle (E) Is not essential for muscle contraction in the intact muscle Answer: B. Because of this spread, the muscle can contract as a unit. All other choices are opposite. . . . . . . .. . .