USMLE-Type Examination Questions - Cardiac Muscle:
50. Select the mechanism most responsible for the production of the resting transmembrane potential in an axon or skeletal muscle fiber. (A) a pump that extrudes fewer positively charged particles from the cell than it brings into the cell (B) a pump that extrudes more K+ than it brings in Na+ (C) a pump that extrudes more Na+ than it brings in K+ (D) a membrane that is more permeable to Na+ than to K+ (E) a membrane that is more permeable to K+ than to Na+ 87. Skeletal and cardiac muscle are both striated and at resting length contain in each sarcomere an A band. 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 97. Which of the following best defines contraction? (A) a series of chemical reactions that cause the muscle to pull (B) a series of chemical reactions that cause the muscle to shorten (C) a series of chemical reactions in which the muscle responds to stimulation (D) shortening (E) production of tension 102. The all-or-none law stated by Bowditch for the striated muscle cell is that the strength of contraction of a single cell responding to a single stimulus (A) cannot be increased in a healthy subject (B) cannot be changed by changing the environment of the cell (C) cannot be increased by increasing the strength of the stimulus (usually an electrical stimulus) above threshold (D) cannot be increased by increasing the frequency of stimulation (E) is not increased in hypertrophy 103. Which of the following statements best defines the treppe phenomenon described by Henry Bowditch? (A) a maintained contraction associated with multiple action potentials (B) an increase in the force generated in a twitch due to an increased frequency of stimulation (C) a decrease in the force generated by a muscle due to a prolonged period of stimulation (D) a failure to relax following stimulation (E) a maintained bond between the myosin cross-bridges and actin that is not associated with an action potential 124. Which of the following is not a characteristic of the heart? (A) its contraction is initiated by a nerve impulse (B) it conducts impulses from one muscle cell to the next (C) it contains a number of cells with an unstable transmembrane potential (D) it contains a number of cells with a stable transmembrane potential (E) its ventricles are inexcitable for most of their period of contraction 155. With the exception of the following, a sinoatrial node pacemaker cell differs from a ventricular follower cell in that it has (A) a smaller K+ conductance during its diastolic potential (B) a smaller diastolic potential (C) a membrane potential that is produced by elevated potassium conductance (D) a phase 0 potential that is due primarily to Ca++ influx (E) a less steep slope for its phase 0 potential 156. Which of the following statements about heart muscle is correct? (A) K+ gate opening initiates the fast current (in some cardiac muscle cells at a transmembrane potential of about -70 mv) (B) Ca++ gate opening initiates the slow current (in some cardiac muscle cells at a transmembrane potential of about -50 mv) (C) K+ gate closing is the most important cause of the repolarization (phase 3) of a cardiac muscle cell (D) Na+ gate opening is the action of acetylcholine on the cells of the SA node during the middle of ventricular diastole?
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