Chapter 5: Normal Physiology of the Cardiovascular System
Which of the following is true about Figure 5-1?
FIGURE 5-1. Phases of the action potential and major associated currents in ventricular myocytes.
A. Sodium influx during phase 1 depolarization is decreased by membrane depolarization
B. Sodium influx is limited after phase 1 by the closure of inactivation gates
C. The phase 2 plateau of the action potential is the result of a decrease in intracellular calcium through the L-type calcium channel
D. Phase 3 repolarization is the result of potassium influx into the cell through the funny channel (IF)
E. Maintenance of the membrane potential during diastole is an energy-independent process
The answer is B. (Hurst’s The Heart, 14th Edition, Chap. 5). Cardiomyocytes are specialized cells that couple membrane depolarization with cellular contraction. Phase 1 depolarization is the result of sodium influx through Na+ channels. Sodium influx is regenerative, in that increasing membrane depolarization opens more Na+ channels (option A). Rapid depolarization is limited by K+ efflux but also by the closure of inactivation gates on Na+ channels that prevent reopening of individual channels only after the membrane has been fully repolarized (option B). The phase 2 plateau is the result of balanced K+ efflux and Ca2+ influx through L-type Ca2+ channels (option C). Phase 3 repolarization is the result of K+ efflux primarily through the delayed outward K current (IK). The funny current (IF) is a specialized channel found in pacemaker cells that results in spontaneous depolarization of the cell during phase 4 and in automaticity (option D). Phase 4 occurs during diastole and requires the Na+/K+ ATPase to restore and maintain low intracellular Na+ and high intracellular K+ concentrations (option E).
Please select the true statement about Figure 5-2.
FIGURE 5-2. Major components of excitation–contraction coupling in cardiomyocytes. (Adapted with permission from Scoote M, Poole-Wilson PA, Williams AJ, et al. The therapeutic potential of new insights into myocardial excitation-contraction coupling. Heart. 2003 Apr;89(4):371-376.)
A. Calcium-induced calcium release occurs by calcium influx through ion channel B with release of calcium from the sarcoplasmic reticulum via ion channel C
B. The magnitude of the calcium transient is, in part, determined by calcium influx into the cell via channels B and D