Special electrodes are used to detect the change in membrane potential of a specific type of cardiac cell. These changes are recorded on the graph below. The deflection indicated by the arrow is most likely caused by movement of which of the following ions?

Question

Special electrodes are used to detect the change in membrane potential of a specific type of cardiac cell.  These changes are recorded on the graph below.  The deflection indicated by the arrow is most likely caused by movement of which of the following ions?

Quizplus · Accepted Answer

The graph in the question stem illustrates an action potential typical of cardiac pacemaker cells, such as those found in the sinoatrial node. These action potentials are different from non-pacemaker cardiac action potentials in that phases 1 and 2 are absent and phase 4 represents a slow depolarization phase. Pacemaker cells are unique in that they exhibit automaticity (an inherent ability to depolarize without any external influences). Automaticity is made possible by a inward, mixed sodium-potassium current (the funny current) that occurs during phase 4 of the action potential (Choice E). This current gradually brings the membrane potential closer to threshold, at which point phase 0 depolarization occurs due to a large increase in calcium influx (arrow). Pacemaker cell depolarization is then followed by phase 3 (repolarization), which is caused by an outward potassium current (Choice B). In cardiomyocytes and Purkinje cells (non-pacemaker cells), phase 4 corresponds to the resting membrane potential and is typically stable at approximately -90 mV (near the equilibrium potential of potassium). Phase 0 of the non-pacemaker action potential is mediated by rapid sodium influx (Choice A), in contrast to the slower calcium-mediated depolarization of pacemaker cells. Phase 1 is an early short repolarization occurring immediately after depolarization that is caused by increased outflow of potassium and decreased sodium conductance. Phase 2 is the plateau phase of the action potential, during which an inward calcium current counteracts the outward movement of potassium ions that started in phase 1 (Choice F). Phase 3 (repolarization) occurs as the inward calcium current ceases while potassium continues to rush out of the cell. Educational objective: In cardiac pacemaker cells, phase 0 depolarization is mediated by an inward flux of calcium. This differs from phase 0 of cardiomyocytes and Purkinje cells, which results from an inward sodium current.

Special Electrodes Are Used to Detect the Change in Membrane