Final answer:
The statement that K+ movement out of cardiac autorhythmic cells causes repolarization is true. Repolarization is marked by K+ exiting the cell after the peak action potential, leading to a decline in membrane potential towards resting levels.
Step-by-step explanation:
The statement that K+ movement out of the cardiac autorhythmic cell during an action potential causes repolarization is true. During the cardiac action potential in autorhythmic cells, after the initial rise in membrane potential due to an influx of Na+ ions, and a further rapid depolarization due to Ca2+ entering the cell, the repolarization phase begins when Ca2+ channels close and K+ channels open. This allows K+ to move out of the cell, restoring the negative charge inside the cell, which brings the membrane potential back down toward the resting level, in a process known as repolarization.
For example, during the peak action potential, K+ channels open, allowing K+ to exit the cell. This outflow of K+ coincides with the closure of Na+ channels, contributing to the membrane's return to a more negative potential. The net movement of positive charges out of the cell is what characterizes repolarization.
The behavior of these ion channels is critical for the rhythmic contraction of the heart muscle, ensuring that cardiac autorhythmic cells can reset and prepare for the next cycle of depolarization, leading to the heart's autorhythmicity.