Which process occurs when normally negatively charged cells within the heart muscle develop a positive charge?

The correct choice is depolarization, which refers to the process in which normally negatively charged cells within the heart muscle become positively charged. This shift in charge occurs when sodium ions rush into the cells, leading to a less negative internal environment. Depolarization is a critical phase in the cardiac action potential, allowing the heart muscle cells to become activated and prepared for contraction.

When depolarization takes place, it is associated with the initiation of the heartbeat and the transmission of electrical impulses, which are essential for effective heart function. This is particularly important in coordinating contractions in the heart's chambers, thereby ensuring efficient blood flow through the cardiovascular system.

Excitability refers to the ability of cardiac cells to respond to a stimulus, which is a consequence of depolarization but not the process itself. Contractility describes the ability of heart muscles to contract and generate force but is a result of successful depolarization. Conductivity relates to the propagation of electrical impulses through the heart tissue, which also relies on the depolarization of cells as an initial step. Understanding these terms helps to clarify the distinct phases and processes involved in cardiac physiology.