What is the primary ion contributing to the resting membrane potential of cardiac pacemaker cells due to its greater concentration outside the cell?

The primary ion contributing to the resting membrane potential of cardiac pacemaker cells is sodium. In these cells, the resting membrane potential is largely influenced by the equilibrium potential of sodium ions, which are more concentrated outside the cell compared to the inside.

During the process of depolarization, sodium ions flow into the pacemaker cells through specialized ion channels when the membrane potential reaches a certain threshold, resulting in the generation of action potentials that are crucial for the rhythmic contractions of the heart. While potassium also plays a significant role in repolarization and maintaining the resting potential in other cardiac cells, it is the influx of sodium that primarily dictates the initiation of the electrical signal in pacemaker cells.

The other ions mentioned—potassium, calcium, and chloride—have their roles in cardiac electrophysiology but do not primarily dictate the resting membrane potential of these specific cells as sodium does. Potassium is usually found in higher concentrations inside the cell and is more critical during the repolarization phases, calcium influences contraction and further depolarization, while chloride's role is less significant compared to the others in maintaining the resting potential in pacemaker cells.