CVS Practice Test

The ion that is crucial for establishing the resting membrane potential in the cardiac action potential is potassium. The resting membrane potential is primarily determined by the distribution of ions across the cell membrane, and potassium plays a key role in this process.

In cardiac cells, at rest, the membrane is more permeable to potassium ions than to other ions due to the presence of potassium channels. As a result, when potassium ions move out of the cell, they carry a positive charge with them, contributing to a negative charge inside the cell relative to the outside. This movement creates a resting membrane potential that is typically around -90 mV in myocardial cells.

This negative potential is essential for the proper functioning of the heart, as it prepares the cardiac cells to respond to various stimuli leading to action potentials. It is also important to note that while sodium, calcium, and chloride ions contribute to other phases of the action potential and overall excitability of the cardiac tissue, it is the potassium ions that are vital for establishing and maintaining the resting potential.