During the depolarization phase of the nodal action potential, the cardiac myocyte membrane is most permeable to• Ca2+• K+O Nat

During the depolarization phase of the nodal action potential, the cardiac myocyte membrane is most permeable to Ca2+ (Calcium ions). Here's a step-by-step explanation:

1. The action potential in cardiac myocytes begins with the opening of fast voltage-gated sodium (Na+) channels, which causes a rapid influx of Na+ ions and a sharp rise in membrane potential - this is the depolarization phase.

2. However, in nodal cells (like those in the sinoatrial node and atrioventricular node), the action potential is slightly different. These cells do not have fast voltage-gated Na+ channels. Instead, they have slow voltage-gated calcium (Ca2+) channels.

3. When these Ca2+ channels open, Ca2+ ions enter the cell. This influx of positive ions causes the membrane potential to rise - this is the depolarization phase in nodal cells.

4. Therefore, during the depolarization phase of the nodal action potential, the cardiac myocyte membrane is most permeable to Ca2+ ions.

5. After the depolarization phase, the repolarization phase begins with the opening of potassium (K+) channels, allowing K+ ions to leave the cell and causing the membrane potential to return to its resting state.

So, in summary, during the depolarization phase of the nodal action potential, the cardiac myocyte membrane is most permeable to Ca2+ ions.