During the rising phase of an action potential, autorhythmic cells primarily rely on what type of influx?

During the rising phase of an action potential in autorhythmic cells, the primary mechanism driving the depolarization is the influx of calcium ions. Autorhythmic cells, found in structures like the sinoatrial (SA) node of the heart, are responsible for generating the action potentials that initiate the heartbeat.

As these cells reach a threshold level of depolarization, voltage-gated calcium channels open, allowing calcium ions to rapidly enter the cell. This influx of calcium is crucial for the rising phase because it contributes to the strong depolarization that characterizes this part of the action potential.

While sodium influx does play a role in the depolarization of other types of excitable cells, such as neurons and ventricular myocytes, in autorhythmic cardiac cells, calcium is the dominant ion that facilitates the rapid rise in membrane potential. Additionally, during repolarization, potassium ions leave the cell to restore the membrane potential, but this occurs after the peak of the action potential. Thus, within the context of autorhythmic cells, the crucial ion contributing to the rising phase of the action potential is calcium.