Final answer:
The predominant channel during depolarization of SA nodal cells is the calcium channel, while the sodium channel is dominant during the depolarization of Purkinje fibers. Calcium channels lead to a rapid depolarization of SA nodal cells, whereas fast Na+ channels cause a quick depolarization of Purkinje fibers before repolarization happens via opening of potassium channels.
Step-by-step explanation:
The calcium channel is dominant during depolarization of SA nodal cells and the sodium channel is dominant during depolarization of Purkinje fibers. During the action potential of the SA node, the prepotential phase begins with a slow influx of Na+ until the membrane potential reaches about -40 mV. At this threshold, calcium ions enter the cell through calcium channels, leading to a more rapid depolarization until the membrane potential is approximately +15 mV. This rapid influx of calcium is what characterizes the depolarization of SA nodal cells.
In contrast, the depolarization of Purkinje fibers involves a different mechanism. Here, fast Na+ channels open swiftly in response to a triggering action potential, causing a quick influx of Na+ into the cells and a sharp rise in membrane potential. This rapid influx of sodium is the main feature of the depolarization of the Purkinje fibers.
After the cells depolarize, repolarization occurs when potassium channels open, allowing K+ to leave the cell, which brings the membrane potential back down, returning the cell to its resting state.