Final answer:
Phase-4 depolarization in SA nodal cells is caused by a gradual influx of Na+ ions until the threshold is reached, initiating an action potential, without involving the Na/Ca exchanger or the Na-K pump.
Step-by-step explanation:
Phase-4 depolarization of SA nodal cells is primarily caused by a slow influx of sodium ions (Na+). This increment in sodium ions leads to the SA node reaching the threshold potential, triggering an action potential. The specific cause for phase-4 depolarization in SA nodal cells is not an increase in the activity of the Na/Ca exchanger nor a decrease in the activity of the Na-K pump. Instead, it is due to the gradual influx of Na+ ions that increases the internal charge of the cell, leading to depolarization. Once the threshold is reached, there is a rapid opening of additional sodium channels and depolarization occurs swiftly.
Depolarization in the SA node does not involve a stable resting potential, as seen in other cardiac cells. Instead, the membrane potential steadily increases during phase 4 until it triggers the action potential. The other steps of the action potential involve repolarization and hyperpolarization, where various ion channels and transporters play significant roles, including the Na+/K+ transporter, which helps to restore the resting membrane potential after an action potential has occurred.