Final answer:
In the sinoatrial (SA) node, phase 4 depolarization is attributable to an increase in Na⁺ conductance.
Step-by-step explanation:
In the sinoatrial (SA) node, phase 4 depolarization (pacemaker potential) is attributable to an increase in Na⁺ conductance.
Conductive cells in the heart, including the SA node, have a series of sodium ion channels that allow a slow influx of sodium ions, causing the membrane potential to rise. This slow influx of Na⁺ results in the prepotential depolarization, which initiates the spontaneous depolarization and contraction of the cell. At the threshold, calcium ion channels open, further depolarizing the cell, until they close and potassium channels open, leading to repolarization.