Final answer:
The channel responsible for the initiation of phase-4 depolarization in SA nodal cells is primarily permeable to Na+ ions, known as the funny current or pacemaker current. It is not opened by membrane depolarization, nor is it directly influenced by norepinephrine or vagal nerve stimulation.
Step-by-step explanation:
The initiation of phase-4 depolarization in SA (sinoatrial) nodal cells is a critical event in the generation of the heart's rhythmic contractions. The channel responsible for this initial depolarization is known as the funny current or pacemaker current, which is primarily permeable to sodium (Na+) ions.
When examining the impact of neurotransmitters, it should be noted that the activation of these channels is not directly influenced by norepinephrine; instead, norepinephrine and other sympathetic stimuli work to increase the rate of depolarization through other mechanisms, like the beta-adrenergic pathway, which enhances calcium influx during later phases of action potential.