Ion Channels and Voltage Dependent Gates:
The ion channels responsible for the movement of Na+, K+, Ca++, etc. are thought to have two gates (Figure 4). This permits a resting state, an active state, and an inactive state. Only when both gates are open in the active state, can ions move through the channels in the plasma membrane.
Opening and closing of the gates is voltage dependent. In SA nodal and AV nodal cells, for example, the resting potential is only -60 mv (Table 2). This prevents the Na+ channel from opening, so that Ca++ movement is primarily responsible for the rather slow Phase 0 in these cells. Tetrodotoxin (TTX) inhibits Na+ channels, while nifedipine, verapamil and other Ca++ entry blockers inhibit Ca++ channels.
It can be argued that pacemaker cells do not have a phase 0, since depolarization is not powered by Na+ ion movement, but rather by Ca++ movement. I would prefer to also consider this change in membrane potential as Phase 0.
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