Final answer:
The upstroke of action potentials in SA and AV nodes and most smooth muscles is mainly driven by the slow calcium channels. These channels allow calcium ions to enter the cells and contribute to the prolonged plateau phase necessary for cardiac muscle function and trigger Ca2+ release for muscle contraction.
Step-by-step explanation:
The voltage channel primarily involved in the upstroke of action potentials in the SA and AV nodes, as well as in most smooth muscles, is the slow calcium channel. During the action potential of pacemaker cells, such as the SA node, the upstroke is a result of calcium ions entering through these voltage-gated calcium channels. The calcium influx contributes to the prolonged plateau phase, which is essential for the proper functioning of cardiac muscle and also triggers calcium-induced calcium release (CICR) from the sarcoplasmic reticulum, providing the Ca2+ necessary for cardiac muscle contraction.
In contrast to skeletal muscle, where depolarization is primarily due to fast voltage-gated sodium channels, the cardiac pacemaker cells rely on the slower calcium channels for depolarization. This mechanism contributes to a slower rate of depolarization compared to other muscles, and it is vital for ensuring that the heart beats at an appropriate and consistent rate.