<prompt> Rapid depolarization phase 0 of the action potential of ventricular muscle results from opening of:Voltage-gated Ca2 channelsVoltage-gated Na channelsAcetylcholine-activated K channelsInward rectifying K channels  <response>: Ans. B. Voltage-gated Na channelsVoltage-gated Na channels are responsible for phase 0 in ventricular muscle. Voltage gated Ca2 channels are responsible for phase 0 in nodal cells. The potassium channels mentioned do not play a role in mediating depolarization. 