Final answer:
An action potential in cardiac muscle fiber is not identical to that in skeletal muscle fiber due to the longer plateau phase in cardiac muscles, which leads to a longer contraction time and a greater influx of extracellular calcium ions.
Step-by-step explanation:
False. An action potential in a cardiac muscle fiber is not identical to an action potential in a skeletal muscle fiber. Cardiac muscle fibers have a longer action potential due to a sustained depolarization plateau, which is produced by the entry of calcium ions through voltage-gated calcium channels.
This plateau phase leads to a longer contraction time in cardiac muscles compared to skeletal muscles, where the action potential is shorter. Additionally, in cardiac muscle, a significant amount of the calcium that initiates contraction comes from outside the cell, unlike in skeletal muscle where it mainly comes from the sarcoplasmic reticulum (SR).