Final answer:
The upstroke phase of action potentials in cardiac ventricular muscle cells involves calcium ions entering through voltage-gated calcium channels, causing a sustained plateau in the action potential and allowing a longer contraction period.
Step-by-step explanation:
The flow of calcium into the cell is a key component of the upstroke phase of action potentials in different types of muscle and nerve cells. Specifically, for cardiac ventricular muscle, the influx of calcium ions (Ca2+) through voltage-gated calcium channels in the sarcolemma during action potential is crucial.
This Ca2+ entry causes a sustained depolarization plateau, elongating the action potential, and allows for a longer contraction relative to skeletal muscle action potentials. This is distinct from skeletal muscle, where the majority of Ca2+ comes from the sarcoplasmic reticulum (SR), and contraction is generally triggered more rapidly without the plateau phase.