Skeletal muscle cells are different than the pacemaker cells we have in our cardiac muscle. When a skeletal muscle contracts, it is due to the voltage gated sodium channels opening and triggering the voltage gated calcium channels to open, bind to troponin, expose the tropomyosin binding site, and have myosin bind to actin to complete one cross bridge cycle. On a graph, before depolarization in a skeletal muscle cell, it is a flat line until there is a stimulus, then it rises, and repolarizes back to normal. Without stimulation, the skeletal muscle will not contract.
In cardiac pacemaker cells, the heart begins to repolarizes even when it is generating maximum force. This is because the calcium channels that are triggered via the release of sodium are slow calcium channels. Due to this, the absolute refractory period is longer than a skeletal muscle’s absolute refractory period, and is the reason why the graph of a pacemaker potential does not have a straight line because the heart is overlappingly beating and resting. In cardiac cells, even without stimulation, the heart will beat.
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