Figure 4 shows an action potential typical of what would be recorded from a contractile cell in the ventricle. Superficially, this action potential appears similar to the action potentials seen in nerve, skeletal, and smooth muscle cells (Chapters 6, 7, and 8). One major difference, however, is its duration. Note that the action potential remains in a depolarized state for about 300 msec (almost a third of a second) giving it a shape like a plateau. In contrast, an action potential in a nerve or skeletal muscle cell lasts only about 1 msec. It is the plateaued action potential that keeps the heart activated long enough to develop a forceful contraction from a single action potential. The rapid phase of depolarization is termed phase 0. Phase 1 is a trasient repolarization following the overshooting of Phase 0. Phase 1 is followed by a long period during which the membrane potential remains depolarized, the plateau phase or phase 2. After the plateau phase, a phase of repolarization occurs during which the membrane potential returns to its resting level. This phase is termed phase 3. The resting potential between beats is referred to as phase 4.
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