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PART 4 Regulation and Maintenance

cardiac muscle take approximately 200–500 ms to complete(figure 20.14).The longer action potentials in cardiac muscle can be dividedinto several phases, each associated with specific changes in ionmovement across the membrane. In cardiac muscle, the actionpotential consists of a rapid depolarization phase, followed by arapid but partial early repolarization phase. Then a prolongedperiod of slow repolarization occurs, called the plateau phase. Atthe end of the plateau phase, a more rapid final repolarizationphase takes place, during which the membrane potential returns toits resting level (figure 20.14).Rapid depolarization is the result of changes in membranepermeability to Na + , K + , and Ca 2 + ; however, membrane perme-ability to Na + is the primary determinant of this phase. Membrane

channels, called voltage-gated Na channels, open, bringingabout the depolarization phase of the action potential. As the volt-age-gated Na + channels open, Na + diffuses into the cell, causingrapid depolarization until the cell is depolarized to approximately+ 20 millivolts (mV).The voltage change occurring during depolarization affectsother ion channels in the plasma membrane. Several types of voltage-gated K channels exist, each of which opens and closesat different membrane potentials, causing changes in membranepermeability to K + . For example, at rest, the movement of K + through open voltage-gated K + channels is primarily respon-sible for establishing the resting membrane potential in cardiacmuscle cells. Depolarization causes these voltage-gated K + chan-nels to close, thereby decreasing membrane permeability to K + .