Depolarization of a cardiac muscle cell occurs as the result of:
a. a decrease in the permeability of the cell membrane to ions.
b. the rapid movement of ions across the cell membrane.
c. a blockade by calcium ions.
d. stimuli instigated during the refractory period.
ANS: B
Depolarization is caused by the movement of electrically charged solutes (ions) across cardiac cell membranes. Permeability must be increased for movement to occur. A blockage of calcium ions would decrease depolarization. No cardiac action potential can be initiated in the refractory pe-riod.
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