The cardiac electrical impulse normally begins spontaneously in the sinoatrial (SA) node because it:
a. has a superior location in the right atrium.
b. is the only area of the heart capable of spontaneous depolarization.
c. has rich sympathetic innervation via the vagus nerve.
d. depolarizes more rapidly than other auto-matic cells of the heart.
ANS: D
Cardiac impulses occur in the SA node because its cells depolarize more rapidly than other auto-matic cells, not because of its superior location. The SA node is not the only area of the heart ca-pable of spontaneous depolarization. The SA node does have rich innervations but by parasym-pathetic innervation from the vagus nerve.
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