Final answer:
Depolarization occurs first in the endocardium and then in the epicardium of the heart muscle.
Step-by-step explanation:
The depolarization of the heart muscle occurs first in the endocardium and then in the epicardium.
Depolarization is the process of the heart muscle cells becoming electrically charged to initiate contraction. It starts in the endocardium, the innermost layer of the heart, and then spreads to the epicardium, the outermost layer. This sequential depolarization allows for a coordinated contraction of the heart muscle.
Depolarization in the endocardium is initiated by the sinoatrial (SA) node, the heart's natural pacemaker. The electrical signal then spreads throughout the heart, causing depolarization in the rest of the muscle cells.