The spontaneous opening of Na⁺ channels marks the beginning of ________ of a cardiac muscle cell.

A) depolarization
B) repolarization
C) hyperpolarization
D) isopolarization
E) afterpolarization

Depolarization

The process of reduction in electric charge across a cell membrane, typically initiating a neural impulse or muscle contraction.

Cardiac Muscle Cell

Specialized cardiac muscle cells that involuntarily and rhythmically contract to circulate blood throughout the body.