What ion influx is primarily responsible for the depolarization of cardiac cells?

During the depolarization of cardiac cells, the primary ion that contributes to this process is sodium (Na+). When the cardiac cell membrane is stimulated, voltage-gated sodium channels open, allowing an influx of sodium ions into the cell. This rapid influx of positively charged sodium ions leads to a significant increase in the intracellular positivity, resulting in depolarization.

The depolarization is crucial as it initiates the action potential necessary for heart contraction. It marks the phase of the cardiac action potential where the membrane potential shifts from a negative resting state toward a positive value. This phase is critical for the propagation of electrical signals throughout the heart, leading to synchronized contraction of cardiac muscle.

Calcium ions (Ca2+) do play a significant role in cardiac muscle contraction, particularly during the plateau phase of the action potential, but they are not primarily responsible for the initial depolarization. Potassium (K+) and chloride (Cl-) ions generally work to stabilize the resting membrane potential and contribute to repolarization, rather than to the depolarization phase itself. Thus, sodium ions are the key players in the depolarization process in cardiac cells.