Which channel is primarily responsible for the rapid depolarization in cardiac muscle cells?

• A. Voltage-gated sodium channels
• B. Voltage-gated calcium channels
• C. Sodium-calcium exchange pumps
• D. Ligand-gated potassium channels

Answer: (A)

The rapid depolarization of cardiac muscle cells is driven by a quick, large inward current of sodium ions through fast voltage-gated sodium channels. When the cell is stimulated, these channels open rapidly, letting Na+ rush in and sharply raise the membrane potential, producing the steep upstroke of the action potential (the Phase 0 in ventricular and atrial myocytes). After opening briefly, they inactivate, which ends the rapid depolarization.

Calcium channels, especially the L-type ones, contribute to the later plateau phase by sustaining depolarization, but they are not the primary drivers of the initial rapid upstroke. Sodium-calcium exchange pumps help remove calcium during relaxation rather than initiate the fast depolarization. Ligand-gated potassium channels would influence repolarization or resting potential rather than causing the rapid upstroke of depolarization.