Which tissues rely on fast-activating sodium channels for rapid depolarization?

• A. Pacemaker cells
• B. Ventricular myocardium and Purkinje fibers
• C. Atrial muscle
• D. Smooth muscle

Answer: (B)

The rapid upstroke of the cardiac action potential in certain tissues is driven by fast voltage-gated sodium channels. In ventricular myocardium and Purkinje fibers, these fast Na+ channels open quickly (INa), producing a steep phase 0 depolarization that enables rapid conduction through the ventricles. By contrast, pacemaker cells in the SA and AV nodes rely mainly on calcium influx (L-type and T-type Ca2+ channels) to depolarize, giving a slower upstroke. Smooth muscle depolarization is also primarily calcium-driven rather than sodium-driven. While atrial muscle does have fast Na+ channels, the classic tissues characterized by the rapid Na+-mediated upstroke are the ventricular myocardium and Purkinje fibers.