Slow Na+ channels are associated with which current in cardiac nodal tissue?

• A. Funny pacemaker current
• B. Rapid upstroke current
• C. Plateau current
• D. Refractory current

Answer: (A)

In nodal tissue, automaticity and the pace of firing are set by a slow inward current activated during hyperpolarization, known as the funny current (I_f). This current is carried mainly by Na+ through HCN channels and kicks in after the cell repolarizes, producing a gradual diastolic depolarization toward threshold. The slope and rate of this diastolic depolarization determine how quickly the next action potential occurs, so I_f largely sets the heart rate. It’s also modulated by autonomic signals via cAMP, which shifts activation to less negative potentials and increases rate.

The other currents described don’t fit nodal physiology: the rapid upstroke current is the fast Na+ current responsible for the sharp rise in non-nodal myocardium, not the nodal cells whose upstroke is primarily Ca2+-mediated; the plateau current is the L-type Ca2+ current that maintains the plateau phase; and a separate refractory current isn’t a distinct nodal current. So the slow Na+–related inward current in nodal tissue is the funny pacemaker current.