An electrophysiology study identifies aberrant automaticity in a Purkinje fiber. The ionic current responsible for spontaneous phase 4 depolarization (pacemaker potential) in Purkinje fibers is primarily:

• A L-type calcium current (ICaL) inactivating slowly to drive depolarization
• B Funny current (If, HCN channels) — inward Na+/K+ mixed current activated by hyperpolarization, and T-type calcium current (ICaT) ✓
• C Delayed rectifier K+ current (IKr) gradually increasing to reach threshold
• D Transient outward K+ current (Ito) slowly reversing during diastole

Explanation

Spontaneous phase 4 depolarization in pacemaker cells (SA node, AV node, Purkinje fibers) is driven by multiple overlapping currents. The funny current (If), carried by HCN (hyperpolarization-activated cyclic nucleotide-gated) channels permeable to both Na+ and K+ (net inward depolarizing current), is activated by membrane hyperpolarization at the end of repolarization—it is the primary pacemaker current. As the membrane depolarizes toward threshold, T-type calcium channels (ICaT) activate and contribute additional inward current. Near threshold, L-type calcium channels (ICaL) trigger the action potential upstroke in nodal cells. Simultaneously, the delayed rectifier K+ current (IKr, IKs) decays during diastole, reducing repolarizing current, which also contributes to the net depolarizing drift. If is the primary initiating current.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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