Set the pace.
The sinoatrial (SA) node—the heart’s primary pacemaker—sits in the right atrium near the SVC. Its action potential lacks a true plateau: phase 0 depolarization uses L‑type Ca²⁺ channels, phase 3 repolarizes via K⁺ efflux, and phase 4 exhibits spontaneous diastolic depolarization through If (funny) channels—a mixed Na⁺/K⁺ inward current that underlies automaticity.
Build the beat.
Same heart · Different electrical jobs
SA node
Sets the pace
Phase 0 → Ca²⁺
Phase 4 rises spontaneously.
Ventricular myocyte
Supports contraction
Phase 0 → Na⁺
Phase 2 forms a plateau.
Ventricular myocyte action potential has five phases: phase 0 depolarization is driven by fast Na⁺ channels (target of Class I antiarrhythmics; see Pharmacology), phase 2 is the Ca²⁺-K⁺-balanced plateau that prolongs the refractory period and prevents tetanic contraction; phase 4 rests at ≈ –90 mV. Excitation–contraction coupling begins when L‑type Ca²⁺ entry during phase 2 triggers Ca²⁺‑induced Ca²⁺ release (CICR) from the sarcoplasmic reticulum via ryanodine receptors (RyR2). Cardiac muscle depends heavily on extracellular Ca²⁺, so CCBs → negative inotropy.
Close the loop.
Pause & recall
Which channel carries phase 0 of the SA-node action potential?
L-type Ca²⁺ channels.
Pause & recall
Which channel carries phase 0 of the ventricular myocyte action potential?
Fast Na⁺ channels.
Pause & recall
What does the phase-2 plateau accomplish?
It prolongs the refractory period and prevents tetanic contraction.
Pause & recall
What triggers Ca²⁺-induced Ca²⁺ release, and via which receptor?
L-type Ca²⁺ entry; release is via ryanodine receptors (RyR2).
About this excerpt & further reading
Selected passages and recall questions from “Cardiac Electrophysiology and Conduction,” Cardiovascular / Physiology in The Final Stretch. Selected published passages retrieved September 15, 2026. The diagrams and recall presentation were prepared for this public preview. This is a selection, not the complete chapter. Recall answers stay on this page; no study history is recorded.