Cardiac Anatomy, Physiology, and Electrophysiology
Conduction system and the anatomy questions ask about
The normal impulse starts in the SA node (upper right atrium, supplied by the RCA in ~60% of people), spreads through atrial myocardium, and reaches the AV node within the triangle of Koch. That triangle is bounded by the tendon of Todaro, the coronary sinus ostium, and the tricuspid annulus - a favorite identification question. From the AV node the impulse travels down the His bundle, splits into right and left bundle branches, and terminates in the Purkinje network. Conduction velocity is fastest in Purkinje fibers (~2–4 m/s) and slowest in the AV node (~0.05 m/s); the AV nodal delay is why the PR interval exists and why the AV node is the weak link in rapid atrial rhythms.
Cellular electrophysiology and refractoriness
Ventricular and atrial myocytes depolarize through fast Na+ channels (phase 0), while SA and AV nodal cells depolarize through L-type Ca2+ channels - a classic trap, since questions invert the ionic basis. Phase 3 repolarization is K+ efflux. The effective refractory period (ERP) is the longest coupling interval that fails to propagate; it differs from the relative refractory period, during which conduction is merely slowed.
Reentry and autonomic tone
Reentry needs three ingredients: a unidirectional block, slow conduction through an alternate limb, and recovery of the blocked tissue by the time the impulse returns. Sympathetic tone shortens refractoriness and raises automaticity; vagal tone slows the SA node and AV node. Dual AV nodal pathways (fast and slow) set up typical AVNRT.
Worked example
Convert cycle length to rate: rate = 60,000 ÷ CL(ms). A 400 ms CL is 150 bpm; a 250 ms CL is 240 bpm. Knowing this cold lets you answer rate and tachycardia-mechanism questions quickly.