SA Node Pacemaker & Cardiac Conduction Pathway

SA Node Pacemaker Potential

V_m threshold

SA node firing rate (I_f drive) 72 bpm

AV nodal delay 100 ms

His-Purkinje conduction speed 1.00×

Bundle branch block

Live Readouts

Heart rate

72 bpm

PR interval

— ms

QRS duration (ventricular spread)

— ms

How it works

Sinoatrial (SA) node cells have no stable resting potential. After each action potential they drift back up during phase 4, the pacemaker potential — a slow "funny current" I_f through HCN channels (plus T-type/L-type Ca²⁺ current) depolarizes the cell until it crosses threshold and fires again, with no external stimulus needed:

V(t) = V_min + (V_th − V_min)·(1 − e^(−t/τ))
τ ∝ 1 / (I_f channel activity)   → steeper slope = faster firing = higher heart rate

Each SA spike launches a wavefront that reaches every myocardial cell along the real anatomical hierarchy, each stage with its own conduction velocity:

SA node → internodal tracts   ~1.0 m/s  (fastest of 3 tracts wins)
        → AV node              ~0.05 m/s (deliberate delay — lets atria finish emptying)
        → His bundle            ~2–4 m/s (fast, insulated)
        → Purkinje network      ~2–4 m/s (fans out through both ventricles)
        → ventricular myocardium ~0.3–0.5 m/s (cell-to-cell, slowest stage)

Cell activation time = distance ÷ velocity, summed along the path from the SA node. PR interval is the delay from SA firing to the first ventricular cell activating (dominated by the AV node's slow conduction). QRS duration is the spread of activation times across all ventricular cells — it widens sharply if one bundle branch is blocked, because that side's myocardium can only be reached the slow way, cell-to-cell across the septum, instead of via its own fast Purkinje branch.

SA firing rate — how fast I_f drives the pacemaker potential to threshold; this is exactly what the vagus/sympathetic nerves and drugs like ivabradine (an I_f blocker) modulate.
AV nodal delay — decremental AV conduction; vagal tone or AV nodal disease lengthens it (first-degree AV block).
His-Purkinje speed — scales the fast conduction system; lowering it broadens the QRS.
Bundle branch block — cuts one branch to near cell-to-cell speed, the real electrophysiology behind LBBB/RBBB on a clinical ECG.

SA / internodal AV node / His Purkinje Activated myocardium

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