SA Node Pacemaker & Cardiac Conduction Pathway

The heartbeat begins without any nerve signal: sinoatrial (SA) node cells spontaneously depolarize through a slow "funny current" (I_f) pacemaker potential, then fire and launch an electrical wave that sweeps through the internodal tracts, pauses deliberately at the AV node, races down the His-Purkinje system, and finally spreads cell-to-cell through the ventricular myocardium. This simulator models that full hierarchy with real order-of-magnitude conduction velocities at each stage, renders the wavefront propagating through a 3D conduction network and ventricular mesh, and reports live PR interval and QRS duration — including how a blocked bundle branch widens the QRS by forcing one ventricle to activate the slow way.