ABSTRACT
The pacemaker potential in sinoatrial (SA) node cells normally reaches threshold before the Atrioventricular (AV) node cells and much before Purkinje fibers. The normal heart rhythm, driven by the SA node as the pacemaker, is called "normal sinus rhythm". The intrinsic frequency of the SA node, in the absence of input from the autonomic nervous system, is about 100 beats/min. Cells in the AV node can serve as the heart's pacemaker if, for instance, the SA node fails. The AV node intrinsic rate, with no autonomic nervous system input, is approximately 40-60 beats/min. The intrinsic frequency of Purkinje fibers is about 25-40/min. Purkinje fibers can become the ventricular pacemaker when there is complete failure of transmission of action potentials from the atria to the ventricles. Conduction velocity is the time it takes for conduction of phase 0 from one point to another. Notice the emphasis on "phase 0". Conduction velocity has the usual units for velocity, distance/time, usually mm/ms.
