Page 7, Cardiac Muscle, Dr. D. Penney

Follower and Pacemaker Cells:

Follower and Pacemaker cells show different electrophysiologic patterns (Figure 5). Follower cells show a stable Phase 4 potential, while pacemaker cells show an unstable or rising Phase 4 potential to threshold. Follower cells can be converted to pacemaker cells during pathological conditions (e.g. ischemia), resulting in ectopic foci.

The fastest pacemaker leads, by suppressing all other potential pacemakers (overdrive suppression). Pacemaker firing rate can be altered by hyper- or hypo-polarizing the cell, by altering the slope of the prepotential, and by altering the firing threshold.

Epinephrine increases the Ca++ conductance, thus hypopolarizing the cell and increases the slope of the prepotential. Acetylcholine increases K+ conductance, hyperpolarizing the cell and decreasing the slope of the prepotential.

A number of factors determine the velocity of conduction of an action potential through the heart: the amplitude and rate of change of the action potential, the anatomy of the conducting cells, and the cable properties of the conducting system.

At the AV junction, catecholamines, atropine, and quinidine increase conduction, while acetylcholine, digitalis, Ca++ antagonists, and an increased number of impulses decrease conduction and/or increase refractoriness.

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