Cardiac pacemaker and eventual action potential in contractile cells. Having cardiomyocytes connected via gap junctions allow all contractile cells of the heart to act in a coordinated fashion and contract as a unit. All the while being in sync with the pacemaker cells; this is the property that allows the pacemaker cells to control contraction in all other cardiomyocytes. Cells in the SA node spontaneously depolarize, ultimately resulting in contraction, approximately 100 times per minute. This native rate is constantly modified by the activity of sympathetic and parasympathetic nerve fibers via the autonomic nervous system, so that the average resting cardiac rate
Cardiac glycoside changes to heart chronotropic activity, resulting in multiple kinds of dysrhythmia and potentially fatal ventricular tachycardia. These dysrhythmias are an effect of an influx of sodium and decrease of resting membrane potential threshold in cardiac muscle cells. When taken beyond a narrow dosage range specific to each particular cardiac glycoside, these compounds can rapidly become dangerous. In sum, they interfere with fundamental processes that regulate membrane potential they are toxic to the heart, the brain, and the gut, at doses that are not difficult to reach. (In the heart, the most common negative effect is Premature ventricular contraction.) Cardiac glycoside