Which of the following is the single most impoant factor in controlling automatic contractility of hea?

Autonomic nervous system both sympathetic and parasympathetic is involved in regulating the automatic contractility of hea. Sympathetic nerve fibers stimulate all pas of the atria and ventricles. Stimulation of the sympathetic nervous system causes the release of various chemical messengers including epinephrine and norepinephrine. Epinephrine activates the beta 1 adrenergic receptors. Sympathetic stimulation causes: ? Increase in the rate of sinus nodal discharge. ? Increase the rate of conduction as well as the level of excitability in all poions of the hea. ? Increase the force of contraction of all the cardiac musculature, both atrial and ventricular Maximal stimulation can almost triple the frequency of heabeat and can increase the strength of hea contraction as much as twofold. Ref: Guyton and Hall Medical Physiology, 11th Edition, Pages 121-122; Arrhythmia Recognition: The A of Interpretation By Tomas B. Garcia, Page 24-5.