The cardiac autonomic nervous system consists of 2 branches—the sympathetic and the parasympathetic systems—that work in a delicately tuned, yet opposing fashion in the heart. This extrinsic control mechanism can dominate intrinsic regulatory mechanisms that modulate heart rate and cardiac output. These branches differ in their neurotransmitters (norepinephrine and acetylcholine) and exert stimulatory or inhibitory effects on target tissue via adrenergic and muscarinic receptors. Stimulation of the sympathetic branch exerts facilitatory effects on function, increasing heart rate and myocardial contractility, whereas the stimulation of the parasympathetic branch exerts inhibitory effects that decrease heart rate and contractility. The interplay between these two branches is complex and susceptible to control at several levels, from centrally mediated baroreceptors and chemoreceptors to local interneuronal interactions.

Alterations in autonomic function occur in several interrelated cardiac conditions including sudden cardiac death, congestive heart failure, diabetic neuropathy, and myocardial ischemia. Although the full extent of these changes has not been elucidated, multiple autonomic remodeling mechanisms have been observed at both the neuronal fiber and myocardial cellular level that contribute to an arrhythmogenic substrate. We describe the anatomy of both systems in this review. However, the review will premdominantly focus on the sympathetic system, whose role in the modulation of cardiac arrhythmias is slightly better delineated.