Autonomic innervation of the heart

This brief review describes recent advances in the areas of myocardial receptors that discharge into nonmyelinated, afferent, vagal C-fibers and the regional distribution of sympathetic postganglionic neurons to the myocardium. Complex, nonencapsulated nerve endings discharging into myelinated afferents have been known to exist for many years. More recently, however, indirect evidence for the existence of cardiac receptors that are either silent or exhibit low resting rates of activity, and discharge into slowly conducting C-type fibers, has been demonstrated. The receptors themselves have not yet been identified histologically. Cardiac receptors include subpopulations that are preferentially activated by chemical stimuli, including a variety of exogenous chemicals as well as prostaglandins. Another subpopulation is preferentially activated by mechanical stresses in the physiological range. Further investigation may reveal their participation in overall cardiovascular regulation, and mediation of responses to exogenous chemical stimuli. Four principal cardiac sympathetic nerves have been identified in the right thoracic region and three on the left. Most carry sympathetic and parasympathetic fibers. Stimulation of individual nerves, before and after parasympathetic blockade, results in regionally, well-defined myocardial responses.