Gross and microscopic anatomy of the vagal innervation of the rat heart.
Intrinsic cardiac ganglia and their vagal innervation are described from gross and microscopic dissections and functional studies in the anesthetized, open-chest, adult rat. Dissecting microscope sketches of the ventral and dorsal aspects of the rat heart provide gross descriptions of the anatomical course of the vagal cardiac nerves. Histological sectioning of adipose tissue packets surrounding the terminal endings of vagal branches distributed to the roots of the great cardiac vessels (aorta, pulmonary artery, precaval veins) revealed clusters of autonomic ganglia. These packets or "fat pads" were located: (1) along the dorsal surface of the right precava and extending medially toward the aortic root, (2) deep to the aortic arch, (3) in the angle between the root of the left precava and the pulmonary artery on the superior-dorsal surface of the left atrium, and (4) in the rostro-dorsal interatrial septum. Vagal distributions of small terminal branches were traced to each of these pads, which contained numerous autonomic ganglia. Electrical excitation of right or left cervical vagus elicited varying degrees of sinus slowing, slowing of A-V conduction, and suppression in atrial contractile force. Very small quantities (0.5 mg in 10 microliters saline) of the ganglionic blocking agent, hexamethonium (C6) were injected selectively into a single fat pad, followed by repetition of right or left vagal stimulation, with careful analysis of changes in heart rate (paced and unpaced), A-V conduction, and contractile force.