Anatomical and physiological characterization of avian sympathetic cardiac efferents.

In the preceding paper6 describing the right sympathetic cardiac nerve of the pigeon we reported a fast compound action potential component with no chronotropic effect, and single unit recordings from sympathetic postganglionic neurons demonstrated that this action potential component was not reflected in the antidromic postganglionic latencies. Electron microscopy then indicated the presence of numerous myelinated fibers in the nerve, and together these findings suggested the existence of a substantial number of sympathetic cardiac afferents in the right cardiac nerve. The present paper confirms this by demonstrating with retrograde transport of horseradish peroxidase that some fibers of the right sympathetic cardiac nerve have their cells of origin in dorsal root ganglia. This was also shown to be the case for the left sympathetic cardiac nerve. Selective activation of the myelinated fiber contingent was then shown to elicit a short latency decrease in arterial blood pressure that could be further augmented by the activation of the smaller unmyelinated fibers. This reflex depressor response to activation of sympathetic cardiac afferents survived bilateral vagotomy but was blocked by atropine. It is therefore concluded that both the left and right sympathetic cardiac nerves of the pigeon contain afferent fibers, both myelinated and unmyelinated, and that the reflex effect of activating these fibers is a sympathetically mediated vasodilation that is atropine-sensitive.