Physiological studies of small mediastinal ganglia in the cardiopulmonary nerves of dogs

Stimulation of the cranial end of a decentralized canine cardiopulmonary nerve results in the generation of a compound action potential which can be recorded at the caudal end. A region has been identified which, on stimulation, produces a compound action potential with complex configuration which is different from simpler wave forms obtained by stimulating 1 mm or more in either the rostral or caudal direction. Histological examination reveals that clusters of neuronal cell bodies are localized to this region of the nerve, whereas none is found to either side. Characteristics of the different wave forms evoked by changing the site of stimulation and reversing the stimulation and recording electrodes provide evidence for the existence of both afferent and efferent synaptic pathways. The compound action potential evoked by the most rostral stimulations and presumed to contain synaptic components was not altered by intravenously administered cholinergic and adrenergic pharmacological blocking agents (hexamethonium, atropine, phentolamine, or propranolol). It was, however, depressed by local injections of chymotrypsin or manganese into the functionally identified transitional region. It is concluded that synapses, which can be activated at relatively high frequencies (1–10 Hz) and may be important for rapidly changing local neural regulation of the heart and lungs, appear to exist within the course of cardiopulmonary nerves.