Transmission Pathways in the Rat Superior Cervical Ganglion

On isolated preparations of the superior cervical ganglion (SCG, n = 8) taken from 21-day-old rats, we studied the intraganglion pathways and mechanisms underlying generation of synaptic responses of SCG neurons to antidromic stimulation. One of the three nerves connected with the SCG was stimulated, and compound action potentials were recorded simultaneously from the other two nerves; then, the order of stimulated and recorded nerves was changed. Orthodromic stimulation of the cervical sympathetic nerve (CSN) evoked responses in the internal carotid nerve (ICN), which could be completely blocked by hexamethonium, and responses in the external carotid nerve (ECN), which contained a component that was not blocked by this agent. Antidromic stimulation of the ICN did not induce responses in the CSN and ECN, whereas stimulation of the ECN caused responses in the CSN, which were not blocked by hexamethonium. Effects of superfusion of the SCG with a Ca2+-free solution allowed us to conclude that the hexamethonium-insensitive component of the responses in the CSN and ECN and ECN-CSN conduction can be explained by the presence of direct fibers going from the CSN to the ECN with no synaptic relay. Possible mechanisms underlying antidromic stimulation-induced synaptic responses in SCG neurons are discussed.