Intracellular recordings from "recurrent neurons" in the rat superior cervical ganglion

Intracellular recordings in the isolated superior cervical ganglion of the rat showed that electrical stimulation of the cervical sympathetic trunk elicited in a cluster of neurons localized in the caudal part of the ganglion synaptically driven action potentials, and propagated potentials having the features of typical antidromic spikes. The results demonstrate that these neurons, besides synapsing with common preganglionic fibres, project their axons to the cervical sympathetic trunk. The recurrent neurons showed a very low threshold to direct intracellular stimulation and a high input resistance, suggesting that they have a small size. Almost all recurrent neurons were activated synaptically also by stimulating the postganglionic trunks, indicating that they are innervated by collaterals of preganglionic through-fibres which are known to sustain a direct pathway between pre- and postganglionic nerves. Moreover, some recurrent neurons could also be activated antidromically following stimulation of the external carotid nerve, indicating that their axons divide into collaterals which project not only to the preganglionic trunk but also to a postganglionic nerve. The presence of recurrent neurons in the superior cervical ganglion of the rat provides further evidence for the concept that sympathetic ganglia consist of discrete cell subpopulations which are segregated in different regions and probably subserve different functions.