Competitive reinnervation of the rat superior cervical ganglion by foreign nerves.

The efficacy of foreign nerves to form synapses was studied morphologically and physiologically in the superior cervical ganglion of the rat. The denervated ganglion was anastomosed to the central stump of either the vagus, the hypoglossal or both nerves together. The degree of reinnervation was assessed two to ten months later. We measured the strength of contraction of the nictitating membrane after each type of operation and compared this to the number, type and distribution of synapses in the same ganglion. Both the vagus and hypoglossal nerves preferentially reinnervated a population of neurones that are situated in the cranial pole of the superior cervical ganglion and supply the nictitating membrane. When both nerves were connected to the ganglion only the vagus nerve could be shown to reinnervate it, and no reinnervation by the hypoglossal nerve was detected. However, in this experiment neither foreign nerve did as well in competition as each did alone and the overall result was reduced functional efficiency. We conclude that not all sympathetic neurones are equivalent and that, just like sympathetic afferents, the foreign nerves are capable of selectively reinnervating preferred target cells.