Evidence of neuroanatomical connection between the superior cervical ganglion and hypoglossal nerve in the hamster as revealed by tract-tracing and degeneration methods.

Previous studies have shown the existence of a sympathetic component in some cranial nerves including the hypoglossal nerve. In this study, the horseradish peroxidase (HRP) tract-tracing retrograde technique and experimental degeneration method were used to elucidate the possible neuroanatomical relationship between the superior cervical ganglion (SCG) and the hypoglossal nerve of hamsters. About 10% of the SCG principal neurons were HRP positive following the tracer application to the trunk of hypoglossal nerve. Most of the HRP-labelled neurons were multipolar and were randomly distributed in the ganglion. When HRP was injected into the medial branch of the hypoglossal nerve, some of the SCG neurons were labelled, but they were not detected when HRP was injected into the lateral branch. The present findings suggest that postganglionic sympathetic fibres from the SCG may travel along the hypoglossal nerve trunk via its medial branch to terminate in visceral targets such as the intralingual glands. By electron microscopy, the HRP reaction product was localised in the neuronal somata and numerous unmyelinated fibres in the SCG. In addition, HRP-labelled axon profiles considered to be the collateral branches of the principal neurons contained numerous clear round and a few dense core vesicles. Besides the above, some HRP-labelled small myelinated fibres, considered to be visceral afferents, were also present. Results of experimental degeneration following the severance of the hypoglossal nerve showed the presence of degenerating neuronal elements both in the hypoglossal nucleus and the SCG. This confirms that the hypoglossal nerve contains sympathetic component from the SCG which may be involved in regulation of the autonomic function of the tongue.