Contribution of cranial nerve ganglia to innervation of the walls of the rat external acoustic meatus

The retrograde neuronal tracer, fluorogold, was used to determine the relative contributions of neurons in selected cranial nerve ganglia to the somatosensory innervation of the external acoustic meatus in the rat. Frozen sections from the trigeminal (semilunar), facial (geniculate), glossopharyngeal (superior petrosal) and vagal (jugular) nerve ganglia were examined by epifluoresence microscopy 10 to 14 days after application of the tracer dye fluorogold to the superior, inferior, anterior and posterior walls of the external acoustic meatus. The anterior wall of the canal makes a large contribution to the trigeminal ganglion and a lesser contribution to the other 3 ganglia. The trigeminal ganglion is also a major recipient from the posterior wall with smaller contributions from that wall to the geniculate and vagal ganglia. The superior wall projects a relatively constant supply to all 4 ganglia while the inferior wall primarily relays to the trigeminal and geniculate ganglia sending smaller contributions to the vagal and glossopharyngeal ganglia. In conclusion, neurons in these 4 cranial nerve ganglia contribute an overlapping supply to the 4 walls of the external ear meatus, and there is no evidence of a preferential distribution of nerve supply from one particular ganglion or nerve to specific walls of the external acoustic meatus.