Brainstem projections of different branches of the vestibular nerve. An experimental study by transganglionic transport of horseradish peroxidase in the cat. I. The horizontal ampullar and utricular nerves.

Twenty-seven adult cats were used to study the central projection of the horizontal ampullar nerve and the utricular nerve using transganglionic transport of HRP. The nerve to be investigated was exposed transtympanally, cut, and a capsule containing 50% HRP applied to the proximal stump for one hour; 2-8 days later the animals were perfused. Frozen serial sections were cut from the brainstem and processed for the demonstration of HRP using tetramethyl benzidine as chromogen. Maximal labelling was observed 5-7 days postoperatively. After application of HRP to the horizontal ampullar nerve extensive labelling was found in all four main nuclei except the lateral vestibular nucleus. Furthermore, labelling was observed in the interstitial nucleus of N. VIII, the rostral part of the external cuneate nucleus and in the reticular formation laterodorsal to the abducens nucleus. Small fibers were also seen in the brachium conjunctivum oriented towards the cerebellum, from the superior vestibular nucleus. After exposing the utricular nerve to HRP, labelling was observed in the ventral part of the lateral vestibular nucleus, the central part of the medial vestibular nucleus and in the rostral part of the descending vestibular nucleus. The findings are discussed in relation to known physiological differences between the two kinds of nerves here investigated.