VESTIBULOOCULOMOTOR CONNECTIONS IN AN ELASMOBRANCH FISH, THE ATLANTICSTINGRAY, DASYATIS-SABINA

In elasmobranch fishes, including the Atlantic stingray, the medial re ctus muscle is innervated by the contralateral oculomotor nucleus. Thi s is different from most vertebrates, in which the medial rectus is in nervated by the ipsilateral oculomotor nucleus. This observation led t o the prediction that the excitatory vestibulo-extraocular motoneuron projections connecting each semicircular canal to the appropriate musc le should use a contralateral projection from the vestibular nuclei to the motoneurons. This hypothesis was examined in the Atlantic stingra y by injecting horseradish peroxidase unilaterally into the oculomotor nucleus. It was found that vestibulo-oculomotor projections arise fro m the ipsilateral anterior octaval nucleus and the contralateral desce nding octaval nucleus. The same pattern was observed when the trochlea r nucleus was involved in the injection. There were no cells labeled i n the region of the abducens nucleus, and no candidate for a nucleus p repositus hypoglossus was identified. The presence of compensatory eye movements, the directional sensitivity of the semicircular canals, th e location of the motoneurons innervating each eye muscle, and our res ults indicate that the excitatory input to the extraocular motoneurons is derived from the contralateral descending octaval nucleus, and the inhibitory input is derived from the ipsilateral anterior octaval nuc leus. The absence of both abducens internuclear interneurons and a nuc leus prepositus hypoglossus suggests that eye movements, particularly those in the horizontal plane, are controlled differently in elasmobra nchs than in other vertebrates examined to date.