Cerebellar afferents from neurons in the extraocular motor nuclei: a fluorescent retrograde double-labeling study in the sheep.

The fluorescent retrograde double labeling technique has been used to identify within the extraocular motor nuclei of the sheep the neurons projecting to the cerebellum and to provide evidence whether they are motor neurons sending collaterals to the cerebellum or a separate population of neurons. The study was performed on eight sheep. The fluorescent tracers used were Fast Blue and the diamidino yellow dihydrochloride. In one and the same animal a fluorescent tracer was injected into the extraocular muscles (EOMs) and the other into bilateral points of the vermal folia II-V and paramedian lobule, or into the vermal folia VI, VIIA and VIIB, or into the underlying fastigial nuclei. Within the oculomotor, trochlear, and abducens nuclei, almost all of the motor neurons were labeled by the tracer injected into the EOMs and only a few cells were fluorescent for the tracer infiltrated into the cerebellum. These latter labelings were present bilaterally, and their number and distribution did not show apparent differences after injecting the paramedian lobule and the vermal folia or the fastigial nucleus. Along the rostrocaudal extent of the oculomotor and trochlear nuclei, the neurons projecting to the cerebellum were intermingled with the motor neurons located in the nuclear area facing the medial longitudinal fasciculus. In the abducens nucleus they were restricted to the caudal pole of the nucleus, which is located ventrolaterally to the genu of the facial nerve. Double-labeled neurons were never found. The absence of double-labeled cells, in spite of the efficiency of the tracer infiltration into the EOMs and into the cerebellum, demonstrates that the cerebellar projections from the extraocular motor nuclei are not collaterals of the motor neurons, but axons of a separate population of neurons.