2ND-ORDER VESTIBULAR NEURON MORPHOLOGY OF THE EXTRA-MLF ANTERIOR CANAL PATHWAY IN THE CAT

Second-order vestibular neurons form the central links of the vestibul o-oculomotor three-neuron arcs that mediate compensatory eye movements . Most of the axons that provide for vertical vestibule-ocular reflexe s ascend in the medial longitudinal fasciculus (MLF) toward target neu rons in the oculomotor and trochlear nuclei. We have now determined th e morphology of individual excitatory second-order neurons of the ante rior semicircular canal system that course outside the MLF to the ocul omotor nucleus. The data were obtained by the intracellular horseradis h peroxidase method. Cell somata of the extra-MLF anterior canal neuro ns were located in the superior vestibular nucleus. The main axon asce nded through the deep reticular formation beneath the brachium conjunc tivum to the rostral extent of the nucleus reticularis tegmenti pontis , where it crossed the midline. The main axon continued its trajectory to the caudal edge of the red nucleus from where it coursed back towa rd the oculomotor nucleus. Within the oculomotor nucleus, collaterals reached superior rectus and inferior oblique motoneurons. Some axon br anches recrossed the midline within the oculomotor nucleus and reached the superior rectus motoneuron subdivision on that side. Since these neurons did not give off a collateral toward the spinal cord, they wer e classified as being of the vestibulo-oculomotor type and are thought to be involved exclusively in eye movement control. The signal conten t and spatial tuning characteristics of this anterior canal vestibulo- oculomotor neuron class remain to be determined.