Properties of Vestibular Neurons Mediating Vertical Canal Reflexes.

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The firing properties and projection patterns of secondary vestibular nucleus neurons involved in the vertical vestibulo-ocular pathways were investigated in alert cats. Single unit recordings were made in the medial longitudinal fasciculus (MLF) near the trochlear nucleus from axons that were monosynaptically activated following stimulation of the vestibular nerve. Many identified axons discharged in relation to vertical eye movements when the head was still. The majority of these axons increased their firing rate for downward eye position during fixation (DPVs). During pitch rotation in the light, all the DPVs tested increased their firing rate with upward head rotation, suggesting that they received monosynaptic input from the posterior semicircular canal. DPVs could be divided into two groups on the basis of their firing regularity. There was a tendency for regular DPVs to have a higher firing rate, a higher correlation for the rate-position relationship, and a larger phase lag and a smaller gain re head velocity than irregular DPVs. Spike-triggered average method and intraaxonal HRP techniques demonstrated that ipsilaterally projecting (i-) DPVs made inhibitory connections with up-on extraocular motoneurons, and contralaterally projecting (c-) DPVs made excitatory connections with down-on motoneurons. Virtually all i-DPVs were of regular type, while c-DPVs included both regular and irregular types. Stimulation of the caudal MLF at the level of the obex indicated that all the irregular c-DPVs and some of the regular c-DPVs had a collateral to the spinal cord, while none of the regular i-DPVs had such a collateral.