Electrophysiological analysis of cerebellar corticovestibular and fastigiovestibular projections to the lateral vestibular nucleus in the cat.

In the lateral vestibular nucleus, vestibulospinal tract (VST) neurons were surveyed with microelectrodes in cats anesthetized with sodium pentobarbital. The VST neurons (n = 450) were classified by their properties; axonal courses (LVST and MVST). spinal segmental levels of their axonal termination (C1-3, C4-8, T1-13, L1-4, and L5-neurons), their orthodromic activation by the primary vestibular nerve (second-order and non-second-order vestibular neurons), and their location in the LVN. Inhibitory and excitatory effects of cerebellar stimulation on these classified VST neurons were investigated. 84% (259/308) neurons were observed to receive cerebellar corticovestibular inhibition. The rate was high, and almost the same among classified neurons; C1-3 to L5-neurons, and second-order and non-second-order neurons. However, the rate with MVST neurons (69%) was significantly lower than with LVST cells (87%). These neurons which received cerebellar inhibition were distributed in all areas even deep in the rostroventral region of the LVN, while neurons which did not receive were distributed in the ventral region of the LVN. Electrical stimulation of ipsi- and contralateral fastigial nuclei evoked monosynaptic excitation of the classified VST neurons. Rate of occurrence of crossed fastigiovestibular excitation was higher with cervical neurons (86%) than with lumbar neurons (43%), and higher with second-order neurons (78%) than with non-second-order neurons (41%). Neurons which received monosynaptic excitation from crossed fastigiovestibular fibers were distributed in the ventral region of the LVN. In total, 73% of the neurons were identified to receive either ipsi- or contralateral fastigiovestibular excitation. The results indicated that there was relative scarcity of fastigiovestibular projections in the dorsal region of the LVN. Spinovestibular and other afferents to the LVN were also investigated.