Localization and synaptic effects of inhibitory vestibulocollic neurons activated by the posterior semicircular canal nerve in the cat.

Cellular locations, axonal projections, and synaptic effects of inhibitory vestibulocollic (VC) neurons activated by the ampullary nerve of the posterior semicircular canal (PCN) were studied in anesthetized cats. The inhibitory VC neurons were identified by their monosynaptic responses to PCN stimulation and by their antidromic responses to stimulation of the ipsilateral (i-) and contralateral (c-) neck extensor motoneuron pools, which are inhibitory targets of the PCN. They were classified as VCi (vestibulocollic neuron sending an axon to the i-neck extensor motoneuron pool) and VCc (vestibulocollic neuron sending an axon to the c-neck extensor motoneuron pool) neurons. Neither VCi nor VCc neurons were activated antidromically by localized stimulation of the ascending medial longitudinal fasciculus (asc. MLF) or the 3rd nuclei. Their cell somata were localized in the rostral part of the descending vestibular nucleus and the ventral part of the lateral vestibular nucleus. VCi and VCc neurons produced unitary IPSPs in neck extensor motoneurons in the C1 segment.