Convergence of the horizontal semicircular canal and otolith afferents on cat single vestibular neurons

We studied the convergence of two afferent pairs of single vestibular neuro ns by selective stimulation of the horizontal semicircular canal (HC) and s accular (SAC) nerves, and the HC and utricular (UT) nerves in decerebrate c ats. All recorded neurons were classified as vestibulospinal (VS), vestibul o-oculospinal (VOS) or vestibulo-ocular (VO), by antidromic stimulation fro m the oculomotor/trochlear nuclei and the spinal cord: neurons that could n ot be activated from any test sites were classified as vestibular (V) neuro ns. Of a total of 125 neurons activated by stimulation of the HC/SAC nerves , 21(17%) received convergent inputs. Twelve of 21 neurons received monosyn aptic excitatory inputs from both nerves. About half (9/21, 43%) of the con vergent neurons were classified as VS neurons, the majority of which descen ded through the ipsilateral lateral vestibulospinal tract (i-LVST). The HC/ SAC convergent neurons were located in the rostral part of the descending, the medial and the caudal-ventral part of the lateral vestibular nucleus. I n 80 neurons studied by stimulation of the HC/UT nerves, both inputs conver ged in 12 (15%) neurons, more than half of which were VS neurons. Eight of 12 convergent neurons received excitatory inputs followed by inhibition fro m both the HC and UT nerves. A few convergent neurons (3/12) projected to t he oculomotor/trochlear nucleus. Half of the convergent and non-convergent VS neurons descended to the spinal cord through the i-LVST, and the only on e VOS convergent neuron via the medial vestibulospinal tract. Most of the c onvergent neurons were located in the lateral, the rostral part of the desc ending and medial vestibular nucleus. The percentages of HC/SAC and HC/UT c onvergence were half those of the posterior semicircular canal (PC), PC/SAC (33%) and PC/UT (33%) convergence, respectively. The convergent neurons re ceiving the HC and otolith inputs may contribute at least partly to the ves tibulocollic reflex.