Commissural effects in the otolith system

We examined whether otolith-activated second- and third-order vestibular nu cleus neurons received commissural inhibition from the contralateral otolit hic macula oriented in the same geometric plane. For this purpose we perfor med intracellular recording in vestibular nucleus neurons after stimulation of the ipsi- and contralateral utricular and saccular nerves. More than ha lf (41/72) of the utricular-activated second-order vestibular nucleus neuro ns received commissural inhibition from the contralateral utricular nerve. The remaining neurons (31/72) showed no visible response to contralateral u tricular nerve stimulation. About half (17/36) of utricular-activated third -order neurons also received commissural inhibition from the contralateral utricular nerve. Approximately 10% (7/67) of saccular-activated second-orde r vestibular neurons received polysynaptic commissural inhibition, whereas 16% (11/67) received commissural facilitation. The majority (49/67) of sacc ular second-order vestibular neurons, and almost all (22/23) third-order ne urons, showed no visible response to stimulation of the contralateral saccu lar nerve. The present findings suggest that many utricular-activated vesti bular nucleus neurons receive commissural inhibition, which may provide a m echanism for increasing the sensitivity of vestibular neurons to horizontal linear acceleration and lateral tilt of the head, Commissural inhibition i n the saccular system was less prominent than in the utricular system.