Response characteristics of semicircular canal and otolith systems in cat. II. Responses of trochlear motoneurons.

1. The electrical activity of single trochlear motoneurons (TMns) and axons of second order vestibular neurons presumably terminating on these motoneurons were studied during natural stimulation of semicircular canals and otolith organs in cats anesthetized with Ketamine. 2. Null point analysis showed that TMns received an excitatory canal input from the contralateral posterior canal, and labyrinthine lesion experiments suggested that the functionally synergistic, ipsilateral anterior canal provides an inhibitory input. A small number of motoneurons showed orthogonal canal convergence. 3. In addition to the canal projections most TMns received an otolithic input. Firing rate was proportional to lateral head tilt and was of the beta type. Most units also responded to pitch with an increase and decrease in firing rate on nose-up and nose-down positioning, respectively. Lesion experiments indicated that the otolith responses are the results of reciprocal innervation of TMns by contralateral (excitatory) and ipsilateral (inhibitory) otolith projections. 4. During sinusoidal rotation in yaw (canal only stimulation) the mean phase lag re acceleration of the response of TMns increased from 60 degrees at 0.025 Hz to 126 degrees at 1.0 Hz. In roll (canal plus otolith stimulation) the phase lag of TMn responses measured 180 degrees and 130 degrees at 0.025 and 1.0 Hz, respectively. Phase-lags measured in Vi and Vc axons were less by ca. 15 degrees. 5. The otolith contribution to TMn responses in roll was calculated by vectorial subtraction of the yaw from the roll responses: A phase lag of 10 (0.025 Hz) to 90 degrees (0.5 Hz) re. displacement was noted and gain was constant over the same range. Similar lag dynamics were revealed in TMns when studied during ramp displacement of the head. 6. The possible functional role of central canal-otolith convergence and the differences between the response of primary vestibular afferents and secondary vestibular neurons and TMns will be discussed.