PROPERTIES OF UTRICULAR NERVE-ACTIVATED VESTIBULOSPINAL NEURONS IN CATS

The axonal pathway, conduction velocities, and locations of the cell b odies of utricular nerve-activated vestibulospinal neurons were studie d in decerebrated or anesthetized cats using the collision test of ort hodromic and antidromic spikes. For orthodromic stimulation, bipolar t ungsten electrodes were placed on the utricular nerve and the other ve stibular nerve branches were transected. Monopolar tungsten electrodes were positioned on both sides of the upper cervical segments (C2-4), caudal end of the cervical enlargement (C7-T1), and from the lower tho racic to the upper lumbar segments (T12-L3) and were used for antidrom ic stimulation of the spinal cord. Another monopolar electrode was als o placed in the oculomotor nucleus to study whether utricular nerve-ac tivated vestibulospinal neurons have ascending branches to the oculomo tor nucleus. Of the 173 vestibular neurons orthodromically activated b y the stimulation of the utricular nerve, 46 were second-order vestibu lospinal neurons and 5 were third-order neurons. The majority of the u tricular nerve-activated vestibulospinal neurons were located in the r ostral part of the descending vestibular nucleus and the caudal part o f the ventral lateral nucleus. Seventy-three percent of the utricular nerve-activated vestibulospinal neurons descended through the ipsilate ral lateral vestibulospinal tract. Approximately 80% of these neurons reached the cervicothoracic junction, but a few reached the upper lumb ar spinal cord. Twenty-seven percent of the utricular nerve-activated vestibulospinal neurons descended through the medial vestibulospinal t ract or the contralateral vestibulospinal tracts. Those axons terminat ed mainly in the upper cervical segments. Almost none of the utricular nerve-activated vestibular neurons had ascending branches to the ocul omotor nucleus.