Properties of saccular nerve-activated vestibulospinal neurons in cats

Axonal pathways, projection levels, conduction velocities, and locations of the cell bodies of saccular nerve-activated vestibulospinal neurons were studied in decerebrated cats and anesthetized cats, using a collision test of orthodromic and antidromic spikes. The saccular nerve was selectively stimulated by bipolar tungsten electrodes. Three monopolar electrodes were inserted into the left and right lateral vestibulospinal tract (LVST) and medial vestibulospinal tract (MVST) of the C1 segment, to determine the pathway of axons. Three pairs of similar electrodes were positioned bilaterally in the C3-4, T1, and L3 segments to examine projection levels. Another monopolar electrode was placed in the oculomotor nucleus to determine whether saccular nerve-activated vestibulospinal neurons have branches ascending to the oculomotor nucleus. Of 145 vestibular neurons orthodromically activated by stimulation of the saccular nerve, 46 were activated from the C1 segment antidromically. Forty-three were second-order vestibulospinal neurons and 3 were third-order vestibulospinal neurons. Four saccular nerve-activated vestibulospinal neurons were also antidromically activated from the oculomotor nucleus. Sixty-three percent of the saccular nerve-activated vestibulospinal neurons descended through the MVST; one-third of these terminated in the upper cervical segments, one-third reached the lower cervical segments and the remaining one-third reached the upper thoracic segments. Thirty percent of the saccular nerve-activated vestibulospinal neurons descended through the ipsilateral LVST; most of these reached the upper thoracic segments. Seven percent of the saccular nerve-activated vestibulospinal neurons descended through the contralateral vestibulospinal tracts terminating in the upper cervical segments. Most of the saccular nerve-activated vestibulospinal neurons originated in the caudal part of the lateral nucleus and rostral part of the descending nucleus.