PROPERTIES OF SACCULAR NERVE-ACTIVATED VESTIBULOSPINAL NEURONS IN CATS

Axonal pathways, projection levels, conduction velocities, and locatio ns of the cell bodies of saccular nerve-activated vestibulospinal neur ons were studied in decerebrated cats and anesthetized cats, using a c ollision test of orthodromic and antidromic spikes. The saccular nerve was selectively stimulated by bipolar tungsten electrodes. Three mono polar electrodes were inserted into the left and right lateral vestibu lospinal tract (LVST) and medial vestibulospinal tract (MVST) of the C 1 segment, to determine the pathway of axons. Three pairs of similar e lectrodes 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-activate d vestibulospinal neurons have branches ascending to the oculomotor nu cleus. Of 145 vestibular neurons orthodromically activated by stimulat ion of the saccular nerve, 46 were activated from the C1 segment antid romically. Forty-three were second-order vestibulospinal neurons and 3 were third-order vestibulospinal neurons. Four saccular nerve-activat ed vestibulospinal neurons were also antidromically activated from the oculomotor nucleus. Sixty-three per cent of the saccular nerve-activa ted vestibulospinal neurons descended through the MVST, one-third of t hese 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 vest ibulospinal neurons descended through the ipsilateral LVST; most of th ese reached the upper thoracic segments. Seven percent of the saccular nerve-activated vestibulospinal neurons descended through the contral ateral vestibulospinal tracts terminating in the upper cervical segmen ts. Most of the saccular nerve-activated vestibulospinal neurons origi nated in the caudal part of the lateral nucleus and rostral part of th e descending nucleus.