Properties of vestibular neurones projecting to neck segments of the cat spinal cord*

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Abstract
Vestibular neurons projecting to the upper cervical grey matter (vestibulocollic neurones) were identified by localized microstimulation in the C3 segment of the cat spinal cord. The neurons were found in the lateral (Deiters''), medial and descending nuclei bilaterally and projected to the spinal cord in the lateral and medial vestibulospinal tracts (LVST and MVST). Ipsilateral axons of Deiters'' neurons were mostly in the LVST, axons of medial and descending neurons in the MVST; a few Deiters'' neurons had axons in the MVST; some descending neurons had axons in the LVST. Most axons of contralateral neurons were in the MVST. The axons of 62% of ipsilateral vestibulocollic Deiters'' neurons not only gave off a collateral to C3, but also extended as far as the cervical enlargement (branching); some of these neurons projected as far as the upper thoracic cord, almost none to the lumbar cord. Ipsilateral descending nucleus neurons branch in the same fashion, but there is no branching in the relatively small medial nucleus population. A large majority of vestibulocollic neurons receive monosynaptic excitation from the ipsilateral labyrinth and a number are inhibited by stimulation of the contralateral labyrinth (commissural inhibition). It is possible that commissural inhibition acts on a broad population of vestibular neurons involved in the control of eye, head and trunk movement. Vestibulocollic neurons do not make up a homogeneous population acting only on the neck. Instead it is likely that subpopulations, for example branching and non-branching neurons, have different functions.