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

1. Vestibular neurones projecting to the upper cervical grey matter (vestibulocollic neurones) were identified by localized microstimulation in the C3 segment of the cat spinal cord.2. The neurones 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' neurones were mostly in the LVST, axons of medial and descending neurones in the MVST; a few Deiters' neurones had axons in the MVST; some descending neurones had axons in the LVST. Most axons of contralateral neurones were in the MVST.3. The axons of 62% of ipsilateral vestibulocollic Deiters' neurones not only gave off a collateral to C3, but also extended as far as the cervical enlargement (;branching'); some of these neurones projected as far as the upper thoracic cord, almost none to the lumbar cord. Ipsilateral descending nucleus neurones branch in the same fashion, but there is no branching in the relatively small medial nucleus population.4. A large majority of vestibulocollic neurones 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 neurones involved in the control of eye, head and trunk movement.5. Vestibulocollic neurones do not make up a homogeneous population acting only on the neck. Instead it is likely that subpopulations, for example branching and non-branching neurones, have different functions.