VERTICAL VESTIBULAR INPUT TO AND PROJECTIONS FROM THE CAUDAL PARTS OFTHE VESTIBULAR NUCLEI OF THE DECEREBRATE CAT

1. To investigate the type of vestibular signals that neurons in the c audal parts of the vestibular nuclei transmit to the cerebellum and sp inal cord, we studied their responses to natural vestibular stimulatio n in vertical planes in decerebrate cats with the caudal cerebellum re moved. Most neurons were in the caudal half of the descending vestibul ar nucleus, the remainder at corresponding levels of the medial nucleu s or the medial-descending border. 2. Dynamics of the responses of spo ntaneously firing neurons were studied with sinusoidal tilts delivered at 0.05-1 Hz near the plane of body rotation that produced maximal mo dulation of the neuron's activity (response vector orientation). For m ost neurons the predominant vestibular input could be identified as co ming from otolith organs (46%) or vertical semicircular canals (37%). Some neurons had otolith + canal convergence (9%) and others either ha d such converging input or some other form of central processing (8%). 3. Gain and phase of the responses of otolith neurons were comparable with values obtained in earlier studies on Deiters' nucleus and the r ostral descending nucleus. Many canal neurons had a steeper gain slope and more advanced phase than observed previously for more rostral neu rons. This may be due to more irregular afferent input to many neurons or to the absence of the vestibulocerebellum. 4. Response vector orie ntations of canal neurons were closely bunched near the planes of the ipsilateral vertical canals. The small number of contralaterally proje cting vectors showed evidence of convergence between the two contralat eral vertical canals. As is the case elsewhere in the vestibular nucle i, there was no evidence of convergence from bilateral vertical canals . Response vector orientations of otolith neurons were restricted to t he roll quadrants; the majority pointed ipsilaterally. 5. Antidromic s timulation with an electrode in the restiform body or with several ele ctrodes in the dorsal half of the white matter of the upper cervical c ord was used to identify neurons projecting to the cerebellum and spin al cord, respectively. A substantial number of spontaneously firing ne urons projected to the cerebellum, but there were few spontaneously ac tive vestibulospinal neurons. The properties of the vestibular input t o cerebellar-projecting neurons were the same as those of the populati on as a whole, but the effect of tilt on vestibulospinal neurons appea red weak or absent. 6. Many neurons were inhibited by stimulation of t he restiform body. We suggest that this is mainly due to stimulation o f the axons of vestibulocerebellar Purkinje cells. 7. Our results demo nstrate a robust vertical vestibular input to the caudal parts of the vestibular nuclei. Whereas this input is prominent for cerebellar-proj ecting neurons, it does not appear important for vestibulospinal neuro ns in this region. The results are also consistent with the existence of a loop between the caudal vestibular nuclei and vestibulocerebellum .