TILT RESPONSES OF NEURONS IN THE CAUDAL DESCENDING NUCLEUS OF THE DECEREBRATE CAT - INFLUENCE OF THE CAUDAL CEREBELLAR VERMIS AND OF NECK RECEPTORS

1. In decerebrate cats with intact cerebellums, we studied the respons es of neurons in the caudal areas of the vestibular nuclei to natural vestibular stimulation in vertical planes and to neck rotation. The ac tivity of most neurons was recorded in the caudal half of the descendi ng nucleus. 2. One goal of our experiments was to compare the dynamic and spatial properties of responses to sinusoidal vestibular stimulati on with those seen in previous experiments in which the caudal cerebel lar vermis, including the nodulus and uvula, was removed. This part of the cerebellum receives vestibular input and projects to the caudal a reas of the vestibular nuclei, suggesting that it could influence resp onses to stimulation of the labyrinth. 3. As in our previous experimen ts, most neurons could be classified as receiving predominant input ei ther from the otoliths or from one vertical semicircular canal. When m ean gain and phase and response vector orientations were compared, the re were no obvious differences between the behavior of neurons in the partially decerebellate preparation and the one with the cerebellum in tact, demonstrating that in the decerebrate cat the nodulus and uvula have little or no influence on the processing of vertical vestibular i nput in this region of the vestibular nuclei. 4. Only 23 of 74 (31%) o f neurons tested responded to neck rotation. This contrasts with the m uch larger fractions that respond to this stimulus in Deiters' nucleus and in the rostral descending nucleus. We also recorded from neurons near the vestibular nuclei, mainly in the external cuneate nucleus. Al l of them (9 of 9) responded to neck rotation. 5. Responses to neck ro tation also differed in their dynamics from those found more rostrally in the vestibular nuclei. Dynamics of more rostral neurons resemble t hose of neck muscle spindles, as do those of external cuneate neurons. The dynamics of caudal vestibular neurons, on the other hand, have a steeper gain slope and more advanced phases than do those of neurons i n the more rostral vestibular nuclei. This suggests the possibility of involvement of additional receptors in the production of these respon ses. 6. In the more rostral vestibular nuclei, responses to vestibular and neck rotation are most often antagonistic, so that head rotation results in little or no response. This is not the case in the caudal a reas of the vestibular nuclei, where less than half the neurons tested displayed antagonistic behavior. Further experiments are required to put the neck projection to the caudal vestibular nuclei in a functiona l context.