RESPONSES OF CAUDAL MEDULLARY RAPHE NEURONS TO NATURAL VESTIBULAR STIMULATION

1. Over two thirds of caudal medullary raphespinal neurons respond to electrical stimulation of the vestibular nerve, and it has been sugges ted that these neurons may participate in the generation of vestibulos pinal and vestibulosympathetic reflexes. The objective of the present study was to determine which vestibular endorgans (semicircular canals or otolith organs) provide inputs to these cells. 2. Experiments were conducted on decerebrate cats that were baroreceptor denervated and v agotomized, and that had a cervical spinal cord transection so that in puts from tilt-sensitive receptors outside of the labyrinth did not in fluence the units we recorded. 3. In most experiments, vertical vestib ular stimulation was used to stimulate the anterior and posterior semi circular canals and the otolith organs. The plane of whole body rotati on that produced maximal modulation of a neuron's firing rate (respons e vector orientation) was measured at one or more frequencies between 0.1 and 0.5 Hz. Neuron dynamics were then studied with sinusoidal (0.0 2-1 Hz) stimuli aligned with this orientation. Alternatively, in two a nimals horizontal rotations at 0.5 and 1.0 Hz were employed to stimula te the horizontal semicircular canals. 4. The properties of raphespina l neurons were similar to those of a larger sample of raphe neurons st udied that either could not be antidromically activated from the cervi cal spinal cord or were not tested for a spinal projection. In respons e to vertical vestibular stimulation, >85% of caudal medullary raphe n eurons had response gains that remained relatively constant across sti mulus frequencies, like regularly firing otolith afferents. In additio n, only a small fraction of the tested cells responded to horizontal r otations, and the response gains of the modulated neurons were extreme ly weak. Thus the labyrinthine inputs to caudal medullary raphe neuron s appear to come principally from otolith organs, with little contribu tion from any of the semicircular canals. 5. Over two-thirds of raphes pinal neurons with predominant otolith input had response vector orien tations that were nearer pitch than roll. This is in contrast to media l pontomedullary reticulospinal and vestibulospinal neurons that recei ve mainly otolith signals, most of which respond better to roll than t o pitch. 6. Our data suggest that changes in head position in the sagi ttal plane (pitch) may influence the excitability of spinal cord motor and autonomic function via the caudal medullary raphe nuclei.