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 suggested that these neurons may participate in the generation of vestibulospinal 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 vagotomized, and that had a cervical spinal cord transection so that inputs from tilt-sensitive receptors outside of the labyrinth did not influence the units we recorded. 3. In most experiments, vertical vestibular stimulation was used to stimulate the anterior and posterior semicircular canals and the otolith organs. The plane of whole body rotation that produced maximal modulation of a neuron's firing rate (response vector orientation) was measured at one or more frequencies between 0.1 and 0.5 Hz. Neuron dynamics were then studied with sinusoidal (0.02-1 Hz) stimuli aligned with this orientation. Alternatively, in two animals horizontal rotations at 0.5 and 1.0 Hz were employed to stimulate the horizontal semicircular canals. 4. The properties of raphespinal neurons were similar to those of a larger sample of raphe neurons studied that either could not be antidromically activated from the cervical spinal cord or were not tested for a spinal projection. In response to vertical vestibular stimulation, > 85% of caudal medullary raphe neurons had response gains that remained relatively constant across stimulus frequencies, like regularly firing otolith afferents.(ABSTRACT TRUNCATED AT 250 WORDS)