Responses of ventral respiratory group neurons of the cat to natural vestibular stimulation.

Stimulation of vestibular otolith afferents by fore-aft tilt (pitch) elicits changes in activity of nerves innervating respiratory muscles, including the diaphragm, abdominal muscles, and tongue musculature. To determine the role of ventral respiratory group (VRG) neurons in producing these vestibular-respiratory responses, the activity of VRG neurons was recorded during natural vestibular stimulation in multiple transverse planes. Only a small fraction of VRG neurons with inspiratory (I, 20 of 80 cells), expiratory (E, 11 of 59 cells), or phase spanning (4 of 16 cells) activity responded to tilts up to 15° in amplitude delivered at frequencies from 0.02 to 2 Hz. In particular, responses were infrequent in VRG neurons with projections to the spinal cord (0 of 23 E cells and 2 of 15° I cells), despite the fact that the tilts employed produced robust modulation of the activity of abdominal (expiratory) nerves. Furthermore, the characteristics of responses to tilt of the small fraction of VRG neurons with vestibular inputs did not match those of respiratory muscles. These data suggest that neurons in addition to those in the VRG must participate in generating vestibular-respiratory responses.