Vestibular Inputs to Propriospinal Interneurons in the Feline C1-C2 Spinal Cord Projecting to the C5-C6 Ventral Horn

The resting length of respiratory muscles must be altered during changes in posture in order to maintain stable ventilation. Prior studies showed that although the vestibular system contributes to these adjustments in respiratory muscle activity, the medullary respiratory groups receive little vestibular input. Additionally, previous transneuronal tracing studies demonstrated that propriospinal interneurons in the C(1)-C(2) spinal cord send projections to the ipsilateral diaphragm motor pool. The present study tested the hypothesis that C(1)-C(2) interneurons mediate vestibular influences on diaphragm activity. Recordings were made from 145 C(1)-C(2) neurons that could be antidromically activated from the ipsilateral C(5)-C(6 )ventral horn, 60 of which had spontaneous activity, during stimulation of vestibular receptors using electric current pulses or whole-body rotations in vertical planes. The firing of 19 of 31 spontaneously active neurons was modulated by vertical vestibular stimulation; the response vector orientations of many of these cells were closer to the pitch plane than the roll plane, and their response gains remained relatively constant across stimulus frequencies. Virtually all spontaneously active neurons responded robustly to electrical vestibular stimulation, and their response latencies were typically shorter than those for diaphragm motoneurons. Nonetheless, respiratory muscle responses to vestibular stimulation were still present after inactivation of the C(1)-C(2) cord using large injections of either muscimol or ibotenic acid. These data suggest that C(1)-C(2) propriospinal interneurons contribute to regulating posturally related responses of the diaphragm, although additional pathways are also involved in generating this activity.