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.

Use of electrical vestibular stimulation to alter genioglossal muscle activity in awake cats.

Prior work has shown that the vestibular system contributes to regulating activity of upper airway muscles including the tongue protruder muscle genioglossus. The goal of the present experiments was to determine whether electrical vestibular stimulation could potentially be used to alter genioglossal activity in awake animals. Six adult cats were instrumented for recording of EMG activity from genioglossus, abdominal musculature, and triceps. In addition, a silver ball electrode was implanted on the round window for stimulation of vestibular afferents. Subsequently, stimulation and recordings were conducted while animals were awake. In all cases, stimulation using single shocks or trains of pulses > 100 microA in intensity produced responses in all muscles, including genioglossus. The latency of the genioglossal response was approximately 12 msec, and delivering continuous current trains to the labyrinth chronically elevated the muscle's activity. Although a number of muscles were affected by the stimulus, animals experienced no obvious distress or balance disturbances. Vestibular stimulation remained effective in producing genioglossal responses until experiments were discontinued 1-2 months following onset. These data suggest that electrical vestibular stimulation could potentially be used therapeutically to alter upper airway muscle activity.

Transneuronal tracing of neural pathways influencing both diaphragm and genioglossal muscle activity in the ferret.

In prior experiments that employed the transneuronal transport of isogenic recombinants of pseudorabies virus (PRV), we demonstrated that neurons located ventrally in the medial medullary reticular formation (MRF) of the ferret provide collateralized projections to both diaphragm and abdominal muscle motoneurons as well as to multiple abdominal muscle motoneuron pools. The goal of the present study was to determine whether single MRF neurons also furnish inputs to diaphragm motoneurons and those innervating an airway muscle with inspiratory-related activity: the tongue protruder genioglossus. For this purpose, PRV recombinants expressing unique reporters (beta-galactosidase or enhanced green fluorescent protein) were injected into either the diaphragm or the genioglossal muscle. The virus injections produced transneuronal infection of overlapping populations of MRF neurons. A small proportion of these neurons (<15%) was infected by both PRV recombinants, which indicated that they provide collateralized inputs to genioglossal and diaphragm motoneurons. These findings show that, whereas some MRF neurons simultaneously influence the activity of upper airway and respiratory pump muscles, other cells in this brain stem region independently contribute to diaphragm and genioglossal muscle contraction regulation.