ROLE OF VENTRAL RESPIRATORY GROUP BULBOSPINAL EXPIRATORY NEURONS IN VESTIBULAR-RESPIRATORY REFLEXES

1. Activation of the vestibular system produces reflex modulation of e xpiratory muscle activity. The purpose of the present study was to inv estigate the possible role of bulbospinal expiratory (E) neurons locat ed in the caudal ventral respiratory group (VRG) in mediating vestibul o-respiratory reflexes. Experiments were carried out in decerebrated, paralyzed, and artificially ventilated cats. 2. Electrical stimulation of the vestibular nerve (VN), using short trains of current pulses, e licited bilateral reflex responses on abdominal muscle nerves (ABDNs). This response was not affected by lesions of the cochlear nuclei made by kainic acid injections. The ABDN response typically consisted of a combination of short-latency excitation and long-latency inhibition o n the ipsilateral side and, in contrast, a combination of short-latenc y inhibition and long-latency excitation on the contralateral side. 3. Extracellular recordings were made from 43 caudal VRG bulbospinal E n eurons that were activated antidromically from the contralateral upper lumbar spinal cord. More than 80% of these neurons responded to eithe r ipsi- and/or contralateral VN stimulation. The neuronal response con sisted of either a combination of excitation and inhibition or only in hibition. The majority of neurons had response patterns appropriate to contribute to the response observed on the contralateral ABDN; howeve r, the latency of the VRG E neuron response was too long to initiate t he ABDN response. 4. To further evaluate the contribution of caudal VR G E neurons to the vestibule-abdominal reflex, ABDN responses were com pared before and after sectioning the axons of caudal VRG bulbospinal E neurons where they cross the midline between the obex and first cerv ical spinal segment. These midsagittal lesions abolished expiratory mo dulation of ABDN discharge. The lesions also decreased the amplitude o f the vestibular-evoked ABDN response but could not abolish the respon se. The postlesion amplitude was decreased on average to similar to 70 % of prelesion values. 5. In conclusion, although the present results indicate that the majority of caudal VRG bulbospinal E neurons respond appropriately to contribute to the vestibule-abdominal reflex, the re flex largely is unaffected by the removal of caudal VRG E input. The a dditional descending inputs that are important for mediating the refle x remain to be investigated and may include vestibulospinal and/or ret iculospinal tracts.