Role of the medial medullary reticular formation in relaying vestibular signals to the diaphragm and abdominal muscles

Changes in posture can affect the resting length of respiratory muscles, re quiring alterations in the activity of these muscles if ventilation is to b e unaffected. Recent studies have shown that the vestibular system contribu tes to altering respiratory muscle activity during movement and changes in posture. Furthermore, anatomical studies have demonstrated that many bulbos pinal neurons in the medial medullary reticular formation (MRF) provide inp uts to phrenic and abdominal motoneurons; because this region of the reticu lar formation receives substantial vestibular and other movement-related in put, it seems likely that medial medullary reticulospinal neurons could adj ust the activity of respiratory motoneurons during postural alterations. Th e objective of the present study was to determine whether functional lesion s of the MRF affect inspiratory and expiratory muscle responses to activati on of the vestibular system. Lidocaine or muscimol injections into the MRF produced a large increase in diaphragm and abdominal muscle responses to ve stibular stimulation. These vestibulo-respiratory responses were eliminated following subsequent chemical blockade of descending pathways in the later al medulla. However, inactivation of pathways coursing through the lateral medulla eliminated excitatory, but not inhibitory, components of vestibulo- respiratory responses. The simplest explanation for these data is that MRF neurons that receive input from the vestibular nuclei make inhibitory conne ctions with diaphragm and abdominal motoneurons, whereas a pathway that cou rses laterally in the caudal medulla provides excitatory vestibular inputs to these motoneurons. (C) 2001 Elsevier Science B.V. All rights reserved.