Multimodal medullary neurons and correlational linkages of the respiratorynetwork

Multimodal medullary neurons and correlational linkages of the respiratory network. J. Neurophysiol. 82: 188-201, 1999. This study addresses the hypot hesis that multiple sensory systems, each capable of reflexly altering brea thing, jointly influence neurons of the brain stem respiratory network. Car otid chemoreceptors, baroreceptors, and foot pad nociceptors were stimulate d sequentially in 33 Dial-urethan-anesthetized or decerebrate vagotomized a dult cats. Neuronal impulses were monitored with microelectrode arrays in t he rostral and caudal ventral respiratory group (VRG), nucleus tractus soli tarius (NTS), and n. raphe obscurus. Efferent phrenic nerve activity was re corded. Spike trains of 889 neurons were analyzed with cycle-triggered hist ograms and tested for respiratory-modulated firing rates. Responses to stim ulus protocols were assessed with peristimulus time and cumulative sum hist ograms. Cross-correlation analysis was used to test for nonrandom temporal relationships between spike trains. Spike-triggered averages of efferent ph renic activity and antidromic stimulation methods provided evidence for fun ctional associations of bulbar neurons with phrenic motoneurons. Spike trai n cross-correlograms were calculated for 6,471 pairs of neurons. Significan t correlogram features were detected for 425 pairs, including 189 primary c entral peaks or troughs, 156 offset peaks or troughs, and 80 pairs with mul tiple peaks and troughs. The results provide evidence that correlational me dullary assemblies include neurons with overlapping memberships in groups r esponsive to different sets of sensory modalities. The data suggest and sup port several hypotheses concerning cooperative relationships that modulate the respiratory motor pattern. I) Neurons responsive to a single tested mod ality promote or Limit changes in firing rate of multimodal target neurons. 2) Multimodal neurons contribute to changes in firing rate of neurons resp onsive to a single tested modality. 3) Multimodal neurons may promote respo nses during stimulation of one modality and "limit" changes in firing rates during stimulation of another sensory modality. 4) Caudal VRG inspiratory neurons have inhibitory connections that provide negative feedback regulati on of inspiratory drive and phase duration.