Excitatory connections between upper cervical inspiratory neurons and phrenic motoneurons in cats

This study aimed to determine whether upper cervical inspiratory neurons (UCINs), which are localized in the intermediolateral part of the gray matter of the upper cervical segments, have propriospinal connections to phrenic motoneurons of the ipsilateral lower cervical segment in anesthetized cats. Unit action potentials of UCINs were extracellularly recorded simultaneously with ipsilateral phrenic nerve activity. To eliminate the descending influences from medullary respiratory neurons to phrenic motoneurons, bulbospinal conduction paths were temporarily blocked by focal cooling applied to the ventral caudal medulla at the pyramidal decussation level by means of a cooling thermode (1 mm tip diam). By using a spike-triggered method, during cooling phrenic nerve activities were evoked by UCIN spikes that were elicited by microinjection of L-glutamate for 20 of the 55 (36%) UCIN units examined. The onset latencies of these phrenic motoneuron responses ranged from 1.5 to 7.1 ms (mean 3.6 ms), depending on synaptic transmission delays. These results clearly demonstrate that UCINs have, at least in part, excitatory mono- and paucisynaptic connections with ipsilateral phrenic motoneurons.