EXCITATORY CONNECTIONS BETWEEN UPPER CERVICAL INSPIRATORY NEURONS ANDPHRENIC MOTONEURONS IN CATS

This study aimed to determine whether upper cervical inspiratory neuro ns (UCINs), which are localized in the intermediolateral part of the g ray matter of the upper cervical segments, have propriospinal connecti ons to phrenic motoneurons of the ipsilateral lower cervical segment i n anesthetized cats. Unit action potentials of UCINs were extracellula rly recorded simultaneously with ipsilateral phrenic nerve activity. T o eliminate the descending influences from medullary respiratory neuro ns to phrenic motoneurons, bulbospinal conduction paths were temporari ly blocked by focal cooling applied to the ventral caudal medulla at t he pyramidal decussation level by means of a cooling thermode (1 mm ti p diam). By using a spike-triggered method, during cooling phrenic ner ve activities were evoked by UCIN spikes that were elicited by microin jection 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 trans mission delays. These results clearly demonstrate that UCINs have, at least in part, e xcitatory mono- and paucisynaptic connections with ipsilateral phrenic motoneurons.