Dorsal respiratory group neurons in the medulla of cat: Spinal projections, responses to lung inflation and superior laryngeal nerve stimulation

Abstract

Extracellular activity of 200 dorsal respiratory group (DRG) neurons of the ventrolateral nucleus of the tractus solitarius was recorded in anesthetized, paralyzed, artificially ventilated cats. The effects on neural activity of electrically stimulating the contralateral cervical spinal cord, the superior laryngeal nerves (SLN) bilaterally, and inflating the lungs to various levels during the expiratory pause were tested. 1. (1) Of the 200 cells, 157 were inspiratory cells; 32 cells (termed P cells), fired synchronously with lung inflation but did not have inspiratory rhythm of central origin; and 11 were non-inspiratory respiratory cells. Eighty-eight of the inspiratory cells which were inhibited with inhibition of phrenic inspiratory discharge following lung inflation were classified as α-type. However, most also showed short duration activation (gasp reflex) at the onset of lung hyperinflation. Thirty-five inspiratory cells which were excited by lung inflation in addition to showing the gasp reflex were classified as β-type neurons. The discharge of these β-type cells adapted during lung inflation; their excitation threshold was higher and sensitivity lower than that of the Hering-Breuer expiratory facilitatory reflex. I conclude that they are not the interneurons responsible for that reflex, but could possibly receive excitatory inputs from pulmonary irritant receptors. 2. (2) Approximately 80% of both α- and β-type neurons had axonal projections to the cord. A spinal cord collision test error and relative error were calculated for 93 projecting DRG neurons. Only two neurons had a collison error of zero, the rest had positive values. I conclude that these results imply only spinal axonal projections. 3. (3) Fifty-seven (58%) of the inspiratory cells tested were evoked from the ipsilateral SLN, with a mean latency of 3.9 msec ± 0.3 S.E.. All cells that were evoked with short latency followed stimuli delivered at 100 Hz. No cell was evoked from the contralateral SLN. I suggest that some DRG neurons are part of a disynaptic neural circuit involving SLN afferents, ipsilateral DRG neurons, and contralateral phrenic motoneurons.