Locations of medullary neurons with non-phasic discharges excited by stimulation of central and/or peripheral chemoreceptors and by activation of nociceptors in cat.

The activity of medullary neurons (146 units) with non-phasic discharges was recorded extracellularly in decerebrated, spontaneously breathing cats. The firing rate changes were studied during injections of 100% CO2-saturated saline into the vertebral artery and into the carotid artery. Thirty-nine of the 146 units were excited by the vertebral artery injections in the same time course as ventilatory augmentation. Eighteen of the 39 units did not react to peripheral chemoreceptor stimulation, i.e. they responded exclusively to central chemoreceptor stimulation. These 18 units were distributed in the caudal ('C') chemoceptive area of the ventral surface, in the vicinity of ventral respiratory group (VRG) neurons, and in the dorsal area ventral to the solitary tract. Twenty-one of the 39 units were excited by peripheral chemoreceptor stimulation, i.e. neurons with integrations of central and peripheral chemoreceptor inputs. They were densely packed in the nucleus paragigantocellularis lateralis, although also found in and around the rostral VRG neurons. Fifty-one of the 146 non-phasic units were excited (44) or inhibited (7) immediately after the vertebral artery injections, and their discharges returned to pre-injection levels prior to or during the early period of ventilatory augmentation. They also reacted to skin pinching, i.e. neurons with nociceptive action, and were found scattered variously in the ventral half of the medulla. Fourteen of the 146 units reacted to both chemoreceptor and nociceptor stimulations. The remaining 42 units were non-responsive. In summary, the distributions of the tonically active neurons excited by the central and/or peripheral chemoreceptors were not restricted to the ventral surface of the medulla.