Discharge properties of dorsal medullary inspiratory neurons: relation to pulmonary afferent and phrenic efferent discharge

Abstract

In decerebrate paralyzed cats ventilated with a cycle-triggered pump system that delivered inflation during the central inspiratory (I) phase (indicated by efferent phrenic discharge), activity of 90 neurons with inspiratory-modulated discharge was recorded in the region of the ventrolateral nucleus of tractus solitarius (vlNTS) in the medulla. Of these, 82 had centrally originating I patterns (69 augmenting, 13 decrementing), and 8 were "pump" neurons. The neurons were classified into functional types by the application of inflation tests, the main test being withholding of the inflation usually delivered during the central I phase. This maneuver eliminated phasic pulmonary afferent input and produced lengthening of the I phase. The neurons' responses were heterogeneous, and were classified as: excitation by inflation (inflation(+], no significant effect of inflation (inflation(0], and depression by inflation (inflation(-]. Application of inflation during the expiratory (E) phase only rarely produced excitation of central I neurons. The I neurons excited by inflation correspond to the "I-beta" neurons of earlier studies. On the basis of the multimodal distribution of their discharge onset times relative to phrenic onset, they were classified as early onset or late onset. In contrast, the inflation(0) and inflation(-) neurons, which correspond to the "I-alpha" neurons, almost exclusively had early onsets. The pump neurons could be distinguished from the inflation(+) I neurons by the effects of inflations delivered during E: the former were equally excited by inflations during E and during I, while the latter were not excited or were minimally excited by inflation during E. Thus, the pump neurons had lung afferent inputs but no inputs from the central I pattern generator, while the inflation(+) I neurons had both kinds of input. The neurons having decrementing discharge during I (early inspiratory and expiratory-inspiratory) were depressed by inflation. Cross-correlation histograms (CCH) of unit firing versus whole phrenic (contralateral and ipsilateral) activity on a short time scale (milliseconds) were computed; the majority of these had high-frequency oscillations, with mean period of 14.6 +/- 3.6 (SD) ms. The CCHs for the different groups of neurons were analyzed with respect to a) strength of correlation as a function of laterality and b) lag of the main correlation peak. Both the inflation(0) and inflation(+) neuronal groups had correlation properties suggesting that these neurons monosynaptically excite phrenic motoneurons.(ABSTRACT TRUNCATED AT 400 WORDS)