Different mechanisms involved in supraspinal and spinal reflex regulation of phrenic activity through chest movements

The coordination of breathing activity with chest movements was compared in the same decorticate rabbit preparations prior to and after a transection at the C2 spinal level. Pharmacological activation was induced with a combination of nialamide and DOPA in the latter situation. The preparation was curarized and chest inflations and deflations were induced by a respirator whose parameters could be modified. In decorticate preparations, phrenic activity was coordinated 1:1 with the respirator period over a large range of imposed periods. Beyond the extreme values a new coupling was achieved with a ratio of either 1:2 or 2:1. Throughout the range of 1:1 coordination, phrenic bursting always happened at a preferred time in the respirator period, although this time differed for the various imposed periods. This coordinated activity required vagal inputs. After spinal transection the phrenic nerves were totally silent; DOPA administration allowed rhythmic activity to develop. In some preparations, phrenic bursts were coordinated 1:1 with the respirator period and remained so for all the imposed periods: the phase of these phrenic discharges relative to the respirator cycle was kept unchanged for the different periods. In addition, there was a modulation of amplitude superimposed on this 1:1 coupling. These spinal phrenic bursts were generally suppressed when the respirator was turned off. From these results, the coordination of phrenic activity with the respirator rate appears to be produced by different mechanisms in the decorticate and in the spinal preparations. In the decorticate animal the periodic vagal inflow reset the activity of the medullary inspiratory generator and entrains it at its own rate. The coordination observed in the spinal preparation results from a periodic peripheral activation of premotoneuronal or motoneural phrenic elements during inflation. If the central bursts provided by the spinal "respiration" generator can fire phrenic motoneurons above threshold, their timing is not dependent on the peripheral inflow; when the motoneurons are fired below threshold by these central inputs, they are probably summing together the central and peripheral excitations, which could account for the amplitude modulation of the coordinated phrenic bursts of pure reflex origin. Possible afferent pathways are discussed.