Short time scale correlations between discharges of medullary respiratory neurons.

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Abstract
The interactions on a short time scale among inspiratory (I) neurons and among expiratory (E) neurons of the cat medulla were studied by analyzing simultaneous recordings of spike activity from 2 or more neurons. Microelectrodes were used to record activity in 3 regions: the ventrolateral nucleus of the solitary tract (vINTS), containing I neurons almost exclusively; the rostral portion of the ventral respiratory group (rVRG), in the ventrolateral medulla rostral to the obex, containing mostly I neurons; and the caudal portion of the VRG (cVRG), containing mainly E neurons. Cross-correlation histograms (CCH) between activities of 2 I neurons or 2 E neurons were used to ascertain the existence of correlated discharges on a short time scale (about 1 ms), as indicated by a sharp peak near 0 lag in the CCH. Adjacent neuron pairs (recorded with the same electrode) have a high incidence of such correlated discharges: they were present in 16 of 26 pairs of I neurons and 28 of 45 pairs of E neurons. Such correlations were extremely rare for pairs of contralateral distant neurons (recorded with 2 microelectrodes on opposite sides of the medulla): only 1 of 43 I neuron distant pairs had a clear-cut peak in the CCH and none of the 97 (0 of 97) E neuron distant pairs had such a peak. An additional 18 I neuron distant pairs had common high-frequency oscillations (HFO), with period 9-17 ms. Within local clusters of I or E neurons, a high incidence of local interactions and/or locally shared inputs apparently exists; only a small fraction of distant neuron pairs are oligosynaptically connected. Since respiratory neurons of a particular discharge type are ultimately connected, as indicated by synchronization of firing on a time scale of seconds or milliseconds (HFO), the coordination of activity between distant regions is probably produced by limited and specific connections between specialized subpopulations.