Repeated sequences of interspike intervals in baroresponsive respiratory related neuronal assemblies of the cat brain stem

Many neurons exhibit spontaneous activity in the absence of any specific ex perimental perturbation. Patterns of distributed synchrony embedded in such activity have been detected in the brain stem, suggesting that it represen ts more than "baseline" firing rates subject only to being regulated up or down. This work tested the hypothesis that nonrandom sequences of impulses recur in baroresponsive respiratory-related brain stem neurons that are ele ments of correlational neuronal assemblies. In 15 Dial-urethan anesthetized vagotomized adult cats, neuronal impulses were monitored with microelectro de arrays in the ventral respiratory group, nucleus tractus solitarius, and medullary raphe nuclei. Efferent phrenic nerve activity was recorded. Spik e trains were analyzed with cycle-triggered histograms and tested for respi ratory-modulated firing rates. Baroreceptors were stimulated by unilateral pressure changes in the carotid sinus or occlusion of the descending aorta; changes in firing rates were assessed with peristimulus time and cumulativ e sum histograms. Cross-correlation analysis was used to test for nonrandom temporal relationships between spike trains. Favored patterns of interspik e interval sequences were detected in 31 of 58 single spike trains; 18 of t he neurons with significant sequences also had short-time scale correlation s with other simultaneously recorded cells. The number of distributed patte rns exceeded that expected under the null hypothesis in 12 of 14 data sets composed of 4-11 simultaneously recorded spike trains. The data support the hypothesis that baroresponsive brain stem neurons operate in transiently c onfigured coordinated assemblies and suggest that single neuron patterns ma y be fragments of distributed impulse sequences. The results further encour age the search for coding functions of spike patterns in the respiratory ne twork.