Comparison of activities of medullary respiratory neurons in eupnea and apneusis

We evaluated patterns of antidromic latencies of medullary respiratory neurons in eupnea and apneusis to define how afferent influences from the pneumotaxic center regulate their activities. Apneusis was reversibly produced by cold block in decerebrate, vagotomized, paralyzed and ventilated cats. Most neurons, which discharged during all of eupneic inspiration or expiration, maintained the same pattern in apneusis. However, those active during only portions of these phases or spanning both changed markedly with alterations in periods of discharge, including tonic patterns or cessations of activity. Such marked changes were observed for activities of all laryngeal expiratory neurons. Upon termination of eupneic discharge, most bulbospinal and laryngeal neurons had transient peaks of latencies, indicating hyperpolarizations; declines from these peak values were greatly reduced in apneusis. Moreover, reflecting depolarizations, latencies of some inspiratory and expiratory neurons declined during eupneic expiration and inspiration, respectively; these declines were much reduced in apneusis. We conclude that the pneumotaxic center influences medullary respiratory neuronal activities not only at end-inspiration, but throughout the entire respiratory cycle.