Synchronization of discharge, spontaneous and evoked, between inspiratory neurons

(i) In midcollicular decerebrate, unanaesthetized, vagotomized, paralyzed cats, efferent phrenic discharge was recorded simultaneously with inspiratory unit and inspiratory wave activity in the rostral lateral medulla. Phrenic motoneurons tend to fire in synchronous bursts, as indicated by the occurrence of high frequency oscillation (60-110 per second; median oscillation period 13 ms) in the wholenerve recording. (ii) Crosscorrelation analysis of medullary inspiratory unit vs. phrenic activity showed that most units in the rostral medulla had oscillation of discharge which was locked to phrenic oscillation; peak unit activity usually occurred 3-5 ms before peak phrenic activity. Since this delay is comparable to the latency of a phrenic response evoked by an electric stimulus in the inspiratory regions, it is suggested that these medullary inspiratory neurons are part of the efferent inspiratory pathway from medulla to spinal cord. (iii) In the region near the nucleus ambiguus, wave activity occurring during the inspiratory phase was recorded; its phase relation to phrenic oscillation was similar to the unit-to-phrenic phase relation. It is suggested that the wave activity reflects synchronized synaptic potentials in neighbouring structures. (iv) By electrical stimulation of the rostral pontine pneumotaxic system (nucleus parabrachialis), it was possible to force the oscillation frequency as well as to change the phase relations of activity. (v) Thus, medullary inspiratory activity has a short-term synchrony. It is suggested that this synchrony arises from: (a) reexcitant connections between inspiratory neurons; (b) similar time courses of post-spike inexcitability in different inspiratory neurons. The phenomenon of high-frequency oscillation in inspiratory discharge furnishes an electrical sign of inspiratory activity, which should prove useful in analysis of the mechanisms producing inspiratory discharge.