Transmission of respiratory rhythm: Midline-crossing connections at the level of the phrenic motor nucleus?

We used three methods to test for the existence of transmission of respiratory rhythm across the midline at the level of the phrenic motor nucleus in rats using the in situ preparation over a range of ages from neonatal to juvenile. Stimulus-triggered averages of phrenic activity for stimuli applied to one side of the spinal cord at C2 and C3 produced large peaks in the ipsilateral averages but no discernible peaks in the contralateral averages, unless the stimulating microelectrode was placed close to the midline in the ventral funiculus. Following mid-sagittal section of the medulla, respiratory rhythm was maintained for all ages, with bursts occurring on one phrenic nerve that were absent on the other. Cross-correlations of left and right phrenic discharges displayed peaks indicative of short time-scale synchronisation before the medullary transections but not afterwards. We therefore could not find evidence for transmission of respiratory rhythm across the midline at the phrenic motoneurone level; we did find evidence that that transmission via ipsilaterally descending axons of medullary phrenic pre-motor neurones is present at all ages.