Caudal expiratory neurones in the rat

The main source of expiratory drive to respiratory muscles in the rat is thought to be the caudal expiratory neurones. However, their projections to the spinal cord and the coordination of the population activity with the respiratory cycle are largely unknown. We examined their bulbospinal projections using antidromic activation, and the coordination of their activity using cross-correlation. Of 76 expiratory neurones examined, 40% projected to the C2 segment of the spinal cord, all unilaterally and all but 4 to the contralateral side. Thirteen contralateral axons were located in the ventromedial funiculus, when activation thresholds were less than 25 microA. For 29 neurones, the mean (+/- SEM) conduction velocity, calculated from the single-point activation and a measure of the direct distance between recording and stimulating electrodes, was 7.4 +/- 0.4 m/s. We calculated 88 cross-correlograms from ipsilateral pairs of expiratory neurones recorded on the same side of the medulla, and 176 contralateral pairs recorded from opposite sides of the medulla. All of the latter were featureless, but 23% of cross-correlograms for ipsilateral pairs displayed broad peaks at time zero, which we interpreted as due to activation of both neurones from a common source. We conclude that in the adult rat, approximately half of the caudal expiratory neurones project unilaterally and contralaterally to the spinal cord and, although common activation serves to coordinate some of the ipsilateral population, we suggest that neither common activation nor excitatory cross-connections exist as sufficient means for coordinating left and right populations to the same respiratory rhythm.