THE DISTRIBUTION OF MONO-SYNAPTIC CONNECTIONS FROM INSPIRATORY BULBOSPINAL NEURONS TO INSPIRATORY MOTONEURONES IN THE CAT

Monosynaptic connexions were sought between bulbospinal inspiratory neurones and inspiratory motoneurones of the C5 phrenic nerve or the intercostal nerves of up to six segments in the anesthetized cat; they were identified by the presence of narrow peaks in cross-correlation histograms constructed from the spontaneous discharges of these neurons. About half of the bulbospinal neurons tested with the phrenic gave a monosynaptic connexion, as did just under half of those tested with the intercostal nerves. Some neurons gave connexions to both. For pairs of bulbospinal neurons and intercostal nerves, connexions were seen in 50 out of 295 instances (17%). No evidence was found of systematic patterns of connexions to different segments. Some neurons gave connexions to adjacent segments, others up to five segments apart. Bulbospinal neurons were not equipotent at giving connexions to the intercostal motoneurons. If a neuron gave a connexion to one segment, it was more likely to give connexion to another segment than chance would predict. The neurons with strong connexions were concentrated rostral to obex in the ventral respiratory group; neurons in both the caudal part of the ventral group and in the dorsal group gave weaker connexions. Regional variation within the medulla in the proportions of neurons giving connexions to the phrenic motoneurons was not seen, in particular neurons of the dorsal group (eight out of sixteen) and the ventral group (five out of eleven) were equipotent. When the strength of the connexions was assessed by k, the ratio of the peak count in the cross-correlation histogram to the base-line count, the mean strength was similar for the peaks seen for the phrenic (k = 1.113) and the intercostal (k = 1.114) motoneurons. However, because, on a segment-by-segment basis, fewer connexions were seen for the intercostals, the over-all mean value of k (absence of a narrow peak taken as k = 1.0) was stronger for the phrenic (k = 1.055) than for the intercostals (k = 1.019). For the phrenic, the apparent strength of connexion (assessed by k) was considerably stronger for the motoneurons giving large spikes than for those giving small ones. The same result was probably also true for the intercostals. With several assumptions, the total depolarization during inspiration derived monosynaptically in phrenic or intercostal motoneurones was calculated and found to be low, about 1 mV for the intercostals and 3-4 mV for the phrenic. The functional implications of this, i.e., that most of the synaptic depolarization of the motoneurons is likely to be derived from interneurons, is discussed.