Motoneurons in Thoracic and Upper Lumbar Segments of the Spinal Cord

34 35 Cross-correlation of neural discharges was used to investigate the connections between 36 expiratory bulbospinal neurons (EBSNs) in the caudal medulla and expiratory motoneurons 37 innervating thoracic and abdominal muscles in anesthetized cats. Peaks were seen in the cross38 correlation histograms for around half of the EBSN-nerve pairs for the following: at T8, the nerve 39 branches innervating internal intercostal muscle and external abdominal oblique muscle and a more 40 distal branch of the internal intercostal nerve; at L1, a nerve branch innervating internal abdominal 41 oblique muscle and a more distal branch of the ventral ramus. Fewer peaks were seen for the L1 42 nerve innervating external abdominal oblique, but a paucity of presumed alpha motoneuron 43 discharges could explain the rarity of the peaks in this instance. Taking into account individual EBSN 44 conduction times to T8 and to L1, as well as peripheral conduction times, nearly all of the peaks were 45 interpreted as representing monosynaptic connections. Individual EBSNs showed connections at both 46 T8 and L1, but without any discernible pattern. The overall strength of the monosynaptic connection 47 from EBSNs at L1 was found to be very similar to that at T8, which was previously argued to be 48 substantial and responsible for the temporal patterns of expiratory motoneuron discharges. However, 49 we argue that other inputs are required to create the stereotyped spatial patterns of discharges in the 50 thoracic and abdominal musculature. 51 52 53