Ascending and descending pathways of reflex straining in the dog

The location of ascending and descending pathways of the straining reflex was studied by making bulbar cuts in dogs. Rhythmic increases, which were elicited simultaneously in nervous outflows to the diaphragm and rectus abdominis by activation of pelvic afferent fibers, were used to indicate rhythmic straining. The increases were not affected by cutting the dorsal funiculus at the bulbospinal junction, but stopped after cutting both lateral funiculi and by longitudinally cutting the roof of the central canal just caudal to the obex. The increases were impaired by unilateral section of the bulbospinal junction, but were abolished completely after successive ipsilateral hemisection of the rostral bulb. Moreover, rhythmic increases, which appeared synchronously with rhythmic straining in unit discharges recorded from the Kölliker-Fuse nucleus and neighboring structures, disappeared after the longitudinal bulbar cut. However, reflex discharges elicited in pelvic vesical and rectal branches by stimulation of corresponding pelvic afferent fibers were not reduced. These results show that the indispensable part of the ascending pathway of rhythmic straining passes through the lateral funiculus and decussates at the lower bulb, but the pathway of micturition and defecation reflexes do not cross at that level.