Location of the reflex centre for straining elicited by activation of pelvic afferent fibres of decerebrate dogs

Abstract

The reflex centres for straining for defaecation, micturition and presumably for parturition were located electrophysiologically in decerebrate dogs. Stimulation of pelvic afferent fibres initially induced a sustained increase in nervous outflow to the diaphragm, rectus abdominis and lateral cricoarytenoid muscles and subsequently induced rhythmic increases which were superimposed on the sustained increase. The rhythmic increases occurred even after transection at the most rostral pons, but they were abolished by a partial cut at the most lateral part of the rostral pons following transection of the contralateral half of the rostral medulla oblongata. The sustained increase continued after transection 1.5 mm caudal to the obex, but disappeared after transection about 5 mm caudal to the obex. This result shows that straining is brought about by both sustained and rhythmic straining reflexes. Both sustained and rhythmic straining, but not defaecation and micturition, could be elicited by stimulation of an area of the Ko¨lliker-Fuse nucleus. The discharges of about half of the units in the nucleus and the neighbouring rostrolateral pontine area which responded to stimulation of the pelvic afferent fibres changed synchronously with the rhythmic straining. These results show that the rhythmic and sustained straining reflex centres are located in the Ko¨lliker-Fuse nucleus and in the lower medulla oblongata, respectively.