Pelvic floor structures and sacral innervation influence rectal compliance

Abstract

Structural anatomy and the surrounding pelvic floor musculature influence the compliance of the rectum. Additionally, afferent inputs and spinal reflexes from sacral spinal roots SI-S3 modulate rectal compliance. The individual contribution of these mechanisms has not been clearly established. Aims. The current study sought to evaluate the influence of pelvic floor muscles and sacral innervation, both afferent and efferent, on rectal compliance in the cat. Methods. Following surgical preparation, animals were decerebrated and ventilated. After a lumbo-sacral laminectomy, the sacral roots were exposed. All measures were obtained after stabilization of respiration and arterial pressure. Pressure-volume curves were determined during slow ramp distention (0 to 50 mmHg) in animals with intact pelvic floor (n= 10) and following isolation of the rectum from the pelvic floor musculature (n= 10). To assess the influence of spinal outflow and afferent inputs, rectal compliance was measured in the isolated rectum following transection of the dorsal and ventral sacral roots sequentially from SI to S3 and from dorsal to ventral (n=4). Results. Isolation of the rectum from the pelvic floor musculature resulted in a significant interaction effect (F(9,189)=6.12, p<O.OOI) due to a rightward shift of the pressure-volume curve in the lower range of distention (0-25 mmHg) without affecting compliance at higher pressure. Bilateral section of dorsal root SI further shifted the pressure-volume curve to the right (F(l,3)= 13.69, p=O.03). Subsequent sections to ventral root S3 failed to significantly influence rectal compliance. Section of ventral root S3 resulted in an additional rightward shift (F(l,3)=41.35, p=0.008) of the compliance curve. Conclusion. The rightward shift of the compliance curve in the lower distention range after isolation of the rectum from its pelvic floor anchors may have resulted from an alteration in adaptive relaxation of the longitudinal muscle. Section of sacral spinal afferents in the dorsal root S I and efferent inputs from ventral root S3 further shifted the compliance curve to the right. These observations suggest that afferent inputs and spinal reflexes may exert inhibitory influences on the rectum during distention, thereby initiating adaptive responses to rectal filling (Supported by DFG).