A new concept of the anatomy of the anal sphincter mechanism and the physiology of defecation: mass contraction of the pelvic floor muscles.

We have previously demonstrated both anatomically and physiologically that the external anal (EAS) and urethral (EUS) sphincters and the bulbocavernosus muscle (BC) originate from the puborectalis muscle (PR). It is hypothesized that stimulation of any of these muscles would lead to contraction of all the others. Because the levator ani (pubococcygeus) muscle (LA) also has the same innervation as the above-mentioned muscles, it is further suggested that it, too, contracts reflexly upon stimulation of any of those muscles. The purpose of this study was to test this hypothesis. The study comprised 18 healthy volunteers (mean age 36.6 +/- 8.4 years; 10 men, 8 women). The EAS was stimulated and the response of the EUS, PR, LA and BC was determined. Each muscle was thereafter stimulated separately and the response of the other pelvic floor muscles registered. Stimulation of any of the pelvic floor muscles effected an increased EMG activity of the rest of the muscles. The muscle contraction was instantaneous with no latency in all the muscles except the LA EMG activity, which showed a mean latency of 21.3 +/- 6.6 ms. The pelvic floor muscles' response seems to be attributable to muscle stimulation both directly and indirectly through activation of pudendal nerve fibers in the muscles. The study demonstrated that the pelvic floor muscles behave as one muscle: they contract or relax en masse. This 'mass contraction' might explain some of the physiologic phenomena that occur during pelvic organ evacuation. However, besides this mass contraction, a voluntary 'selective' individual muscle activity exists by which each individual muscle acts independently of the others.