BACKGROUND: The anorectal physiology is still highly controversial.

OBJECTIVEE: The aim of this study was to assess the interaction of the pelvic floor muscles and to interpret the mechanism of anal continence and defecation.

DESIGN AND SETTING: Forty normal volunteers (20 men, aged between 19-35 years) were examined in the supine position by dynamic magnetic resonance imaging defecography with 200 mL of ultrasonographic gel in the rectum during rest, squeeze and true defecation; and with an empty rectum during simulated defecation.

MAIN OUTCOME MEASURES: From each subject, anal coronal static images and dynamic videos were obtained. It shows the displacement of abdominal wall, intra-abdominal organs, pelvic floor muscles, anus and perineal soft-tissues.

RESULTS: At rest, both the puborectalis and the external anal sphincter squeezed the levator ani muscle and the anal canal. During squeeze, the puborectalis elevated the levator ani muscle and the external anal sphincter closed the anal canal more tightly. During true defecation, the moved abdominal wall pushed the intra-abdominal organs and the pelvic floor downwards; the descended levator ani muscle overstretched the puborectalis and the external sphincters; the descended intra-abdominal organs forced the rectal contents (the gel) into the anal canal; and the anal contents (the gel) opened the anal canal. During simulated defecation, the anus descended in 74% subjects and ascended in 26% subjects, and there was no anal content to open the anus.

LIMITATIONS: Only the coronal morphological information was obtained in this study. 

CONCLUSIONS: The puborectalis and the external anal sphincter have the function to elevate and squeeze the levator ani muscle keeping the anal canal (internal anal sphincter) under control; the abdominal muscle and the diaphragm have the opposite function. The up-and-down reciprocating motion of the levator ani muscle is actually passive.

KEYWORDS: Abdominal muscle; Levator ani muscle; External anal sphincter; Internal anal sphincter; Magnetic resonance imaging defecography.

Anal anatomy and anorectal physiology is still controversial,^1,2 and our former imaging studies^3~5 have yielded lots of new insights.

Textbooks of anatomy, endoanal ultrasound⁶ and endoanal coil MRI studies⁷ have described the anal region as possessing 5 anatomical layers. However, our former body coil studies⁵ have shown 7 MRI layers including the mucosa, submucosa, anal smooth muscle, inner space, vertical levator, outer space and the external anal sphincter (EAS).

Some anatomists regarded that the longitudinal rectal muscle (LRM) and the levator ani muscle merges to form the conjoined longitudinal anal muscle, and the puborectalis is a part of the levator ani muscle or a part of the EAS muscle.^8,9 Conversely, our MRI studies⁵ demonstrated that the fat spaces keep them apart and they are separate muscles.

Histological technology¹⁰ can easily distinguish the internal anal sphincter (IAS) from the LRM muscle. However, modern MRI technology⁵ is unable to separate the IAS or LRM muscle from the anal smooth muscle by a visible fat space in vivo.   

Shafik¹¹ suggested that the levator ani muscle lifts the anus during defecation. Instead, our seated CT defecography⁴ has shown that the puborectalis actually lifts the anus during squeeze. So the puborectalis is worthy of the name “levator ani muscle”.

Furthermore, the new imaging anatomical signs have suggested that the imaging anal canal (the direct extension of rectum) and the levator ani muscle are controlled by the puborectalis and the EAS muscle bundles;⁵ and the new imaging physiological signs have suggested that the puborectalis gives a centripetal force to the levator ani muscle during squeeze.⁴