Anatomy And Physiology
Susan M. Congilosi, MD
Clinical Instructor in Surgery
University of Minnesota
Division of Colon and Rectal Surgery
Anatomy and Physiology
The patient with a complaint related to the pelvic floor (e.g. incontinence, constipation, prolapse, or obstructed defecation) can be studied with a variety of tests to help determine the etiology of the disorder and direct therapy. Anorectal physiology testing, endoanal ultrasound, and defecography are used to evaluate patients prior to surgery (e.g. anal sphincter defects, rectal prolapse), help delineate difficult anatomy (e.g. recurrent anal fistulas, rectovaginal fistulas), and to determine if a non-surgical approach or biofeedback is appropriate (nonrelaxing puborectalis, incontinence). An understanding of the anatomy and physiology of the pelvic floor is necessary to both choose the appropriate tests and interpret the results.
Pelvic Floor Muscles
The pelvic floor consists of a striated muscular sheet through which viscera pass. This striated muscle, the paired levator ani muscles, is actually subdivided into four muscles defined by the area of attachment on the pubic bone. The attachments span from the pubic bone, along the arcus tendineus (a condensation of the obturator fascia), to the ischial spine. The components of the levator ani are therefore named the pubococcygeus, ileococcygeus, and ischiococcygeus. The pubococcygeus is further subdivided to include the puborectalis. Between the urogenital viscera and the anal canal lies the perineal body. The perineal body consists of the superficial and deep transverse perinei muscles and the ventral extension of the external sphincter muscle to a tendinous intersection with the bulbocavernosus muscle. The postanal plate lies between the anus and the veterbral column and consists of the presacral fascia, the anococcygeal ligament, anococcygeal raphe (midline condensation of the ileococcygeus), and the dorsal extension of the puborectalis and external anal sphincter fibers to the coccyx.
Pelvic Floor Innervation
The fourth sacral nerve innervates the levator ani muscles. Controversy continues regarding the innervation and origin of the puborectalis muscle. Cadaver studies differ from in vivo stimulation studies as to whether the puborectalis muscle receives innervation only from the sacral nerve or also from the pudendal nerve. Comparative anatomy and histological studies of fiber typing also support the inclusion of the puborectalis muscle with the sphincter complex and not as a pelvic floor muscle. In addition, EMG studies of the external anal sphincter and puborectalis muscle indicate that the muscles function together with cough and strain.
Rectum and Anus
The anal canal is approximately 4 cm long and extends from the anal verge to the top of the external anal sphincter complex or ring. This is a clinical and surgical description that corresponds to the digital exam and findings seen on ultrasound but not to the histological changes along the canal. It differs from a purely anatomical description of the anal canal as extending from the anal verge to the dentate line or a histological one which describes the anal canal as ending at approximately 10 mm above the anal valves where it is lined by rectal type mucosa. The dentate line lies at or just distal to the anal valves. It is a mucosal boundary and not the proximal boundary of the transitional zone. The anal transitional zone, as defined by Fenger, is the "zone interposed between uninterrupted colorectal type mucosa above and uninterrupted squamous epithelium below."
Rectal Muscles
The rectal smooth muscle consists of the muscularis mucosa, and inner circular layer and an outer longitudinal layer. The inner circular muscle forms the valves of Houston and in the proximal 30 mm of the 40-mm anal canal it becomes the internal anal sphincter. Ultrasound examination of the anal canal clearly shows that the internal anal sphincter ends approximately 10 mm proximal to the most distal portion of the external anal sphincter. The outer longitudinal layer surrounds the sigmoid colon but is thicker the areas known as the taenia coli. This same layer continues down to the anorectal junction where it forms the conjoined longitudinal muscle along with fibers from the pubococcygeus muscle. Distally, this muscle lies in the intersphincteric plane and fibers may fan out and cross both the internal and external anal sphincter muscles.
Internal Anal Sphincter
The internal anal sphincter is an involuntary, smooth muscle. It receives sympathetic innervation via the hypogastric and pelvic plexus. Parasympathetic innervation is from S1, S2, and S3 via the pelvic plexus. There is considerable evidence that the sympathetic innervation is excitatory but conflicting information regarding the parasympathetic effect. There is an unclear contribution to the innervation from nonadrenergic, noncholinergic pelvic nerves. Spinal anesthesia decreases rectal tone by 50% and the decreased resting tone seen in diabetic patients may be due to an autonomic neuropathy. The internal anal sphincter contributes 55% to the anal resting pressure. The myogenic activity contributes 10% and 45% is due to the sympathetic innervation. The remainder of the resting tone is from the hemorrhoidal plexus (15%) and the external anal sphincter (30%). The internal anal sphincter has slow waves occurring 6-20 times each minute. Ultraslow waves occur less than 3 times a minute and are not present in all individuals. Ultraslow waves are associated with higher resting pressures, hemorrhoids and anal fissures.
External anal sphincter
The external anal sphincter was classically described as having three components (subcutaneous, superficial, and deep), but is now regarded as one continuous muscle. The proximal aspect is in intimate relationship with the puborectalis muscle. The pudendal nerve, specifically the inferior rectal branch, innervates the external anal sphincter.
Rectal and anal sensory innervation
Anal canal sensation to touch, pin-prick, heat, and cold are present from the anal verge to 2.5 - 15 mm above the anal valves. This sensitive area is thought to help discriminate between flatus and stool but local anesthesia does not obliterate that ability. The rectum is only sensitive to distention. Rectal sensation may be due to receptors in the rectal wall but also in the pelvic fascia or surrounding muscle. The sensory pathway for rectal distention is the parasympathetic system via the pelvic plexus to S2, S3 and S4. Below 15 cm rectal distention is perceived as flatus but above 15 cm air distention causes a sensation of abdominal discomfort. Anal canal sensation is via the inferior rectal branch of the pudendal nerve that arises from S2, S3 and S4. This is the first branch of the pudendal nerve and along with the second branch, the perineal nerve, arises from the pudendal nerve in the pudendal canal (Alcocks canal). The remainder of the pudendal nerve continues as the dorsal nerve of the penis or clitoris.
Vascular supply of the rectum and anus
The superior and inferior rectal arteries supply the rectum and anal canal. The superior rectal artery is a continuation of the inferior mesenteric artery as it passes over the left common iliac artery. The inferior rectal artery is a branch of the pudendal artery, which arises from the internal iliac artery. The middle rectal artery is a branch of the anterior division of the internal iliac artery Controversy is associated with the contribution of the middle rectal artery. Conflicting cadaver studies suggest that it is present 5 - 70% unilaterally or bilaterally. The size and presence of the middle rectal artery may be in inverse relationship to the superior rectal artery. As a practical matter, a particularly small superior rectal artery may be an indication during a low anterior resection that the middle rectal arteries are substantial. Less controversial is the paucity of vascular supply to the anterior and particularly posterior aspects of the rectum from the superior rectal artery. The extent of crossover of vascular supply between the inferior and superior rectal arteries is also debated however it is accepted that the rectum and anus can survive ligation of the superior and middle rectal arteries. Venous drainage follows the main arterial supply.
Reflexes
Rectoanal inhibitory reflex and Inflation reflex
Rectal distention and electrical stimuli induce relaxation of the internal anal sphincter with a resultant lowering of the resting pressure. The internal anal sphincter recovers within one minute from small volumes of air or stool. . A reflex contraction of the external anal sphincter (inflation reflex or rectoanal excitatory reflex) is seen at the time of the internal anal sphincter relaxation except during sleep. This inflation reflex is lost at high rectal volumes (400 ml). The rectoanal inhibitory reflex is absent in Hirschsprung's disease, Chagas disease, and after rectal mucosal excision. It may return after initially being abolished by a low rectal anastomosis. The afferent receptors are therefore not only in the anorectal epithelium and rectal wall, but probably also in the pelvic floor. The reflex is intramural involving myenteric plexus neurons that are noncholinergic and nonadrenergic.
Sampling Reflex
The sampling reflex is incompletely understood but a vital component of continence. Stool or flatus entering into the rectum causes a reflex relaxation of the internal anal sphincter. The length of the functional high-pressure zone of the anal canal decreases and this allows contact of the rectal contents with the highly sensitive anal canal. A deficit in this sensory function can be a contributing factor in patients with fecal incontinence in the absence of motor deficits. Biofeedback can improve rectal balloon distention thresholds.
Anal Cough Reflex
The external anal sphincter contracts with coughing, sneezing, and standing. The squeeze pressure with cough is higher than when the patient voluntarily contracts the external anal sphincter muscle.
Cutaneoanal reflex
The cutaneoanal reflex or "anal wink" has its afferent and efferent pathways in the pudendal nerve and is abolished if S4 is transected. The latency can vary widely with short, intermediate and long latencies. Short latencies are thought to be due to direct stimulation of nerve fibers near the sphincter. Intermediate latencies are probably due to stimulation to a branch point that then travels back to the muscle failing to complete the complete reflex arc at the spinal level. This variability in latency limits usefulness of measurement of this reflex.
Defecation
The awareness of the need to defecate or urinate occurs in the superior frontal gyrus and anterior cingulate gyrus. Rectal distention stimulates internal anal sphincter relaxation and the sampling reflex. If defecation is to be deferred voluntary contraction of the external anal sphincter and levator ani muscles occurs. Accommodation refers to the relaxation of the rectal ampulla after an initial increase in pressure. At the appropriate time for defecation or when rectal pressure is high, the levator ani muscle, puborectalis muscle and external anal sphincter relax. Pelvic floor relaxation, along with a squatting position, straightens the anorectal angle. An increase in abdominal pressure along with colonic and rectal contractions allows expulsion of a fecal bolus. Increased effort is necessary to expel a smaller bolus than an optimal 2-cm fecal bolus.
Continence
The interplay of all the aforementioned anatomy and physiology ensures continence. It does not follow that a deficit in any one area ensures incontinence. Continence achieved in the ileoanal pouch patient is proof the rectum is not essential. An intact and functional puborectalis muscle can provide continence in the pediatric imperforate anus patient, but incontinence can ensue during adulthood. Even profound deficits do not necessarily lead to incontinence if stool consistency is solid, while minor deficits can easily lead to incontinence to gas and a percentage of normal adults have gas incontinence. This complex problem requires a systematic approach focusing on identifying the specific deficits present and then directing therapy accordingly.
References
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Henry MM, Swash M (ed): Coloproctology and the Pelvic Floor. Oxford: Butterworth-Heinemann Ltd, 1992: 3 - 249.
Netter F: Interactive Atlas of Clinical Anatomy CD-ROM. Novartis, DxR Development Group, Inc. 1997
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