Anatomy and Physiology of Continence 1 Adil E. Bharucha, Roberta E. Blandon Introduction ration between the rectum and the urogenital tract. The upper portion of the anal canal is derived from Webster’s dictionary defines continence as “the abil- endoderm and is supplied by the inferior mesenteric ity to retain a bodily discharge voluntarily”. The artery, which supplies the hindgut. The lower third of word has its origins from the Latin continere or the anal canal has ectodermal origins and is supplied tenere, which means “to hold”. The anorectum is the by the rectal arteries, which are branches of the inter- caudal end of the gastrointestinal tract, and is nal pudendal artery [2]. responsible for fecal continence and defecation. In humans, defecation is a viscero somatic reflex that is often preceded by several attempts to preserve conti- Anatomy nence. Any attempt at managing anorectal disorders requires a clear understanding of the anatomy and Pelvic Floor the integrated physiologic mechanisms responsible for maintaining continence. The pelvic floor is a dome-shaped muscular sheet [4] that predominantly contains striated muscle and has midline defects enclosing the bladder, the uterus, and Embryology the rectum. These defects are closed by connective tissue anterior to the urethra, anterior to the rectum The primitive gut is formed during the third week of (i.e., the perineal body), and posterior to the rectum gestation. The anorectal region in humans derives (i.e., the postanal plate). Together with the viscera (i.e., from four separate embryological structures: the the bladder and anorectum), the pelvic floor is respon- hindgut, the cloaca, the proctodeum, and the anal sible for storing and evacuating urine and stool. tubercles [1]. The hindgut forms the distal third of The levator ani and the coccygeus muscle com- the transverse colon, the descending colon, the sig- prise the two muscular components of the pelvic moid, the rectum, and the upper part of the anal floor or pelvic diaphragm. The muscles that consti- canal to the level of the anal valves [2]. The end of the tute the levator ani complex are the puborectalis, the hindgut enters into the cloaca, an endoderm-lined pubococcygeus, and the ileococcygeus. These mus- cavity that is in direct contact with the surface ecto- cles originate at different levels of the pubic bone, the derm. The cloaca is initially a single tube that is sub- arcus tendineus fascia pelvis (condensation of the sequently separated by caudad migration of the obturator internus muscle fascia), and the ischial urorectal septum into anterior urogenital and poste- spine. These muscles are inserted at the level of the rior intestinal passages. During the 10th week of rectum, the anococcygeal raphe (levator plate), and development, the external anal sphincter is formed the coccyx (Fig. 1). from the posterior cloaca as the descent of the uro- It is unclear whether the puborectalis should be genital septum becomes complete. By the 12th week, regarded as a component of the levator ani complex the internal anal sphincter is formed from a thick- or the external anal sphincter. Based on developmen- ened extension of rectal circular muscle [3]. The tal evidence, innervation, and histological studies, proctodeal portion of the cloacal membrane disinte- the puborectalis appears distinct from the majority grates to form the anal tubercles that join posteriorly of the levator ani [1]. On the other hand, the pub- and migrate ventrally to encircle a depression, orectalis and external sphincter complex are inner- known as the anal dimple or proctodeum. The anal vated by separate nerves originating from S2–4 (see tubercles join the urorectal septum and genital tuber- below), suggesting phylogenetic differences between cles to form the perineal body, completing the sepa- these two muscles [5]. 4 A.E. Bharucha, R.E. Blandon Fig. 1. Pelvic view of the levator ani de- monstrating its four main components: puborectalis, pubococcygeus, iliococ- cygeus, and coccygeus. Reprinted with permission from [6] rectums are separated by a horizontal fold. The Rectum and Anal Canal upper rectum is derived from the embryological hind gut, generally contains feces, and can distend toward The rectum is 15- to 20-cm long and extends from the the peritoneal cavity [7]. The lower part, derived recto sigmoid junction at the level of third sacral ver- from the cloaca, is surrounded by condensed extra tebra to the anal orifice (Fig. 2). The upper and lower peritoneal connective tissue and is generally empty Fig. 2. Diagram of a coronal section of the rectum, anal canal, and adjacent structures. The pelvic barrier includes the anal sphincters and the pelvic floor muscles. Reprinted with permission from [8] Chapter 1 Anatomy and Physiology of Continence 5 in normal subjects, except during defecation. In ing from ventral rami of the second, the third, and humans, there are fewer enteric ganglia in the rectum often the fourth sacral nerves to form the inferior compared with the colon and very few ganglia in the hypogastric plexus, which is located posterior to the anal sphincter [9, 10]. urinary bladder. The inferior hypogastric plexus The anal canal is an anteroposterior slit, with its gives rise to the middle rectal plexus, the vesical lateral walls in close contact. The literature describes plexus, the prostatic plexus, and the uterovaginal a longer (approximately 4.0–4.5 cm) “surgical” or “cli- plexus. The nerve supply to the rectum and anal nical” anal canal and a shorter (approximately 2.0 cm) canal is derived from the superior, middle, and infe- “anatomical” or “embryological” anal canal. The anal rior rectal plexus. Parasympathetic fibers in the supe- valves and the distal end of the ampullary part of the rior and middle rectal plexuses synapse with post- rectum mark the proximal margin of the “short” and ganglionic neurons in the myenteric plexus in the “long” anal canal, respectively. The proximal 10 mm rectal wall. In addition, ascending fibers from the of the anal canal is lined by columnar, rectal-type inferior hypogastric plexus travel via superior mucosa. The next 15 mm (which includes the valves) hypogastric and aortic plexuses to reach the inferior is lined by stratified, or a modified columnar, epithe- mesenteric plexus, ultimately innervating the lium. Distal to that is about 10 mm of thick, non descending and sigmoid colon. After entering the hairy, stratified epithelium (i.e., the pecten). The colon, these fibers form the ascending colonic nerves, most distal 5–10 mm is lined by hairy skin. traveling cephalad in the plane of the myenteric The anal canal is surrounded by the internal and plexus to supply a variable portion of the left colon. external anal sphincters. The internal sphincter is a Sacral parasympathetic pathways to the colon thickened extension of the circular smooth muscle have excitatory and inhibitory components [14]. layer surrounding the colon that contains discrete Excitatory pathways play an important role in muscle bundles separated by large septa [11]. In the colonic propulsive activity, especially during defeca- rectum, the interstitial cells of Cajal (ICC) are organ- tion. In other species (e.g., guinea pig), feces trans- ized in dense networks along the submucosal and port may be entirely organized by the enteric nervous myenteric borders. In the internal anal sphincter, the system; spinal and supraspinal reflexes are also ICCs are located along the periphery of the muscle involved in the process [15]. Inhibitory pathways bundles within the circular layer. allow colonic volume to adapt to its contents, and The external sphincter is composed of superficial, they also mediate descending inhibition that initiates subcutaneous, and deep portions; the deep portion colonic relaxation ahead of a fecal bolus. blends with the puborectalis [7]. In men, this trilam- inar pattern is preserved around the sphincter cir- cumference. In contrast, the anterior portion of the Somatic Motor Innervation external sphincter in women is a single muscle bun- dle. External sphincter fibers are circumferentially Cortical mapping with transcranial magnetic stimu- oriented, very small, and separated by profuse con- lation suggests that rectal and anal responses are nective tissue [12]. bilaterally represented on the superior motor cortex, i.e., Brodmann area 4 [16]. There are subtle differ- ences in the degree of bilateral hemispheric repre- Nerve Supply to the Pelvic Floor sentation between subjects. Motor neurons in Onuf’s nucleus, which is located in the sacral spinal cord, Autonomic Innervation innervate the external anal and urethral sphincters. Though they supply striated muscles under volun- The anorectum and pelvic floor are supplied by sym- tary control, these motor neurons are smaller than pathetic, parasympathetic, and somatic fibers [13]. usual α-motor neurons and resemble autonomic Sympathetic pre ganglionic fibers originate from the motor neurons [17]; however, the conduction veloci- lowest thoracic ganglion in the paravertebral sympa- ty in pudendal nerve fibers is comparable with that of thetic chain and join branches from the aortic plexus peripheral nerves. In contrast to other somatic motor to form the superior hypogastric plexus. Because the neurons in the spinal cord, these neurons are rela- superior hypogastric plexus is not a single nerve, the tively spared in amyotrophic lateral sclerosis but are alternative term for this plexus, i.e., “presacral affected in Shy-Dräger syndrome [18, 19]. Somatic