of the Normal Rectum and Sphincters
The second, third, and fourth sacral segments of the spinal cord are the nerve centers of the arcs that subserve the receptors and effectors of the rectum, anus, bladder, and urethra, and, together with higher centers in the brain, are responsible for continence. These centers in the spinal cord also subserve cutaneous sensation in the anal canal to the level of the valves and in the perianal region. The sympathetic supply, however, arises in the second, third, and fourth lumbar segments. Malformations of the spinal cord pertaining to the sacral segments involve all systems, but damage of nerves within the pelvis or perineum may have more localized effects.
7.5.1 Parasympathetic Nerves
The parasympathetic nerves to the bowel arise on either side of the pelvis from the anterior divisions of the third and fourth sacral nerves, with twigs sometimes from the second. These preganglionic nerve fi-
Fig. 7.3 Defecography in a healthy child in sagittal position. A Normal anorectal angle formed by the puborectalis sling and the deep part of the external anal sphincter; B internal sphincter relaxation starting with opening of the proximal one-third of the anal canal. The middle and superficial parts of the external anal sphincter are still closed; C complete opening of the internal anal sphincter with simultaneous reflex inhibition of puborectalis/levator ani and external sphincter muscles leading to defecation; D almost complete emptying of the rectum after defecation and restoring of the anorectal angle (reproduced from Holschneider and Puri [23])
Fig. 7.3 Defecography in a healthy child in sagittal position. A Normal anorectal angle formed by the puborectalis sling and the deep part of the external anal sphincter; B internal sphincter relaxation starting with opening of the proximal one-third of the anal canal. The middle and superficial parts of the external anal sphincter are still closed; C complete opening of the internal anal sphincter with simultaneous reflex inhibition of puborectalis/levator ani and external sphincter muscles leading to defecation; D almost complete emptying of the rectum after defecation and restoring of the anorectal angle (reproduced from Holschneider and Puri [23])
bers usually join to form two nervi erigentes, which give short branches directly to the rectum at the level of the ischial spine (Fig. 7.5 A-C) and continue as longer trunks to the inferior hypogastric or Hypogastric Plexus Spinal Nerve" href="/spinal-nerve/neuroanatomy-kan.html">pelvic plexus, where they are redistributed to pelvic organs, directly or via blood vessels. In the wall of the rectum, these fibers relay in the ganglia of Auerbach's plexus. Other small parasympathetic nerves from the anterior divisions of the third and fourth sacral nerves join and ascend in the presacral sympathetic nerve and then follow the ramifications of the inferior mesenteric artery.
These delicate, tenuous nervi erigentes run lateral to the rectum, directly attached to the rectal fascia [16] close to the ischial spine or, in the newborn baby, at the level of the pubococcygeal (PC) line [56]. The main trunks can be separated safely from the rectum because a natural plane of cleavage can be found between the perirectal connective tissue, rectal fascia, and nerves. Hence, bladder and urethral function is spared in excision of the rectum for nonmalignant conditions.
The nervi erigentes in rectal deformities are separated throughout their course by the rectum if it descends to the level of the PC line (see chapter 25).
When the rectum is located higher in the pelvis than the PC line, these nerves run a more medial course with the perirectal connective tissue (perirectal
Fig. 7.4 A Sagittal section of a normal pelvis at the level of the pubic arch (P). The longitudinal muscle (L) is thickened and blended with the external anal sphincter. DE Deep external anal sphincter, SE superficial anal sphincter, I internal anal sphincter. B Transverse section of a normal male pelvis at the level of the pubic arch. Inner circular muscle (I) and longitudinal muscle (L) are thickened at this level. P Puborectalis muscle, U
urethra. C Rectourethral fistula in a boy with a high anorectal malformation. At this level, 2 cm above the connection with a fistula, the thickening of the inner circular muscle can be seen. The puborectalis muscle is just adjacent to the rectal wall. A-C reproduced from Yokoyama et al. [21] with the permission of the publisher
Fig. 7.4 A Sagittal section of a normal pelvis at the level of the pubic arch (P). The longitudinal muscle (L) is thickened and blended with the external anal sphincter. DE Deep external anal sphincter, SE superficial anal sphincter, I internal anal sphincter. B Transverse section of a normal male pelvis at the level of the pubic arch. Inner circular muscle (I) and longitudinal muscle (L) are thickened at this level. P Puborectalis muscle, U
urethra. C Rectourethral fistula in a boy with a high anorectal malformation. At this level, 2 cm above the connection with a fistula, the thickening of the inner circular muscle can be seen. The puborectalis muscle is just adjacent to the rectal wall. A-C reproduced from Yokoyama et al. [21] with the permission of the publisher fascia) beneath the blind ending rectum to reach the region of the bladder base and neck. In this situation, they are more vulnerable, especially if mobilization of the rectum is attempted from the sacrococcygeal approach [55]. Furthermore, in some patients the nervi erigentes and nerves to levator ani have a common stem or origin before dividing and diverging in their different fascial investments, in which event, if the common trunk is damaged, the function of both the bladder and the levator ani would be affected [3].
7.5.2 Sympathetic Nerves
The sympathetic nerves arise in the second, third, and fourth lumbar ganglia and the preaortic plexus. They unite on either side and form the hypogastric plexus in front of the fifth lumbar vertebra and then continue down the posterolateral pelvic walls as the presacral nerves, which join the pelvic ganglion on either side of the pelvis. Several fine sympathetic nerves from the second and third ganglia of the sacral sympathetic chain also join the pelvic ganglion in close company with the parasympathetic nervi erigentes.
The pelvic ganglion is a flat pannus that lies closely applied to the base of the bladder and prostate, the
Fig. 7.5 A-C Schematic view of the compartments of the female pelvis. A Dorsal compartment with hatched perirectal subcompartment [57]; B ventral compartment with marked (yellow/hatched) paravisceral fat body [57]; C middle compartment with hatched paracervical, adventitial connective tissue and sacrouterine ligament [57]. VBladder, Uuterus, R rectum, PS os coccyx, Co canalis obturatorius, Moi Musculus obturato-rius internus, Lsu sacrouterine ligament (see also chapter 25, Fig. 25.6)
region of the uterine cervix, and the adjoining anterolateral wall of the rectum. The ureter passes through it to get to the bladder. The ganglion has two posterior dog ears, one in the line of the presacral nerves and one adjacent to the third and fourth sacral segments, reaching backwards toward the contributions from the nervi erigentes. The pelvic ganglion is composed of multiple convoluted nerves and large clusters of ganglion cells packed into the tessellated pannus. It lies in the parietal layer of pelvic fascia and can be separated from the rectum, which can be freed and resected without interference with function of the urinary or genital tracts (see Fig. 7.5A-C).
The sympathetic and parasympathetic nerves to the rectum and anal canal are responsible through the ganglion plexuses of Auerbach and Meissner for organized peristalsis and tone in the internal sphincter. The sympathetic fibers are said to be inhibitors of the bowel wall and motor to the involuntary internal sphincter, whereas the parasympathetic nerves are motor to the bowel and inhibitors of the sphincters [22, 23]. The parasympathetic nerves carry, in addition, sensory fibers conveying knowledge of distention of the rectum [24], which are supposed to be located at the ventral rectal wall [9].
- B
Fig. 7.6 A Pudendal nerves and arteries and perineal branches of S4 (reproduced from Stelzner [3] with the permission of the publishers). Course of the pudendal nerve with radial branches to the pubococcygeus and puborectalis muscles. Peri-neal branch of S4 to the puborectalis and external sphincter muscles. Note that the midline zone around and in front of the coccyx is free from nerves and safe for dissection. 1 Pudendal artery, 2 anal branch, 3 perineal branch, 4 perineal nerve, 5 dorsal nerve of the penis (4-6 branches of the pudendal nerve). B Nervi erigentes and nerves to the levator ani (reproduced from Stephens and Smith [55] with the permission of the publishers). Right half of the pelvis from within. a Right nervi erigentes arising from the roots of S3 and S4, b branch of S3 and S4 to cranial aspect of levator ani, c pudendal nerve giving branch to the caudal aspect of levator ani and to the external anal sphincter, d perineal branch of S4 to puborectalis and external sphincter
7.5.3 Nerves to the Levator Ani Muscles and the External Sphincter
Branches from the anterior roots of the third and fourth sacral nerves unite to form the main nerve pathway to the ilio- and pubococcygeus muscles. The trunk runs a lateral course on the cranial or pelvic surface of the levator ani muscle, not far from and parallel to the white line of origin. Its branches run obliquely, anteriorly, and medially on these muscles. This nerve may be single with peripheral oblique branchings, a single stem with two main branches, or may be represented by two separate nerves running parallel to each other, arising independently from the nerve roots of third and fourth sacral nerves.
The pudendal nerve, which arises from the anterior divisions of the second, third, and fourth sacral nerves, clings to the lateral wall of the pelvis in the pudendal, or Alcock's canal. It supplies branches to both the ilio- and pubococcygeus muscles and to the puborectalis [25] through its inferior hemorrhoidal and perineal branches, which cross the ischioanal space to enter the muscles (Fig. 7.6).
The perineal branch of the fourth sacral nerve, a nerve that must be distinguished from the perineal branches of the pudendal nerve, enters the ischio-rectal fossa medial to the ischial spine on the caudal and lateral aspect of the coccygeus muscle, and its branches are directed medially to the posterior fibers of the puborectalis sling and external sphincter [15]. This nerve is at surgical risk only when deep lateral cuts are directed from the vicinity of the coccyx and anococcygeal body.
The coccyx and the distal sacral vertebrae are absent in many patients exhibiting ARM, and the coccy-geal nerves and corresponding sacral nerves in some such patients are also defective. Generally, it can be observed that bilateral loss of all sacral nerve fibers S2-S4 leads to complete incontinence. There are no longer anorectal reflex mechanisms or sensitivity.
If only the sacral nerve supply of S1 and S2 is developed bilaterally, the feeling of fullness and the ability to discriminate solid, liquid, or gaseous stools is disturbed, as well as the rectosphincteric reflex mechanism to the external anal sphincter and the pu-borectalis muscles. The complete unilateral loss of the sacral nerves has almost no consequences [26].
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