Organs of the Digestive System and their Neurovasculature

3 Organs of the Digestive System and their Neurovasculature


3.1 Stomach: Location, Shape, Divisions, and Interior View



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A Projection onto the trunk


Anterior view.


The stomach is intraperitoneal and located in the left upper quadrant (epigastrium).


Note the transpyloric plane (halfway between the superior border of the pubic symphysis and superior border of the manubrium, see p. 360). It serves as an important anatomical landmark: the pylorus is located at or slightly below the transpyloric plane. Unlike other parts of the stomach, the pylorus hardly moves at all since it is connected to the duodenum, which is retroperitoneal (and thus relatively immobile).



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B Topographical relationships


Transverse section at approximately the T 12/L 1 level. Viewed from above.


Note the relationship of the stomach to the spleen, pancreas, liver, and omental bursa: The greater curvature extends to the spleen; the left lobe of the liver extends in front of the stomach and into the left upper quadrant. When the abdomen is opened, very little of the stomach is visible as most of it is obscured by the liver. Posterior to the stomach lies a narrow peritoneal space called the omental bursa. Its posterior wall is largely formed by the pancreas. Due to its peritoneal covering, the stomach is very mobile relative to the neighboring organs. This is important for facilitating the stomach’s peristaltic movements. Due to its embryonic placement in the ventral and dorsal mesogastrium (see p. 42), the stomach has direct peritoneal attachments to the spleen and liver.



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C The stomach in situ


Anterior view of the opened upper abdomen. The liver has been retracted superolaterally, and the esophagus has been pulled slightly downward for better exposure. The arrow points to the omental foramen, the opening in the omental bursa behind the lesser omentum. Peritoneal adhesions are visible between the liver and the descending part of the duodenum. The lesser omentum is visibly subdivided into a relatively thick hepatoduodenal ligament (transmitting neurovascular structures to the porta hepatis) and a thinner hepatogastric ligament, which is attached to the lesser curvature of the stomach. A hepatoesophageal ligament can also be identified. The greater curvature of the stomach is closely related to the spleen in the left upper quadrant (LUQ). The greater omentum is a duplication of peritoneum that covers the transverse colon and drapes over the loops of small intestine (not visible here).




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E Interior of the stomach


Anterior view of the stomach with the anterior wall removed. For clarity, small portions of the esophagus and duodenum are also shown. The gastric mucosa forms prominent folds (rugal folds) that serve to increase its surface area. These folds are directed longitudinally toward the pylorus, forming “gastric canals.” The rugal folds are most prominent in the body of the stomach and along the greater curvature and diminish in size toward the pyloric end. The mucosa imparts a glossy sheen to the stomach lining.


Note: The pyloric orifice is quite large in this dissection. Normally, the orifice usually opens to a luminal diameter of only 2–3 mm.


3.2 Stomach: Wall Structure and Histology



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A Muscular layers


Anterior view of the anterior stomach wall with the serosa and subserosa removed. The muscular coat of the stomach has been windowed at several sites. The entire stomach wall ranges from 3 mm to approximately 10 mm in thickness (see B for individual layers). Most of its muscular coat consists not of two layers (as in other hollow organs of the gastrointestinal tract) but of three muscular layers:


An outer longitudinal layer, which is most pronounced along the greater curvature (greatest longitudinal expansion)


A middle circular layer, which is well developed in the body of the stomach and most strongly developed in the pyloric canal (anular sphincter, see p. 229)


An innermost layer of oblique fibers, which is derived from the circular muscle layer and is clearly visible in the body of the stomach.


The three-layered structure of its muscular wall enables the stomach to undergo powerful churning movements. The muscles can forcefully propel solid food components against the stomach wall in the acidic gastric juice, breaking the material up into particles approximately 1 mm in size that can pass easily through the pylorus. The longitudinally-oriented rugal folds (reserve folds that disappear when the stomach is distended) form channels, called gastric canals, that rapidly convey liquids from the gastric inlet to the pylorus.



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B Structure of the stomach wall and gastric glands


a The structure of the stomach wall illustrates the layered wall structure that is typical of the hollow organs throughout the gastrointestinal tract. The stomach is unique, however, in that its muscular coat consists of three rather than two layers (see A).


Note: The serosa (visceral layer of the peritoneum) and subserosa (connective-tissue layer giving attachment to the serosa and transmitting neurovascular structures for the muscular coat) are present only in areas where the organ in question is covered by visceral peritoneum. In wall areas that lack a peritoneal covering (e.g., large portions of the duodenum and colon), the serosa and subserosa are replaced by a fibrous adventitia, which connects the wall of the organ to the connective tissue of surrounding structures.


The mucosa contains specialized cells that are aggregated into glands (visible microscopically). The glandular orifices open at the base of the gastric pits (see b). In the body and pylorus of the stomach these glands extend down to the muscular layer of the mucosa, the muscularis mucosae (deeper glands = more cells = higher secretory output). The submucosa (layer of connective tissue transmitting neurovascular structures for the muscular coat) contains the submucous plexus for visceromotor and viscerosensory control of the hollow organs in the gastrointestinal tract. This plexus, like the myenteric plexus (located in the muscular coat for visceromotor control of the visceral muscle, not shown here), is part of the enteric nervous system, which contains, in total, millions of scattered ganglion cells.


b Structure of the gastric glands (after Lüllmann-Rauch) (simplified schematic of a gland from the body of the stomach). Several types of cells are distinguished in the fundus and body of the stomach:


Surface epithelial cells: cover the surface of the mucosa and secrete a mucous film.


Neck cells: produce mucin to strengthen the mucous film (make it more anionic).


Parietal cells: produce HCl and intrinsic factor, which is necessary for vitamin B12 absorption in the ileum.


Chief cells: produce pepsinogen, which is converted to pepsin (for protein breakdown) in the stomach.


Enteroendocrine cells: different subtypes producing gastrin (G cells), somatostatin (D cells), or other factors controlling motility and secretion


Stem cells: reservoir for replenishing the surface epithelial cells and gland cells



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C Endoscopic appearance of the gastric mucosa


a, b Healthy gastric mucosa with a glistening surface; c Gastric ulcer.


a View into the body of the stomach, which has been moderately distended by air insufflation. The mucosa is raised into prominent, tortuous rugal folds that form the gastric canals.


b Inspection of the pyloric antrum shows less prominent folds than in the body of the stomach.


c Fibrin-covered gastric ulcer with hematin spots. A gastric ulcer is defined as a tissue defect that extends at least into the muscularis mucosae, but many ulcers extend much deeper into the stomach wall. Most gastric ulcers are caused by infection with Helicobacter pylori, a bacterium that is resistant to stomach acid (from Block, Schachschal and Schmidt: The Gastroscopy Trainer. Stuttgart: Thieme, 2004).


3.3 Small Intestine: Duodenum



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A Projected onto the vertebral column


The duodenum is a C-shaped loop of small intestine lying predominantly on the right side of the vertebral column in the right upper quadrant (RUQ) and encompassing the L 1 through L 3 vertebrae and occasionally extending to L 4. The concavity of the duodenum normally encloses the head of the pancreas at the L 2 level (see D).



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B Parts of the duodenum


Anterior view. The anatomical parts of the duodenum (superior, descending, horizontal, and ascending parts with intervening flexures) have a total length of approximately 12 finger-widths (L. duodeni = “twelve at a time”).


Note the suspensory ligament of the duodenum (called also the ligament of Treitz), which often contains smooth-muscle fibers. Mobile loops of small intestine may wrap around this ligament and become entrapped between the ligament and the vessels behind it (most notably the abdominal aorta). This “Treitz hernia” may cause mechanical obstruction of the affected bowel loop and strangulate its blood supply.



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C Wall structure and duct orifices


Anterior view. Most of the duodenum has been opened. The pyloric orifice (here greatly dilated) opens to a luminal diameter of only about 2–3 mm for the passage of chyme. The duodenum has basically the same wall structure as the other hollow organs of the gastrointestinal tract (see B, p. 231). The structure of the mucosa is shown in F. The descending part of the duodenum has two small elevations along its inner curve: the minor duodenal papilla, which bears the orifice of the accessory pancreatic duct, and the major duodenal papilla (called also the papilla of Vater), which has a common orifice for the pancreatic duct and common bile duct. Thus, the release of bile and pancreatic juice to aid digestion takes place in the upper part of the duodenum.




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E Endoscopic view


The endoscope is pointing down into the descending part of the duodenum. The papillary region where the bile duct and pancreatic duct open into the duodenum is visible on the left side of the image at approximately the 10 o’clock position. The circular folds (valves of Kerckring) are typical of those found in the small intestine, diminishing in size in the proximal to distal direction (from Block, Schachschal and Schmidt: Endoscopy of the Upper G I Tract. Stuttgart: Thieme, 2004).



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F Histological structure


Longitudinal section through the duodenal wall. The duodenum has basically the same histological structure as the other hollow organs of the gastrointestinal tract (see B, p. 231), with some notable differences such as the presence of duodenal (Brunner) glands (secrete mucins and bicarbonate to neutralize the acidic gastric juice) and valves of Kerckring (specialized circular folds). Other features that distinguish the duodenum from the jejunum and ileum are its more prominent mucosal folds, which diminish in size toward the end of the small intestine.


Note: The muscularis externa of all portions of the intestine, unlike that of the stomach, consists of only two layers: an inner layer of circular muscle fibers and an outer layer of longitudinal muscle fibers.


3.4 Small Intestine: Jejunum and Ileum



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A Parts of the small intestine: overview (a) and anatomical constrictions (b)


Anterior view. The large intestine surrounds the loops of small intestine like a frame. Because the small bowel loops are intraperitoneal and therefore very mobile, it is not possible to define their location by reference to skeletal landmarks. If the intestinal loop rotates normally during embryonic development (see p. 46), the duodenum lies behind the transverse colon. If the intestinal loop rotates in the wrong direction, the duodenum will come to lie in front of the transverse colon.


Note the following normal anatomical constrictions:


Junction of the pylorus and duodenum (luminal diameter of the pyloric orifice is only about 2–3 mm)


Duodenojejunal flexure


Ileocecal orifice


Swallowed foreign bodies may become lodged at these sites, obstructing intestinal transit and causing mechanical intestinal paralysis (mechanical ileus, a life-threatening condition that is an absolute indication for surgical treatment).



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B Wall structure of the jejunum and ileum


The wall layers of the small intestine are displayed in a “telescoped” cross-section. The mucosal layer has been incised longitudinally and opened. The jejunum and ileum have basically the same wall structure as the other hollow organs of the gastrointestinal tract (see B, p. 231), but local differences are observed in the circular folds (see C) and vascular supply (see p. 268).



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C Differences in the wall structure of the jejunum and ileum


Macroscopic views of the jejunum (a) and ileum (b), which have been opened longitudinally to display their mucosal surface anatomy.


Note: The transversely oriented circular folds in the jejunum are spaced much closer together than in the ileum. Lymphatic follicles are particularly abundant in the wall of the ileum (from the lamina propria to the submucosa) for mounting an immune response to antigens in the intestinal contents (“aggregated lymph nodules,” Peyer’s patches).



3.5 Large Intestine: Colon Segments



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A Projection of the large intestine onto the skeleton


Because of the embryonic rotation of the primary intestinal (midgut) loop, the large intestine typically forms a frame encompassing the small intestine. The position and length of the colon segments may vary, however, depending on the course of intestinal rotation. For example, when the intestinal loop rotates normally, the ascending colon acquires a “normal” length (as shown here). If intestinal rotation is incomplete, the ascending colon is shortened. The transverse colon is particularly mobile owing to its mesocolon, while the ascending and descending colon are less mobile because they are fixed to the posterior wall of the peritoneal cavity. The left colic flexure usually occupies a somewhat higher level than the right colic flexure due to the space occupied by the large right lobe of the liver. Also, the descending colon is usually more posterior than the ascending colon.


B Distinctive morphological features of the large intestine


There are four morphological features—three visible externally and one internally—that distinguish the large intestine from the small intestine. It should be noted that these features do not occur equally in all parts of the large intestine and are absent in the cecum, vermiform appendix, and rectum.



















Taeniae coli


In most portions of the large intestine, the longitudinal muscle fibers do not form a continuous layer around the intestinal wall but are concentrated to form three longitudinal bands, the taeniae (see C). Taeniae are not present in the rectum or vermiform appendix. The three taeniae converge to form the muscularis externa of the appendix.


Epiploic appendices


Fat-filled protrusions of the serosa, scattered over the surface of the large intestine except on the cecum (absent or sparse) and rectum (absent).


Haustra (haustrations)


Saccular wall protrusions between the transverse folds of the large intestine (see p. 238), absent in the rectum.


Semilunar folds


Visible only internally, in contrast to the external features above. They are functional features caused by contraction of the muscular coat. The internal folds correspond to external constrictions that separate the haustra.



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C The three taeniae of the colon


Sagittal section, viewed from the left side. The three taeniae are named for their position on the colon:


Free taenia (Taenia libera)


Omental taenia (the taenia at the attachment of the greater omentum)


Mesocolic taenia (the taenia at the attachment of the mesocolon)


D Anatomical divisions of the large intestine



The large intestine consists of the following divisions in the proximal-to-distal direction:


Cecum with the vermiform appendix


Colon, consisting of four parts:


Ascending colon


Transverse colon


Descending colon


Sigmoid colon


Rectum


Note: For various reasons, some authors consider the rectum to be a separate section of the intestine, and not a part of the large intestine. However, according to the Terminologia Anatomica, which serves as the international standard on human anatomic terminology, the rectum is a segment of the large intestine.



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E Large intestine: segments, shape, and distinctive features


Anterior view, large intestine. The terminal part of the ileum and portions of the transverse and sigmoid mesocolon are shown. The ascending and descending colon have been rotated to display their taeniae. Note: Colorectal cancer, which has become one of the most common cancers in industrialized countries, has a special predilection for the rectosigmoid junction and the rectum itself (i.e., sites distal to the left colic flexure see p. 248).


The various colon segments possess all the morphological characteristics of the large intestine (haustra, taeniae, epiploic appendices, see B). Typically these features disappear past the rectosigmoid junction. As the taeniae disappear, they are replaced on the rectum by a continuous layer of longitudinal muscle fibers. Instead of haustra, the rectum has three permanent constrictions that are produced by internal transverse folds (see p. 241). The peritoneal reflection on the anterior rectal wall represents the site where the peritoneum is reflected onto the posterior wall of the uterus (in the female) or onto the upper surface of the bladder (in the male).


Note: The ascending and descending colon are (secondarily) retroperitoneal and therefore, unlike the sigmoid and transverse colon, they do not have a mesocolon and are covered only anteriorly by peritoneum. The rectum is extraperitoneal in the lesser pelvis, lacks a “suspensory ligament,” and bears other unique features.


3.6 Large Intestine: Wall Structure, Cecum, and Vermiform Appendix



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A Cecum and terminal ileum


Anterior view. The cecum is unique in its end-to-side connection with the terminal part of the small intestine (ileum) and the presence of the vermiform appendix. As a result, there are two openings in the wall of the cecum: the ileocecal orifice on a small papilla (ileal papilla) and, below that, the orifice of the vermiform appendix. The ileocecal orifice is approximately round in the living individual but is often slit-like in the postmortem condition. It is bounded by superior and inferior flaps or “lips,” the ileocolic labrum (superior lip) and the ileocecal labrum (inferior lip). Both are continued as a narrow ridge of mucosa, the frenulum of the ileocecal orifice.


Note: Inflammation of the vermiform appendix (appendicitis) is one of the most common surgically treated diseases of the gastrointestinal tract. If acute appendicitis goes untreated, the inflammation may perforate into the free peritoneal cavity (a “ruptured appendix” in popular jargon). This creates a route by which bacteria in the bowel lumen can enter the peritoneal cavity and gain access to the large peritoneal surface, quickly inciting a life-threatening inflammation of the peritoneum (peritonitis).



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B Ileocecal Orifice


Anterior view of a longitudinal coronal section of the cecum and ileum. The ileocecal orifice hermetically seals the terminal ileum from the cecum and prevents the reflux of contents from the large intestine (structural constriction, see A, p. 234). At the ileocecal orifice, the end of the ileum evaginates the circular muscle layer of the large intestine into the cecal lumen. All layers of the ileal wall except the longitudinal muscle and peritoneum contribute to the structure of the ileocecal orifice. The circular muscle layers of the ileum and cecum function as a sphincter, which periodically opens the orifice. This allows the contents of the small intestine to enter the large intestine while effectively preventing reflux. The function of the sphincter is similar to that of the pylorus.




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D Wall structure of the colon and cecum


Longitudinal section through the bowel wall. All the typical wall layers of the gastrointestinal canal are present: the mucosa, submucosa, muscularis externa, and serosa (or adventitia in the retroperitoneal parts of the colon, see B, p. 231). There are several features, however, that distinguish the wall structure of the colon and cecum from that of the stomach and small intestine:


The mucosa is devoid of villi (i.e., the total surface area is not enlarged as much as in the small intestine). Instead of villi, there are large numbers of deep crypts (Lieberkühn crypts), more numerous than in the small intestine.


The epithelial layer of the mucosa contains large numbers of goblet cells (for clarity, not shown here).


The colonic mucosal surface undulates in large-scale, crescent-shaped, semilunar folds (see C).


The muscularis externa consists of an inner circular layer and an outer longitudinal layer, which is concentrated in three longitudinal bands, the taeniae (see p.236).



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E Variants in the position of the vermiform appendix


Disturbances in the rotation of the embryonic gut can result in numerous positional variants of the cecum and vermiform appendix. The appendix may even come to lie in the left side of the abdomen. The inflammation of an appendix in the typical position is characterized by tenderness at two points:


McBurney point: Position on a line connecting the umbilicus and the right anterior superior iliac spine. The McBurney point is one-third of the distance along this line from the iliac spine.


Lanz point: Position on a line connecting the anterior superior iliac spines. The Lanz point is one third of the distance along this line from the right spine.


Although very useful, these are not definitive clinical signs. Tenderness may be felt at other abdominal sites, especially if the appendix is in an atypical position.



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F Wall structure of the vermiform appendix


The vermiform appendix has the typical wall structure of an intraperitoneal intestinal tube. One striking feature is the abundance of lymphatic follicles in the submucosa (also present in the colon and cecum, but in much smaller numbers). With its high degree of immunological activity, the appendix has been characterized as the “intestinal tonsil.” The mucosa has numerous deep crypts that are in intimate contact with the lymphatic follicles in the lamina propria and the submucosa (crypts and lymphatic follicles are not visible here). Since the vermiform appendix is intraperitoneal, it possesses a small mesentery, the mesoappendix, which transmits neurovascular structures.


3.7 Large Intestine: Location, Shape, and Interior View of Rectum




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B The rectum in situ


Coronal section of the female pelvis, anterior view, with the rectum opened from about the level of the middle transverse inferior rectal fold. The taeniae of the sigmoid colon are not continued onto the rectum. The constrictions in the outer wall of the rectum correspond to the transverse folds on the inner wall. The rectum (which would appear in this form only if the ampulla were full) is shown in a slightly raised position. Below the levator ani muscle is the powerful external anal sphincter, the muscular component of the rectal continence organ. The pararectal connective tissue below the peritoneal cavity contains numerous vessels that supply the rectum.


This drawing was made from the dissection of a female cadaver. Thus, the peritoneum would be reflected from the anterior wall of the rectum onto the posterior wall of the uterus. Although both the anterior rectal wall and uterus are not visible here (anterior to this plane of section), parts of the rectouterine folds are still visible.


C Distinct morphological features of the rectum



The mucosa and wall structure of the rectum does not differ from the large intestine, including the colon and cecum. Nevertheless, it lacks several colonic characteristics:


no taeniae, the rectum has a continuous longitudinal muscle layer;


no omental appendices;


no haustra;


no semilunar folds, the rectum has transverse rectal folds;


the wall of rectum is devoid of ganglion cells;


embryonic development: The part above the anorectal line, like the colon, is derived from endoderm, the anal canal is derived from ectoderm (which is why some authors don’t consider it part of the rectum).



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D Rectum and anal canal: divisions, internal surface and wall structure


Anterior view of the rectum in coronal section with the anterior wall removed. Instead of semilunar folds, the rectum contains three permanent transverse folds. The distal portion of the rectum, also known as the anorectum, is recognizable by a palpable protrusion (puborectalis sling or anorectal junction) which is visible on the mucosal surface. The anorectum is divided into two segments, the rectal ampulla and the anal canal.


Rectal ampulla: the lowest portion of the rectum between the middle transverse rectal fold (Kohlrausch fold) and the anorectal junction. The rectal ampulla is the most distensible part of the rectum and, contrary to popular opinion, does not serve as a reservoir for holding stool but is usually empty (see mechanism of defecation, p. 237). The middle transverse fold, which projects into the rectum from its right posterior wall, is approximately 6–7 cm from the anus and can just be reached with the palpating finger. Rectal tumors located below the Kohlrausch fold may therefore be palpable.


Anal canal: located below the anorectal junction at the distal perineal flexure (see A). It is approximately 4 cm long and normally kept closed by the anal sphincter muscles. The clinically important “surgical anal canal” begins at the level of the anorectal junction and extends to the anocutaneous line, which is also palpable. It is a groove located between the margins of the internal and external anal sphincters (intersphincteric groove) at the junction of the anoderm (white zone), a region with very dense somatic innervation, and the pigmented perianal skin (see p. 242 B). Above the anoderm are located 8–10 longitudinal mucosal folds (anal columns), produced by the arterial cavernous body of the rectum (hemorrhoidal plexus), located in the submucosa (see p. 242). The distal ends of the mucosal folds are connected by valve-like transverse folds (anal valves). All anal valves together form the dentate line, which is an important landmark because it is visible. Behind the anal valves are pocket-like depressions (anal sinuses or pouches of Morgagni), into which empty 6–8 outflow ducts of the rudimentary mucus-secreting anal glands (proctodeal glands). The most common site of these glands is the posterior commissure (approximately at 6 o’clock in the lithotomy position), either in the submucosa or intersphincteric (between the internal and external anal sphincter) space, so that the outflow ducts partially transverse the internal anal sphincter.


Note: Bacterial infections of the glands may cause perianal abscesses and anal fistulas, which are difficult to treat (see p. 247).


3.8 Continence Organ: Structure and Components



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A Components of the continence apparatus


Midsagittal section at the level of the anal canal in the male, viewed from the left side.


The continence apparatus, or continence organ, controls the closing (continence) and opening (defecation) of the rectum and provides a tight closure before and after evacuation of solid, liquid and gas bowel contents.


It consists of a distensible hollow organ as well as vascular and muscular continence mechanisms, including their neural control. These angiomuscular continence mechanisms are integrated into a structurally narrow segment, which begins at the level of the perineal flexure and continues along the anal canal:


Distensible hollow organ:


rectum with stretch receptors, mainly in the rectal ampulla (viscerosensory innervation)


anus with distensible skin in the anal canal (somatosensory innervation);


Muscular continence:


internal anal sphincter (visceromotor innervation)


external anal sphincter (somatomotor innervation)


levator ani, especially the puborectalis muscle (somatomotor innervation);


Vascular continence:


hemorrhoidal plexus (permanently distended cavernous tissue that subsides only during defecation;


Neural control:


visceral and somatic nervous system (mainly from S2–S4) with the pelvic splanchnic nerves, pudendal nerve and rectal plexus.


Functionally, both continence and defecation are the result of a finetuned feedback loop between receptors and effectors of the continence apparatus with involvement of the central nervous system (see p. 244 f).



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B Epithelial regions of the anal canal (after Lüllmann-Rauch)


In the anal canal, the unilayered columnar epithelium of the colorectal mucosa at the level of the transition zone is continuous with the stratified squamous epithelium of the anoderm and perianal skin. The transition occurs near characteristic landmarks. The anal canal can be divided into the following epithelial regions:


colorectal zone between anorectal junction and supratransitional line; homogeneous colorectal mucosa with crypts;


transition zone at the level of the anal columns (between supratransitional line and dentate line) mosaic patterns of colorectal mucosa, unilayered columnar epithelium and stratified squamous epithelium;


squamous zone between dentate line and anocutaneous line: evenly covered by stratified, nonkeratinized squamous epithelium, which is intimately attached to the underlying internal anal sphincter, hence its whitish appearance (white zone). Deep sensory innervation with touch, pressure, temperature and mainly pain receptors (clinically: anoderm);


perianal skin below the anocutaneous line: start of the stratified squamous epithelium of the outer layer of the skin (heavy pigmentation, eccrine and apocrine sweat glands and hair follicles).


Note: Knowledge about the epithelial regions of the anal canal is important mainly for the differentiation between rectal (usually adenocarcinoma) and anal carcinoma (of keratinizing or non-keratinizing squamous epithelium).



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C Structure of the muscular continence mechanism


a Midsagittal section, viewed from the left side; b Puborectalis sling and anorectal angle: relaxed muscle (left) and contracted muscle (right).


The complex system of anal sphincters involves both smooth and striated muscles. Whereas the smooth muscles represent the direct continuation of the muscles of the rectal wall, the striated muscles are formed by specialized areas of the pelvic floor muscles. Thus, these muscular continence mechanisms are maintained under both somatic-voluntary and visceral-involuntary control.


Involuntarily innervated smooth muscles:


Internal anal sphincter: most significant smooth muscle; as the continuation of the circular muscle layer of the rectum, it forms a strong circular ring. Sympathetic nerve fibers and a significantly reduced number of enteric ganglion cells (hypoganglionosis) allow it to maintain constant tonic activity to help constrict the anal canal (the internal sphincter is responsible for 70% of fecal continence);


Muscularis mucosae of the anal canal: as the continuation of the muscular layer of the mucosa, it extends beyond the hemorrhoidal plexus and ends at the dentate line; stabilizes the hemorrhoidal plexus and holds it in place;


Corrugator ani: as the continuation of the longitudinal muscle layer of the rectum, the muscle fibers extend beyond the anal canal, permeate through the subcutaneous part of the external anal sphincter and insert into the perianal skin. Corrugator ani owes its name to the fact that muscle contraction produces radial wrinkles on the perianal skin.


Voluntarily innervated striated muscles:


External anal sphincter: cylindrical muscle that encircles the outside wall of the anal canal, made up of three recognizable parts: deep, superficial, and subcutaneous. Whereas the deep and subcutaneous parts are arranged in circular layers, the superficial part extends between the anteriorly located perineal body and the posterior anococcygeal ligament and surrounds the anal canal and serves as a clamp. It is largely composed of type I fibers, which are slow, durable, and fatigue resistant.


Puborectalis muscle: as the innermost portion of the levator ani muscle, it forms a strong sling of muscle, which loops around the rectum at the level of the anorectal junction and is closely aligned to the deep part of the external anal sphincter. It arises from the fixed end of the pubic bone, so when the puborectalis muscle contracts it creates a “kink” between anal canal and rectum at the anorectal angle.



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D Structure of the vascular continence mechanism


a Longitudinal section of the anal canal with the hemorrhoidal plexus windowed; b and c Hemorrhoidal plexus at rest and during defecation. Above the dentate line at the level of the anal columns in the submucosa lies a cavernous body, the hemorrhoidal plexus. Its elasticity largely ensures liquid- and gas-tight closure of the rectum. The circular configuration of the hemorrhoidal plexus is similar in structure to the cavernous body of the penis but differs in that it is permanently distended. The hemorrhoidal plexus is a network of cavernous tissue and is almost exclusively supplied by three branches of the superior rectal artery (at 3, 7 and 11 o’clock in the lithotomy position), which further divide near the anal columns (see p. 273). Blood reaches the venous drainage system via arteriovenous anastomoses through transsphincteric veins—largely along the internal anal sphincter—and reaches the drainage area of the inferior mesenteric vein (and is carried to the portal vein) but also partially through the middle and inferior rectal veins to the perianal veins of the external venous plexus. When the sphincter apparatus relaxes during defecation, it allows blood to drain from the hemorrhoidal plexus. Note: Abnormal dilation (hyperplasia) of the hemorrhoidal plexus beyond the physiological range leads to hemorrhoidal disease, one of the most common proctological disorders (see p. 246 f).


3.9 Continence Organ: Function



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A Innervation (after Stelzner)


a Somatomotor and somatosensory innervation; b Visceromotor and viscerosensory innervation.


Somatomotor: pudendal nerve for the external anal sphincter, levator nerves for the levator ani muscle (especially the puborectalis). They provide active, partially voluntary innervation of the external anal sphincter and levator ani.


Somatosensory: inferior rectal nerves for the anus and perianal skin. Arising from the pudendal nerve, they transmit touch and especially pain sensation. The skin of the anus is extremely sensitive to pain. Even small tears in the anal skin, which often show inflammatory changes, tend to be extremely painful.


Visceromotor: pelvic splanchnic nerves (S2–S4) for the internal anal sphincter. The resting tone of the internal sphincter (lumbar and sacral splanchnic nerves) helps to maintain closure of the anal canal and inhibits venous drainage from the hemorrhoidal plexus; the cavernous body remains distended, contributing to fecal continence and flatus control. Topographically, the pelvic splanchnic nerves are closely related to the rectal plexuses.


Viscerosensory: pelvic splanchnic nerves (S2–S4) supply the wall of the rectum, particularly the stretch receptors in the rectal ampulla. Stretching of the ampulla by the fecal column triggers a subjective awareness of the need to defecate.



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B Mechanism of defecation (after Wedel; see right page)


a Filling of the rectal ampulla; b Relaxation of the voluntarily controlled sphincters and propulsion of fecal column.


Both defecation and continence are under central nervous system control involving different anatomical structures ranging from the cerebral cortex to the perianal skin, with the anorectum being one of multiple effectors. Directly involved are the pelvic floor, muscles used during squatting, the abdominal press as well as autonomic and sensory nerves along with their higher nerve centers.


Filling of the rectal ampulla and stimulation of local stretch receptors in the ampullary wall: when the fecal bolus is propelled into the rectal ampulla by anterogradely propagating waves, mechanoreceptors detect distension and transmit the information via visceral afferents in the posterior funiculus to the sensory cortex, which perceives the urge to defecate. Olfactory, visual, or acoustic stimuli can either accelerate or decelerate the perception and subsequent voluntary action, which results in defecation.


Rectoanal inhibitory reflex and relaxation of the voluntarily innervated sphincters: When the ampulla fills with feces, the intrarectal pressure increases and the internal anal sphincter relaxes, followed by voluntary relaxation of the puborectalis sling and the external anal sphincter. As a result, the anorectal angle straightens and the anal canal widens.


Propulsion of fecal column: Rectal evacuation is assisted by a direct involuntary increase in pressure in the rectal area and by simultaneous increase in pressure by the contraction of voluntarily innervated muscles: abdominal (abdominal press), perineal (pelvic floor lift), diaphragmatic (diaphragm contraction) and glottic (glottis closure) muscles. The squatting position further increases abdominal pressure (flexor reflex). With the propulsion of the fecal column, the hemorrhoidal cushions are drained and pushed out.


Completion of defecation: After the sphincter apparatus allowed the fecal column to pass through, it comes in contact with the highly sensitive anoderm, which perceives the volume, consistency, and location of the stool. This perception initiates the voluntary process of completing defecation. Defecation is completed once the sphincter apparatus contracts and the hemorrhoidal plexus fills up.


3.10 Disorders of the Anal Canal: Hemorrhoidal Disease, Anal Abscesses, and Anal Fistulas



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A Hemorrhoidal disease


Hemorrhoidal disease is one of the most common proctological disorders. The site of origin is the circular hemorrhoidal plexus of the cavernous body of the rectum located above the dentate line. It is largely responsible for the fine adjustment of anal continence. Hemorrhoid is a general term used to describe hyperplasia (enlargement) of a cavernous body with arterial blood supply, a condition which initially does not cause any symptoms. Hemorrhoids become pathological once they become symptomatic (bleeding is bright red from arterial blood, mucus discharge, itching, burning, fecal soiling, etc.) and when they require treatment (hemorrhoidal disease). Most commonly, hemorrhoids result from increased pressure on the anus during defecation, often caused by chronic constipation as a result of a lack of fiber and fluids in the diet. Another cause is impaired venous return due to increased anal sphincter tone as this may lead to the hemorrhoidal plexus taking on a gnarled appearance. Diagnosis and classification of hemorrhoids are based on examination, palpation, and proctoscopy of the anal canal. Depending on the severity of the hemorrhoids and their symptoms, they are divided into four grades:


Grade I (a): swollen, elastic cushions of tissue that are visible only on proctoscopy (located above the dentate line) and may cause painless bright red bleeding (painless because the swollen cushions are located above the anoderm);


Grade II (b): visibly hyperplastic vascular cushions, which can prolapse inside or outside of the anal canal during defecation or while pressing but retract immediately after emptying the bowels. Dripping blood and mucus discharge may cause oozing or itching, a condition also known as perianal eczema;


Grade III (c): during defecation or while the intra-abdominal pressure is increased, hemorrhoids prolapse spontaneously and require manual repositioning. Possible thrombosis or incarceration of the prolapsed knot may cause significant pain;


Grade IV (d): at this stage, the nodular enlargements and large parts of the anal canal, including the highly pain-sensitive anoderm, are permanently prolapsed (irreducible) and attached to the anal margin (also known as an anal prolapse).


Note: Unlike in the German medical terminology, the Anglo-American and Swiss terminology distinguish between internal and external hemorrhoids. Internal hemorrhoids originate from the internal rectal venous plexus, external hemorrhoids are subcutaneous clots at the margin of the anus (e.g., perianal thromboses). In our assessment, however, external hemorrhoids are simply hyperplastic vascular cushions of the rectal cavernous body with their arterial blood supply that have prolapsed to the outside.



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B Conditions of the perianal skin with and without hemorrhoidal disease


a Normal anatomy of the perianal area in a 38 year old female patient;


b Grade IV hemorrhoids in a 54 year old female patient: mucosal prolapse at the anterior commissure combined with right- and left-lateral anal skin tags (harmless, generally asymptomatic perianal skin folds) (from Rohde, H,: Lehratlas der Proktologie. Thieme, Stuttgart 2006).

Aug 4, 2021 | Posted by in GENERAL SURGERY | Comments Off on Organs of the Digestive System and their Neurovasculature
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