Overview of Lower Digestive Tract

Blood Supply of Small and Large Intestines

The blood supply to the small and large intestines is extremely variable and unpredictable. The variations concerning the origin, course, anastomoses, and distribution of the intestinal vessels are so frequent and so significant that conventional textbook descriptions are inadequate and, in many respects, even misleading, a situation much the same as that of the blood supply of the upper abdominal organs. Because of this variability, the surgeon should have an intimate acquaintance with the entire spectrum of the gut's arterial supply in order to avoid operative errors, such as devascularization of intestinal sections, which might inadvertently induce necrosis leading to rupture and peritonitis. In this overview, we will review the most typical branching patterns of the vessels associated with the small and large intestines. In the sections devoted to each organ, we will consider the complex variations that may be encountered.

The digestive tract within the abdominal cavity receives nearly all of its blood supply from three unpaired branches of the abdominal aorta. The foregut organs (distal esophagus, stomach, liver, gallbladder, spleen, pancreas, and proximal duodenum) are supplied by the celiac trunk , a large artery that leaves the abdominal aorta shortly after passing through the diaphragm. The small and large intestines receive their blood supply from the other two unpaired vessels of the aorta, the superior mesenteric and inferior mesenteric arteries.

The superior mesenteric artery arises from the anterior wall of the abdominal aorta immediately inferior to the celiac trunk. It supplies the midgut organs (distal duodenum, part of the head of the pancreas, jejunum, ileum, cecum, vermiform appendix, ascending colon, and transverse colon) before forming anastomoses with the inferior mesenteric artery. One of the first arteries to arise from the superior mesenteric artery is the middle colic artery , which supplies the most distal midgut structure, the transverse colon. The position of the transverse colon is a remnant of the rotation and extreme elongation of the embryonic gut tube that occurs as it reaches its mature state. We will revisit this artery again as it anastomoses with the blood supply to the ascending colon.

Another proximal branch of the superior mesenteric artery is the inferior pancreaticoduodenal artery . It splits into an anterior branch and a posterior branch that sandwich the inferior aspect of the head of the pancreas. These vessels, as their name implies, also supply the distal duodenum as it transitions to become the jejunum. The anterior and posterior branches of the pancreaticoduodenal artery anastomose with two adjacent branches of the celiac trunk, the anterior and posterior branches of the superior pancreaticoduodenal artery . The inferior pancreaticoduodenal artery also forms an anastomosis with the next branch off of the superior mesenteric artery that supplies the jejunum.

The next branches from the superior mesenteric artery are 15 to 18 intestinal branches that are either jejunal arteries or ileal arteries . These exit the left side of the superior mesenteric artery and travel within the intestinal mesentery until they reach the jejunum and ileum. The point at which these vessels become covered by the two peritoneal folds that become the small intestine's mesentery is termed the root of the mesentery . As these vessels approach the small intestine, they interconnect to form a series of loops, or arterial arcades . These arcades ensure redundancy in the blood supply to the small intestine so that a blockage in one region will not cause ischemia of the nearby intestine. The arterial arcades give off straight arteries (vasa recta) that actually reach the gut tube. Although there is no clear transition between the jejunum and ileum, the appearance of the arterial arcades and straight arteries can assist in differentiating one from the other. The jejunum tends to have simple arcades with one loop between adjacent jejunal arteries and fairly elongated straight arteries. In contrast, the ileum has more complex arcades with two or more loops and relatively short straight arteries that reach the ileum.

Exiting the right side of the superior mesenteric artery are the ileocolic and right colic arteries. The ileocolic artery travels inferiorly and to the right, branching into an ileal branch and a colic branch . The ileal branch anastomoses with the arterial arcades of the ileal artery, supplying the terminal ileum. The ileal branch also typically gives off the important appendicular artery , which travels posteriorly to the ileal arterial arcades, through the mesoappendix, and finally to the vermiform appendix itself. In the vicinity of the appendicular artery, the ileal branch of the ileocolic artery gives off an anterior cecal artery and a posterior cecal artery , which supply blood to their respective sides of the cecum. The right colic artery exits the superior mesenteric artery superior to the ileocolic artery and travels transversely toward the ascending colon. It forms significant anastomoses with the colic branch of the ileocolic and middle colic arteries. The anastomosis occurs primarily through a large artery, the marginal artery (of Drummond), which parallels the entire large colon and receives blood from the ileocolic, right colic, middle colic, left colic, and sigmoid arteries. The marginal artery is a vessel that allows blood to reach the entire length of the colon even if one of the feeder arteries is compromised. From the marginal artery, blood reaches the colon itself via straight arteries (vasa recta).

The inferior mesenteric artery is the last of the three unpaired vessels that supply the digestive tract. It arises from the left anterior aspect of the abdominal aorta approximately 3 to 5 cm above the bifurcation of the right and left common iliac arteries. It quickly gives off several branches, the left colic, sigmoid, and superior rectal arteries. The left colic artery travels transversely to the left, giving off an ascending artery that travels toward the left colic flexure to anastomose with branches of the middle colic artery. The rest of the left colic artery contributes blood primarily to the descending colon via the marginal artery and straight arteries. The sigmoid arteries are a series of three to four arteries branching off of the inferior mesenteric artery which travel within the sigmoid mesocolon to reach the sigmoid colon. The branches of the sigmoid artery form interconnecting arterial arcades before giving off straight arteries that enter the sigmoid colon itself. Typically, the marginal artery does not continue as a distinct structure into the sigmoid mesocolon.

The final direct branch of the inferior mesenteric artery is the superior rectal artery . It communicates with the sigmoid arterial arcades and may also supply some distinct branches to the sigmoid colon, termed the rectosigmoid arteries . The sigmoid mesocolon disappears as the sigmoid colon transitions to become the retroperitoneal rectum. As this transition occurs, the superior rectal artery bifurcates into two lateral rectal arteries that parallel the rectal wall and supply blood to the organ. These branches of the superior rectal artery anastomose with the middle rectal artery , a branch of the internal iliac artery, and to a lesser degree, the inferior rectal artery , a branch of the internal pudendal artery.

Venous Drainage of Small and Large Intestines

The veins that drain blood from the small and large intestines largely parallel the arteries that supply each organ and share the same names. However, because the veins eventually drain to the hepatic portal vein and liver, there are some notable departures from the arterial scheme. The superior mesenteric vein drains the midgut organs and receives blood from the inferior pancreaticoduodenal , jejunal and ileal , ileocolic, and right and middle colic veins . These veins run parallel to the concordantly named arteries as they leave the superior mesenteric artery, although blood is flowing in the opposite direction from blood in the arteries. Because of its position near the superior mesenteric vein and the fact that there is no celiac vein, the right gastroomental vein drains to the right side of the superior mesenteric vein shortly before the latter drains into the hepatic portal vein. The artery of the same name is a branch of the gastroduodenal artery, which branches from the common hepatic artery and celiac trunk.

The jejunal and ileal veins conform in number and appearance of their arcades and straight arteries with those of their respective arteries. Lying, as a rule, to the right of the arteries, the veins from the small intestine extend from the duodenojejunal junction to the close vicinity of the ileocecal junction, where the anterior and posterior cecal veins and the appendicular veins unite to form the ileocolic vein . The first or first two jejunal veins frequently receive, either via a common trunk or as a separate vessel, an inferior pancreaticoduodenal vein running alongside the corresponding arteries. The venous drainage of the first part of the jejunum is often (again, in accord with the varying origins of the first jejunal arteries) not into the superior mesenteric vein but rather into either the anterior or posterior pancreaticoduodenal arcade. The venous pancreaticoduodenal arcades are fashioned in the same way as the arcades of the corresponding arteries. The posterior pancreaticoduodenal arcade, lying over the arterial arcade, is covered by a thin layer of connective tissue, composed of remnants of the fetal dorsal mesoduodenum, which may be readily seen during surgery when one is mobilizing the duodenum and the head of the pancreas.

Around the head of the pancreas, two venous arcades are formed (similar to the situation with the arteries), one anteriorly by the anterior superior and anterior inferior pancreaticoduodenal veins and the other posteriorly by the posterior superior and posterior inferior pancreaticoduodenal veins . Both inferior pancreaticoduodenal veins (the anterior and posterior veins) empty predominantly into the first and second jejunal veins (70%) or into the superior mesenteric vein (30%), either separately or via a common trunk. Deviating from the arterial arrangement, the posterior pancreaticoduodenal vein joins the portal vein directly behind the head of the pancreas, its entry point lying shortly ahead of that of the left gastric vein. The anterior superior pancreaticoduodenal vein joins the right gastroepiploic vein , which, after passing behind the first part of the duodenum, enters into the superior mesenteric vein at the pancreatic notch, shortly before the latter empties into the hepatic portal vein formed by the union of the superior mesenteric and splenic veins.

Starting in the region of the terminal ileum, the superior mesenteric vein first follows an oblique course and then a straight superior course, lying to the right of and somewhat anterior to the accompanying artery. In this way, both vessels describe a curve, with the convexity to the left. Both also cross anterior to the third portion of the duodenum, a fact worth remembering in cases of duodenal obstruction due to compression by the vessels, perhaps caused by the excessive weight of a neoplastic growth or weakness of the anterior abdominal wall.

The inferior mesenteric vein starts as a continuation of the superior rectal vein, which brings blood from the rectum and superior part of the anal canal. During its upward course, it receives venous blood from the rectosigmoid , sigmoid , and left colic veins , which drain the sigmoid colon and descending colon. All these tributaries follow closely the corresponding arteries, lying mostly to their left. Their anastomosing and arcade formations are the same as those described for the respective arteries. However, the main trunk of the inferior mesenteric artery lies to the right, where it branches from the abdominal aorta. Instead of paralleling it, the inferior mesenteric vein continues superiorly as it receives blood from the left colic and upper sigmoid veins, separating from the artery. The vein ascends anterior to the left psoas muscles, just to the left of the fourth portion of the duodenum. It continues behind the body of the pancreas to enter most frequently (in 38% of observed cases) the splenic vein . The splenic vein, in turn, combines with the superior mesenteric vein (at times, 3 to 3.5 cm from the union of the inferior mesenteric and splenic veins). Sometimes (29%), the inferior mesenteric vein enters the superior mesenteric vein, and at other times (32%), it joins the superior mesenteric vein and the splenic vein at their junction. In a few instances, a second inferior mesenteric vein has been found.

The splenic vein emerges from the hilus of the spleen as several fanlike veins that converge into a single large vessel. It has an average length of 15 cm and is never, unlike its accompanying artery, tortuous or coiled. It demonstrates a great number of different divisional patterns, which may include the short gastric veins and a superior polar splenic vein.

The portal vein and, especially, the variations of its tributaries are extremely important when considering a portocaval shunt for redirecting the portal blood flow in order to relieve or ameliorate the consequences of portal hypertension. It seems, therefore, appropriate to discuss in this volume variations that particularly involve the venous drainage of the intestine in which the superior or inferior mesenteric veins participate. The superior pancreaticoduodenal vein is sometimes (38%) only a single vessel but more frequently (50%) is duplicated, with one branch terminating in the portal vein and the other in the superior mesenteric vein. The gastroomental veins , corresponding to arteries of the same name, which are branches to the celiac arterial trunk, usually terminate (in 83% of cases) in the superior mesenteric vein but will, in some instances, join either the end of the splenic vein or the first part of the portal vein. The left gastric vein , also corresponding to an artery branching from the celiac trunk, can join the rest of the portal circulation at the superior aspect of the union of the splenic and superior mesenteric veins but has been found to join the portal vein directly in some instances, as well as the splenic vein distal to the just-mentioned junction. The right gastric vein , which is usually a small vein, typically terminates in the portal vein within 3 cm of its division into right and left branches, but it may also enter the base of the superior mesenteric vein or, far less frequently, the proximal segment of the right gastroomental vein or the inferior pancreaticoduodenal vein.

The veins serving the rectum and anal canal are the unpaired superior rectal veins , as well as the right and left middle rectal veins and the right and left inferior rectal veins . These vessels follow the same course as the arteries of the same name, but they return the blood into two different systems. The superior rectal vein drains blood into the portal system via the inferior mesenteric vein, whereas the middle and inferior rectal veins drain into the internal iliac vein and then the common iliac veins before entering the inferior vena cava. Blood in the inferior rectal veins follows a similar course, first draining to the internal pudendal vein before entering the internal iliac veins.

The contributions to the rectal veins begin in three venous plexuses situated in the walls of the rectoanal canal. The lowest of these plexuses, the external rectal plexus , lies in the perianal space, in the subcutaneous tissue surrounding the lower anal canal near the external opening of the anus. The internal rectal plexus is located in the submucosal space of the rectum superior to the pectinate line. These two plexuses are sometimes collectively referred to as the submucosal plexus or the superior and inferior submucosal plexuses . The third venous plexus surrounds the muscular wall of the rectum below its peritoneal reflection and is called the perimuscular rectal plexus , though some authors refer to it as the external rectal plexus, a term that leads to confusion with the first of the three plexuses described above. The perimuscular rectal plexus withdraws blood chiefly from the muscular wall of the rectum and evacuates the upper portion into the superior rectal vein, although the chief route of drainage of the perimuscular plexus is to the middle rectal veins.

The internal and external rectal plexuses serve the mucosal, submucosal, and perianal tissues. The former encompasses the rectal circumference completely, but the greatest aggregation of small and large veins is in the rectal columns (of Morgagni). Dilatation of the internal rectal plexus results in internal hemorrhoids, and dilatation of the external rectal plexus or thrombosis of its vessels results in external hemorrhoids. The two plexuses, the internal and external rectal plexuses, are separated by the internal anal sphincter as well as the dense tissue of the pecten, but they communicate with each other through these tissues by slender vessels; the vessels increase in size and number with age and are also more voluminous in the presence of hemorrhoids.

These connections between the external and internal rectal plexuses as well as the perimuscular plexuses constitute anastomoses between the inferior and superior veins and between the caval and portal venous systems. The significance of this situation is enhanced by the fact that the inferior and middle rectal veins and their collecting vessels, the internal pudendal veins, have valves, whereas the superior rectal vein is devoid of such valves, so that when the pressure in the portal vein rises, perhaps owing to portal hypertension, the circulation in the superior rectal vein may be reversed and portal blood may traverse the rectal plexuses and be carried away by the inferior rectal vein. This shunts portal blood via the internal iliac vein to the caval system. When this collateral venous circulation develops, the increased blood volume and pressure in the vessels dilate them to the extent that internal and/or external hemorrhoids may result.

In the absence of portal hypertension, spasms of the anal sphincter may also cause external hemorrhoids, because they may shut off the outflow of blood to the inferior rectal veins. Internal hemorrhoids, on the other hand, may develop when alterations (dilatation as well as constriction) occur within the apertures of the rectal wall through which branches of the internal rectal plexus pass.

Innervation of Small and Large Intestines

The nerves supplying the small and large intestines (and their vessels) contain both visceromotor (sympathetic and parasympathetic efferent) fibers and viscerosensory (afferent) fibers. Although nerve cells scattered through the entire alimentary tract and locally produced hormones can maintain some degree of intrinsic intestinal activity, it is the activity of the visceromotor and viscerosensory nerves that markedly affects the activity or quiescence of the intestines. Generally, increased sympathetic activity will constrict the blood supply to the intestines and decrease their activity. In contrast, increased parasympathetic activity will increase the rate of peristalsis and glandular secretion. Viscerosensory signals to either painful or nonpainful stimuli prompt the central nervous system to modulate visceromotor activity.

Although the visceromotor and viscerosensory sys­tems operate as complex, continuous-feedback loops, we can describe the hypothalamus as a source and ter­minus of pathways concerned with visceral activities. The hypothalamus is a small region of the central nervous system located superior to the pituitary gland and anteroinferior to the thalamic nuclei. It is primarily concerned with regulating the body's homeostatic drives by adjusting the body temperature, appetite, and blood flow, as well as the level of aggression. It has extensive cortical connections with the premotor areas of the frontal cortex, the cingulate gyrus, and the orbital surfaces of the frontal lobes. It sends axons to many structures; for our purposes, the most important are the reticular formation, dorsal motor nucleus of the vagus, and specific regions of the thoracic, lumbar, and sacral levels of the spinal cord. The nuclei of the reticular formation also project to the dorsal motor nucleus of the vagus and spinal cord, and these are the distinct locations where visceromotor (both parasympathetic and sympathetic) impulses to the organs are generated. We will go over the differences between the two systems shortly, but first let us examine their similarities.

Visceromotor activity occurs along a two-neuron chain. The preganglionic nerve cell body is always found in the central nervous system, and it projects an axon to reach a second nerve cell body somewhere in the body. The preganglionic axon synapses with this preganglionic cell, which projects another axon to reach the target structures that are actually being innervated, such as smooth muscle or glandular cells.

Preganglionic sympathetic nerve cell bodies are located in the gray matter of the spinal cord in an area between the anterior and posterior horns, the intermediolateral column (nucleus), which stretches from the first thoracic segment of the spinal cord inferiorly to the second or third lumbar segment of the spinal cord. The preganglionic sympathetic nerve cells that spe­cifically innervate the small and large intestines arise, respectively, from segments T8-T10 and T10-L2/L3 of the intermediolateral column. The preganglionic sympathetic neurons from each segment of the intermediolateral column send their axons to exit the spinal cord via the anterior rootlets , which coalesce to form an anterior root from each segment. The anterior roots, and the preganglionic sympathetic axons within them, join posterior roots to form the spinal nerve , which leaves the spinal canal and traverses the intervertebral foramen. The spinal nerve splits to form posterior and anterior rami, which innervate somatic structures of the back and the trunk and limbs, respectively. The preganglionic sympathetic axons travel briefly into the anterior rami before leaving as a separate structure, the white rami communicans , which connects to a nearby longitudinal string of nerve cell bodies, the sympathetic chain (sympathetic trunk and paravertebral ganglia). The preganglionic sympathetic axons that innervate the sweat glands, arrector pili, and precapillary sphincters of the somatic body will synapse with cells in each of these sympathetic chain ganglia; however, the preganglionic sympathetic axons that innervate abdominal organs, such as the small and large intestines, pass through these ganglia without synapsing. These axons leave the medial aspect of the sympathetic chain ganglia as thoracic, lumbar, or sacral splanchnic nerves . Although there is considerable variation in the exact branching pattern, generally the preganglionic sympathetic axons of T5-T9 combine to form greater thoracic splanchnic nerves on each side. Similarly, the T10 and T11 contributions fuse to form the lesser thoracic splanchnic nerve and the T12 axons form the least thoracic splanchnic nerve . These thoracic splanchnic nerves travel medially and inferiorly on each side of the thoracic vertebral bodies before passing posterior to the diaphragm and entering the abdomen to reach postganglionic ganglia. The lumbar splanchnic and sacral splanchnic nerves leave the sympathetic chain ganglia in the lumbar and sacral regions but do not merge; instead, they travel medially to reach their ganglia.

The targets for all of these splanchnic nerves are the prevertebral ganglia that lie anterior to the abdominal aorta and vertebral column. Each of these ganglia is a collection of postganglionic sympathetic nerve cell bodies that will send their axons to the target tissues of the abdominal organs. The members of this group are the celiac, aorticorenal, superior mesenteric, and inferior mesenteric ganglia . There are additional postganglionic sympathetic nerve cell bodies scattered throughout the intermesenteric plexus (located between the superior and inferior mesenteric ganglia), as well as the superior hypogastric and inferior hypogastric plexuses , located within the pelvis. The celiac ganglion gives off axons that contribute to the celiac plexus , which innervates the foregut organs (distal esophagus, stomach, proximal duodenum, pancreas, spleen, liver, and gallbladder). Similarly, the aorticorenal ganglia on the right and left contribute nerves to the aorticorenal plexus , supplying targets in the suprarenal gland, kidney, proximal ureters, and gonads. The superior mesenteric and inferior mesenteric ganglia contribute axons to the superior and inferior mesenteric plexuses , which supply the midgut (distal duodenum, part of the pancreatic head, jejunum, ileum, cecum, appendix, ascending colon, and transverse colon) and hindgut (descending colon, sigmoid colon, and rectum), respectively. The intermesenteric plexus connects the superior and inferior mesenteric ganglia and is composed of visceral afferent axons passing between the two, as well as lumbar splanchnic nerves contributing preganglionic sympathetic axons to the abdominal organs. Postganglionic sympathetic nerve cells may be found within the intermesenteric plexus. Extending inferiorly from the inferior mesenteric ganglion is the superior hypogastric plexus . It is similar to the intermesenteric plexus but also carries a large number of preganglionic parasympathetic axons. The superior hypogastric plexus extends inferiorly and splits into two bundles, the right and left hypogastric nerves , which pass into the true pelvis and terminate as the left and right inferior hypogastric plexuses . The inferior hypogastric plexus consists of preganglionic sympathetic axons from the sacral splanch­nic nerves, postganglionic sympathetic nerve cell bodies, preganglionic parasympathetic axons, and viscerosensory axons. It wraps around the pelvic organs and may be referred to as having subplexuses such as the rectal, vesical, prostatic, vaginal, and uterine plexuses .

Although there is variation in their targets, the sym­pathetic splanchnic nerves tend to follow a certain pattern. The greater thoracic splanchnic nerves contribute preganglionic sympathetic axons to synapse with postganglionic nerve cells in the celiac and superior mesenteric ganglia. Thus, they innervate foregut and midgut organs. The lesser thoracic splanchnic nerves contribute preganglionic sympathetic axons to synapse with postganglionic nerve cells in the aorticorenal and superior mesenteric ganglia. Therefore, they innervate midgut structures, gonadal structures, and structures associated with the kidneys. The lumbar splanchnic nerves contribute to the superior mesenteric and inferior mesenteric ganglia, as well as the intermesenteric and superior hypogastric plexuses. Thus, the lumbar splanchnic nerves innervate midgut and hindgut organs and may contribute to the innervation of other pelvic organs. The preganglionic sympathetic axons in the sacral splanchnic nerves exit the sympathetic chain ganglia and enter the inferior hypogastric plexus. They may synapse with postganglionic nerve cells within it or ascend to the superior hypogastric plexus and synapse there. The sacral splanchnic nerves supply postganglionic sympathetic axons to the distal rectum and other pelvic organs.

In regard to parasympathetic efferent activity, descending fibers arising from the anterior region of the hypothalamus synapse with cells in the dorsal vagal motor nuclei in the medulla oblongata and also with cells in the second to fourth sacral segments of the spinal cord. The dorsal vagal motor nucleus contributes preganglionic parasympathetic axons to the left and right vagus nerves , which leave the jugular foramen to innervate thoracic and abdominopelvic organs. We will ignore the activity of the vagus nerve in the thorax other than to mention that the left and right vagus nerves are closely associated with the esophagus and interweave to produce the anterior and posterior vagal trunks , which pierce the diaphragm alongside the esophagus to enter the abdomen. The anterior vagal trunk travels on the anterior aspect of the stomach, across the hepatogastric ligament , to innervate some of the liver and gallbladder. When the trunk reaches the target organ, it synapses with postganglionic parasympathetic nerve cell bodies located within its wall. The posterior vagal trunk moves further posteriorly from the esophagus to run into the nearby celiac ganglion. In contrast to the preganglionic sympathetic axons entering the celiac ganglion, the preganglionic parasympathetic axons do not synapse there but instead pass through the ganglion to enter the celiac plexus . From there, these axons, alongside the postganglionic sympathetic axons from the celiac ganglion and the viscerosensory axons, travel along branches of the celiac trunk to reach the foregut organs. Once the celiac plexus reaches the foregut organs, the preganglionic parasympathetic axons within it synapse with postganglionic parasympathetic nerve cells within the wall of the organ.

Preganglionic parasympathetic axons from the posterior vagal trunk also pass inferiorly through the celiac ganglion to reach the aorticorenal and superior mesenteric ganglia. As before, the preganglionic parasympathetic axons do not synapse in these ganglia but instead join the aorticorenal and superior mesenteric plexuses, which follow the blood supply from the aorta to reach each of the target organs. In the case of the intestines, preganglionic parasympathetic axons from the posterior vagal trunk pass through the superior mesenteric ganglion to join postganglionic sympathetic and viscerosensory axons within the superior mesenteric plexus . The superior mesenteric plexus follows the branching of the superior mesenteric artery along its course through the mesentery and can be subdivided into plexuses named for each artery : inferior pancreaticoduodenal, middle colic, jejunal, ileal, ileocolic, appendicular, and right colic plexuses , which innervate the organs supplied by each vessel. The preganglionic parasympathetic axons synapse with a series of postganglionic parasympathetic nerve cell bodies that are scattered throughout the walls of the organs. These nerve cells make up the enteric nervous system .

The vagus nerve supplies preganglionic parasympathetic axons that innervate structures of the thorax, foregut, and midgut, but the hindgut and other pelvic organs receive preganglionic parasympathetic axons from nerve cell bodies located within the sacral spinal cord. These cells project axons through the anterior rootlets, anterior roots, spinal nerves, and anterior rami of levels S2-S4. Once the anterior rami have exited the anterior sacral foramina, the preganglionic parasympathetic axons leave as pelvic splanchnic nerves and join the nearby right and left inferior hypogastric plexuses. Some of these axons form synapses in minute ganglia located near, or in the wall of, the pelvic structures (distal colon, rectum, bladder, and external and internal genital organs), to which they carry the impulses for visceromotor activity, vasodilatation, and glandular secretion. These preganglionic parasympathetic axons may travel within the inferior hypogastric plexuses to synapse with postganglionic nerve cells in the pelvic organs and distal rectum. They may also ascend through the right and left hypo­gastric nerves to reach the superior hypogastric plexus and then the inferior mesenteric ganglion. Preganglionic parasympathetic axons pass through the inferior mesenteric ganglion without synapsing and join the inferior mesenteric plexus alongside postganglionic sympathetic axons and visceral afferent axons. The inferior mesenteric plexus follows the branches of the inferior mesenteric artery, branching into left colic, sigmoid, and superior rectal plexuses , depending on the artery that carries the axons. The preganglionic parasympathetic axons synapse with a series of postganglionic parasympathetic nerve cell bodies that are scattered throughout the walls of the organs. These nerve cells form the enteric nervous system, and the axons of these ganglionic cells become the postganglionic parasympathetic axons , which, together with the postganglionic sympathetic axons in the plexus, innervate the smooth muscle of the intestinal wall, the vessels supplying the intestines, and the intestinal glands.

The enteric nervous system stretches through the entire length of the alimentary tract from the esophagus to the rectum. This plexus consists of small groups of nerve cells interconnected by networks of fibers, and it is subdivided into the myenteric (Auerbach) and submucosal (Meissner) plexuses. The myenteric plexus is found between the longitudinal and circular layers of the muscularis externa. The main plexus, or primary plexus, gives off fibers that form finer secondary plexuses and even finer tertiary plexuses, which ramify both within and between the adjacent layers of muscle. Some fibers from the longitudinal intramuscular plexus enter the subserous tissue and constitute a rarefied subserous plexus. The submucosal plexus is located at the interface of the submucosal and muscularis externa layers. It is also subdivided into superficial and deep parts. Fibers from the deep part enter the mucosa, where they form delicate periglandu­lar plexuses. The name of each subdivision of the enteric nervous system describes its location and histologic appearance. These distinctions are somewhat arbitrary, since all parts of the enteric nervous system are interconnected and form an exceedingly complex, self-regulating network. Sympathetic input to the enteric nervous system tends to restrict blood flow and slow the activity of the alimentary tract. Conversely, parasympathetic input tends to increase smooth muscle contraction and peristalsis, as well as glandular secretion into the tract.

Viscerosensory activity related to the small and large intestines falls into two broad categories, visceral pain and normal visceral reflexive stimuli. The intestines are insensitive to ordinary tactile, painful, and thermal stimuli, although they respond to tension, ischemia, and chemical irritations with visceral pain. The free nerve endings of visceral pain axons extend from the intestines and join the plexus related to their blood supply in a retrograde fashion. For example, visceral pain from the jejunum would be carried back to the central nervous system by axons traveling along the jejunal plexus and the superior mesenteric plexus. Thereafter, visceral pain fibers travel in a retrograde fashion along the sympathetic innervation of the target organ. So in the case of visceral pain from the jejunum, after the axon has exited the superior mesenteric plexus, the axon continues along the greater or lesser thoracic splanchnic nerves and through the sympathetic chain ganglia, white rami communicans, anterior ramus, and spinal nerve. At this time, being afferent, the axon travels along the posterior root to reach the spinal cord. Prior to reaching the spinal cord, the axon encounters (but does not synapse within) its nerve cell body. The nerve cell bodies of these viscerosensory axons are located in the posterior (dorsal) root ganglia. Because these nerve cells are pseudounipolar, their axons extend from the target tissue to reach the cell body but also proximally to reach the posterior gray horn of the spinal cord.

Nonpainful, reflexive stimuli from the small and large intestines travel in a retrograde manner along the parasympathetic innervation of each organ. For this reason, a stimulus can follow two pathways. Foregut and midgut organs receive their preganglionic parasympathetic innervation via the vagus nerves; therefore, reflexive visceral afferents from these organs contribute to the bulk of the vagus nerves as they ascend to reach the brainstem and then project to the inferior aspect of the solitary nucleus. The cell bodies for these axons are located in the inferior vagal ganglion , which is near the point at which the vagus nerves exit from the right and left jugular foramina. Hindgut organs do not receive parasympathetic input from the vagus nerves but rather from the pelvic splanchnic nerves. So in this case, reflexive afferent inputs from the hindgut project along the inferior mesenteric plexus, to the superior hypogastric plexus, right and left hypogastric nerves, right and left inferior hypogastric plexuses, pelvic splanchnic nerves, anterior rami of S2-S4, spinal nerves of S2-S4, posterior rami of S2-S4, and, finally, sacral spinal cord. As before, the cell bodies of these afferent axons are located in the posterior root ganglia of S2-S4, and their smaller, proximal extensions project to the posterior horn of the sacral spinal cord.

An exception to this pattern occurs in abdominopelvic organs that are entirely covered by the peritoneum. In these organs (distal rectum, inferior aspect of the bladder, inferior aspect of the uterus, cervix, prostate, and seminal vesicles), both visceral pain and normal reflexive stimuli follow the parasympathetic pathways back to the sacral spinal cord. Once in the spinal cord, the exact course followed by visceral afferent pathways within the central nervous system is very complex.

There is evidence that these pathways behave in a fashion similar to that of the somatic pathways. Some of the visceral axons that enter the cord through the posterior roots form synapses with cells of the posterior horn near their level of entry, and the impulses are conveyed superiorly through tracts that lie near, or are commingled with, the anterolateral system. Other fibers ascend in the posterior white columns and may relay at higher levels in the brainstem. Some of these afferent visceral fibers, as well as the nonpainful reflexive inputs to the solitary nucleus, may pass with somatic fibers to the thalamus and be relayed onward to the postcentral gyrus. Other fibers form synapses in the hypothalamus , from whence fibers project to the premotor areas of the frontal cortex, the orbital surfaces, and the cingulate gyrus. In actuality, many of these hypothalamocortical connections are via relays in the anterior and medial nuclei of the thalamus. Possibly, the hypothalamus plays the same role in visceral afferent pathways as does the thalamus in their somatic counterparts. (To indicate the fact that the hypothalamocortical as well as the corticohypothalamic connections follow similar routes, the lines representing these pathways have arrows on both ends.)

Finally, the innervation of the anorectal transitional region combines features of the autonomic innervation of the rectum and the somatic innervation of the anus and perineum. This area develops from the anal pit, an invagination of the skin and surrounding mesoderm that meets the developing hindgut, forming a continuous tube as the membrane between the two regions ruptures. The two regions retain their distinct epithelium, innervation, and blood supply, transitioning abruptly in the region of the pecten. The portions of the rectum that are covered by peritoneum are innervated by postganglionic sympathetic axons from the inferior mesenteric ganglia and plexus as well as preganglionic parasympathetic axons that will synapse on postganglionic nerve cells in the wall of the rectum, the myenteric and submucosal plexuses. These plexuses supply the involuntary smooth muscles of the rectum and the internal anal sphincter. They dwindle as one progresses inferiorly through the anal region and are entirely absent below the intersphincteric groove (white line of Hilton) within the anal canal. The voluntary external anal sphincter is innervated mainly by somatomotor branches of the inferior anal nerves , branches of the pudendal nerve. These same nerves also carry the somatosensory axons from this region of the anal canal. Therefore, below the pecten this innervation resembles that of the perineal skin and is very sensitive to tactile, painful, and thermal stimuli, whereas the upper part of the anal canal is almost insensitive to such stimuli but responds readily to alterations in tension or ischemia. From a practical point of view, this neuroanatomic situation explains why an anal fissure is so painful and why, in injections for internal hemorrhoids, the puncture is scarcely felt if the needle is inserted through the mucosa.

Secretory, Digestive, and Absorptive Functions of Small and Large Intestines

The purpose of the complex enzymatic reactions to which foodstuffs are exposed within the intestinal lumen is to prepare nutrients for transfer into and assimilation within the organism. The lumen of the digestive system, which is the space encompassed by the wall of the digestive tube, belongs, fundamentally speaking, to the outside world, and the processes by which the products of digestion enter and pass through the intestinal wall into the circulation are called secretion and absorption, respectively. The mucosa of the small intestine throughout its length is lined by cells involved with both secretion and absorption: mucus-secreting cells, neuroendocrine cells, and immune active cells. The incredible efficiency of intestinal function is emphasized by the fact that of the approximately 8 L of fluid that enters the small intestine, only 100 to 200 mL is excreted from the rectum, for an efficiency rate in excess of 98%. In disease states, the large and small intestines absorb even more fluid, sometimes exceeding 25 L per day. Alternatively, in secretory disorders and infection, the volume of diarrhea lost may rapidly pose a life-threatening risk of dehydration, with the loss of many liters of fluids and their accompanying electrolytes.