Neurovascular Supply of the Large Intestine
The large intestines (distal midgut and hindgut) mark the beginning of the terminal segment of the alimentary canal. It is divided into the cecum, vermiform appendix, and the ascending, transverse, descending and sigmoid colons. At this level of the digestive tract, a significant amount of fluid absorption takes place as waste material is prepared for excretion. Vast networks of anastomosing blood vessels supply the tract. The lymphatic return from the large intestines is screened at five different levels before returning to systemic circulation. Additionally, the distal midgut and hindgut receive both extrinsic and intrinsic neuronal stimulation in order to carry out their daily function.
The large intestine is a part of the midgut (from the ileocecal junction, the cecum and vermiform appendix to the proximal ⅔’s of the transverse colon) and the hindgut (from the distal ⅓ of the transverse colon to the sigmoid colon). The midgut region receives arterial supply from the superior mesenteric artery and the hindgut is supplied by the inferior mesenteric artery. As the aorta enters the abdomen through the abdominal hiatus of the diaphragm, it gives off two branches from its anterior surface – the celiac trunk and the superior mesenteric artery. The superior mesenteric artery branches from the abdominal aorta at the inferior border of the L1 vertebra, about 1 cm below the celiac trunk. The artery travels inferolaterally towards (but not reaching) the right iliac fossa.
The superior mesenteric artery gives off the inferior pancreaticoduodenal artery posteriorly, the middle colic, right colic and ileocolic arteries from the right side and several anastomosing jejunoileal branches from the left side. The middle colic artery is the second to leave the superior mesenteric artery. It travels through the substance of the transverse mesocolon and splits into a right and a left branch at the intestinal boundary of the transverse mesocolon. The right and left branches of the middle colic artery forms anastomoses with the ascending branch of the right colic artery and the ascending branch of the left colic artery, respectively. The middle colic artery goes on to supply the colon from the right colic (hepatic) flexure, along the transverse colon almost to the left colic (splenic) flexure. The right colic artery typically arises independently from the superior mesenteric artery; but it may also be given off with the ileocolic artery. It runs almost horizontally to the right side, where it eventually divides into ascending and descending branches. The descending division of the right colic artery anastomoses with the superior branch of the ileocolic artery. The right colic artery supplies the ascending colon above the cecum to the right colic flexure.
The ileocolic artery also arises from the right side of the superior mesenteric artery and travels towards the right iliac fossa. It gives a superior (colic) and an inferior (ileal) branch. The inferior ileocolic branch travels to the ileocolic junction, where it gives of an anterior and a posterior caecal artery, and an appendicular artery before continuing to the left and anastomosing with the terminal segment of the superior mesenteric artery. Each branch of the ileal branch of the ileocolic artery supplies the anatomical section from which it is named.
The remainder of the colon – from the distal transverse colon to the rectosigmoid junction – receive arterial supply from the inferior mesenteric artery. It arises from the left anterior part of the abdominal aorta, about 4 cm superior to the aortic bifurcation and facing the L3 vertebra. The left colic artery, sigmoid arteries and superior rectal artery are all branches of the inferior mesenteric trunk. The ascending and descending branches of the left colic artery arise after a relatively short superolateral course. In addition to its anastomosis with the left branch of the middle colic artery, the ascending left colic artery also anastomoses with the descending left colic artery. The descending left colic artery travels inferolaterally to anastomose with the highest sigmoid artery. The sigmoid arteries are a series of about four loops of vessels resting in the sigmoid mesocolon that anastomose with each other; the last of which anastomoses with the superior rectal artery. Finally, the vessels that course parallel to the colon (branches of the arteries mentioned earlier) are often referred to marginal arteries. These marginal arteries give off arteriae rectae (straight arteries) which supplies the colon directly.
Conveniently, the tributaries to the superior and inferior mesenteric veins adapt the nomenclature of the arteries they accompany. The superior mesenteric vein lies just to the right of its artery. The relatively large trunk receives deoxygenated blood from the ileocecal junction to the proximal 2/3 of the transverse colon via the right colic, ileocolic and middle colic veins.
The superior rectal vein travels superiorly to the left of its artery. Above the level of the pelvic brim, its name changes to the inferior mesenteric vein. Tributaries to the vein also bear the names of the arteries they follow. The inferior mesenteric vein travels superiorly, then medially, to meet with (and terminate in) the splenic vein posterior to the pancreas. The splenic vein continues to enter the superior mesenteric vein. It should be noted that above the point of entry of the splenic vein, the vessel is referred to as the hepatic portal vein. The blood is then processed in the liver then returned to systemic circulation.
The large intestine is innervated by intrinsic and extrinsic sources. The vagus nerve (CNX) enter the abdominal cavity via the oesophageal hiatus of the diaphragm to provide parasympathetic innervation to the large intestines. The pelvic splanchnic nerves (S2-4) also contribute to the large intestines’ parasympathetic supply. The parasympathetic fibers are responsible for increasing secretomotor activity along this segment of the digestive tract. The vagus nerve fulfil this role in the gut to the point of the transverse colon, while the pelvic splanchnic nerves carry on this function from the left colic flexure onwards. The T10-L2 thoracolumbar outflow of sympathetic fibers are responsible for the inhibitory activity along the large intestines. They form synapses at the superior and inferior mesenteric, and the inferior hypogastric plexuses. The superior mesenteric plexus provides sympathetic innervation to the cecum, appendix, ascending and transverse colon (near to the left colic flexure), while the inferior mesenteric plexus innervates the colon from the left colic flexure to the rectum. The inferior hypogastric plexus also innervates the rectum.
In addition to the extrinsic nerve supply to the gut, there are networks of nerve fibers occupying space between the longitudinal and circular muscle layers (myenteric plexus of Auerbach), and in the submucosal layer (submucosal plexus of Meissner). In addition to Auerbach’s and Meissner’s plexuses, there are additional intrinsic plexuses that collectively form the enteric nervous system. Although these networks receive postganglionic inhibitory and preganglionic excitatory fibers, they are fully functional in the absence of those contributions.
In the mucosa of the large intestines, isolated aggregates of lymphatic tissue are distributed along its length as a first line of filtration. These clusters of lymphoid tissue are referred to as mucosa-associated lymphoid tissue (MALT). The lymphatic aggregates are arranged in a tonsillar manner within the appendix. Throughout the large intestines, the lymph nodes are arranged in four general groups. The first group is epicolic nodes that rest on the outer surface of the intestinal wall. The second is paracolic nodes that lie along the intestinal margin. Thirdly, there are intermediate nodes that lie along (and conveniently take the names of) the arterial branches of the superior and inferior mesenteric arteries. Finally, there are the preaortic nodes that are found at the branching point of the coeliac trunk and the superior and inferior mesenteric arteries.
Ischemic colitis resulting from atherosclerosis of main gut arteries is a major concern for senior patients. Patients typically present with bleeding from the rectum, changes in bowel patterns and abdominal pain. It is very difficult to differentiate between ischemic colitis and other inflammatory processes that affect the colon (i.e. infections, ulcers and Crohn’s disease). Subsequent to the ischemia, necrosis of the affected sections is likely. Because the preliminary symptoms typically resolve by themselves, prompt surgical management is not always required.
A more common vascular complication in the Western world is hemorrhoids (pile). Hemorrhoids occur when the submucosal venous plexus (hemorrhoidal) are over dilated. Ensuing imperfections in the vascular walls will predispose the individual to bleeding while lifting heavy objects or straining to pass stool. Piles can be classified as either internal hemorrhoids, if the distension occurs in the (superior hemorrhoidal) venous plexus above the pectinate line, or as external hemorrhoids if it is from the (inferior hemorrhoidal) venous plexus below the line. Fortunately, the piles can be reduced surgically and the associated itching and burning managed medically.