Colonic Physiology
While the primary role of the colon is to absorb excess water and create solid stool, it also plays a critical role in electrolyte homeostasis and immunity. The chapter serves to illustrate the basic tenets in colonic embryology as well as define the hist
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Key Concepts • Colonic innervation is supplied by both extrinsic and intrinsic pathways. The extrinsic pathways are derived from the autonomic nervous system including parasympathetic and sympathetic routes. Parasympathetic input is excitatory while sympathetic input is inhibitory to colonic motor function. The intrinsic colonic nervous system consists of the myenteric plexus. • Short chain fatty acids are produced by the colon as a result of the fermentation of complex carbohydrates by colonic flora. The SCFA, butyrate, is the primary energy source of the colon. • The colon absorbs sodium and water and secretes bicarbonate and potassium. Aldosterone mediates the process of active sodium absorption in the colon. • Colonic contractile events are divided into (1) segmental contractions and (2) propagated contractions (including low-amplitude and high-amplitude propagating contractions, LAPC and HAPC, respectively). The main function of HAPC is to propagate colonic contents towards the anus. • The Interstitial cells of Cajal (ICC) are the primary pacemaker cells governing the function of the enteric nervous system.
Introduction The colon plays a central role in gastrointestinal (GI) physiology. There are multiple functions that the colon and rectum serve. The primary role of the colon is one of absorption of excess water and electrolytes, serving to salvage valuable fluid and unabsorbed nutrients as well as to create solid stool. It also plays a central role in bacterial homeostasis, serving as a home to billions of commensal bacteria whose role is symbiotic in maintaining the health of the colonic epithelium. The rectum has evolved complicated and elegant mechanisms to store feces and accommodate it while
allowing for the selective egress of stool or gas. Understanding the physiologic and histologic components of the colon and rectum are critical to understanding normal and pathologic states.
Embryology Understanding the embryology of the colon and rectum provides essential information for understanding its function. During the third and fourth weeks of gestation, the primitive gut arises from the cephalic caudal and lateral foldings of the dorsal endoderm lined yolk sac. The mucosa arises from the endodermal layer, however the muscular wall, connective tissue and outer serosal surface arises from the mesodermal layer. By the fourth week of gestation, three distinct regions have differentiated based on their blood supply. The midgut, supplied by the superior mesenteric artery, begins distal to the confluence of the common bile duct in the third portion of the duodenum and includes the proximal two-thirds of the transverse colon. This portion of the intestine maintains a connection to the yolk sac via the vitelline duct. Absence of its obliteration results in a Meckel’s diverticulum. The hindgut, which comprises the rest of the distal GI tract, includes the distal transverse colon, descending colon, sigmoid colon, and rectum. This is supplied by the inferior mesenteric artery (IMA). During the fifth week of gestation,
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