Colon Anatomy and Physiology

1. Colon Anatomy and Physiology 7/21/2010

2. Cecum • Blind pouch below the entrance of the ileum • Almost entirely invested in peritoneum • Mobility limited by small mesocecum • Ileum enters posteromedially • Angulation maintained by superior and inferior ileocecal ligaments • Three pericecal recesses or fossae • Superior, inferior, retrocecal

3. Ileocecal valve • Valve de Bauhin • Ileocecal sphincter • Slight thickening of muscular layer of terminal ileum • Relaxes in response to food in the stomach • Competence • Regulates ileal emptying • Angulationplays a role in prevention of reflux

4. Appendix • Vermiform appendix • Elongated diverticulum from posteromedial cecum about 3.0 cm below ileocecal junction • Mean length 8-10cm, approx 5 mm diameter • Mesoappendix contains vessels • 85-95% posteromedial toward ileum • Also can be retrocecal, pelvic, subcecal, pre-ileal, and retro-ileal

5. Ascending colon • 15 cm long, from ileocecal junction to right colic or hepatic flexure • Retroperitoneal • Covered anteriorly and on both sides, not posteriorly • Jackson’s membrane • Adhesions between right abd wall and anterior colon • Hepatic flexure supported by nephrocolic ligament

6. Transverse colon • 45 cm long • Intraperitoneal • Greater omentum fused on anterosuperior aspect • Splenic flexure angle attached to diaphragm by phrenocolic ligament • More acute, higher, and more deeply situated than hepatic flexure

7. Descending colon • 25 cm • Retroperitoneal • Narrower and more dorsally situated than ascending colon

8. Sigmoid colon • 35-40 cm long • Mobile, omega shaped loop • Intraperitoneal • Mesosigmoid attached to pelvic walls in inverted V, resting in intersigmoid fossa • Left ureter immediately below, crossed anteriorly by spermatic, left colic and sigmoid vessels

9. Rectosigmoid junction • Last 5-8 cm of sigmoid and upper 5 cm of rectum • Tinea libera and tinea omentalis fuse and where haustra and mesocolon terminate • 6-7 cm below sacral promontory • Narrowest portion of large intestine • Functional sphincter

10. Blood supply • Superior mesenteric artery (midgut) • Supplies cecum, appendix, ascending colon, proximal 2/3 of transverse colon • Middle, right and ileocolic branches • Inferior mesenteric artery (hindgut) • Supplies distal 1/3 of transverse, descending, sigmoid • Left colic and 2-6 sigmoidal arteries • Becomes superior hemorrhoidalafter crosses left common iliac • Venous drainage follows arterial supply

11. Collateral circulation • Marginal artery of Drummond • Griffiths’ critical point • Sudeck’s critical point • Arc of Riolan • Meandering mesenteric artery • Presence indicates severe stenosis of SMA or IMA

12. Colonic Physiology • Not an essential organ, but has a major role in maintaining health of the body • Extrensic nervous component from autonomic system • Affects motor and sensory • Parasympathetics are excitatory • Motor component through acetylcholineandtachykinins (substance P) • Visceral sensory function • Sympathetic input is inhibitory to colonic peristalsis • Excitatory to sphincters • Inhibitory to non-sphincteric muscle • Mediated by alpha-2 adrenergic receptors • Agonists relax the tone

13. Colonic Physiology • Intrinsic nervous component is enteric nervous system • Mediate reflex behavior independent from brain or spinal cord • Neuronal plexuses in myenteric and submucosal/mucosal layers • Myenteric plexus regulates smooth muscle function • Submucosal plexus modulates mucosal ion transport andabsorptive functions • Acetylcholine, opioids, norepinephrine, serotonin, somatostatin, cholecystokinin, substance P, VIP, neuropeptide Y, and nitric oxide are important neurotransmitters

14. Salvage, Metabolism, and Storage • More than 400 different species of bacteria, most anaerobes • Feed on mucous, residual proteins, complex carbs • Fermentation of carbs produces short chain fatty acids • Acetate, propionate, butyrate • Occurs in rightandproximal transverse colon • Proteins are broken down into SCFAs, branched chain FAs, ammonia, amines, phenols, and indols • Become a nitrogen source for bacterial growth

15. Short Chain Fatty Acids • Butyrate • Least amount produced • Primary energy source for colonocytes • Role in cell proliferation and differentiation • Important in absorption of water and salt • Propionate • Combines with 3 carbon compounds in liver for gluconeogenesis • Acetate • Most abundantly produced • Used to synthesize longer-chain FAs by liver • Energy source for muscle

16. Salvage, Metabolism, and Storage • Proximal colon • More saccular • Acts as a reservoir • Fluid moves through quickly, solid material slower • Principal site for SCFAproduction • Distal colon • More tubular • Acts as a conduit • Protein degredation • Haustral segmentation facilitates mixing, retention of luminal material, formation of solid stool

17. Transport of Electrolytes • Presented 1-2 L of water/day • Absorbs 90% • Only100-150mLeliminated in stool • Can increase to 5-6 L/day when challenged • Important in recovery of salts • Absorbs sodium and chloride • Sodium absorbed against concentration and electrical gradients • Secretes bicarb and potassium

18. Transport of Electrolytes • Chloride is exchanged for bicarb • Secreted into lumen to neutralize organic acids produced • Occurs at luminalborderof mucosal cells • Potassium movement is passive secondary to active absorption of sodium • Active secretion may occur in distalcolon • Coupled with potassium in bacteria and mucous in stool, may explain relatively high concentration of K+ in stool • Secretes urea • Metabolized to ammonia • Majority is absorbed passively

19. Transport of Electrolytes • Aldosterone enhances fluid and sodium absorption • SCFAs are principle ions and stimulate sodium absorption • Absorption of water and salt occurs primarily in ascending and transverse colon • Active transport of sodium creates osmotic gradient and water passively follows • Surface mucosal cells responsible for absorption • Crypt cells involved in fluid secretion

20. Peristalsis • Waves of alternate contraction and relaxation that propel contents, contractile events • No cyclic motility • Segmental contractions, either single or bursts of contractions, rhythmic or arrhythmic • Propagated contractions • Allows slow transit and opportunity for contents to maximally contact mucosal surface • Low-amplitude propagated contraction (LAPC) • Long spike bursts • Related to meals and sleep-wake cycles, passage of flatus

21. Peristalsis • High-amplitude propagated contraction (HAPC) • Migrating long spike bursts • Equivalent of mass movement • Move large amounts of stool toward the anus • Approx 5 times daily • Haustra are static and partially occluding • Disappear with peristalsis • Correspond with mass movement

22. Cellular Basis for Motility • Circular muscle • Longitudinal muscle • Interstitial cells of Cajal (ICC) • Pacemaker cells • Regulation of motility • Electrically active, create ion currents • Basal pathway for slow waves between circular and longitudinal muscle • All electrical activity dependent on stimulation by stretch or chemical mediation • Critical volumes of distention needed for propulsion

23. Colonic Motility • Exhibits circadian rhythm • Decreased activity at night • Increase in activity after waking and after meals (HAPCs) • Regional differences in pressure activity • Transverse and descending have more activity during the day • Rectosigmoid most active at night • Women have less activity in transverse and descending colon • Stress influences function • Induces prolonged propagated contractions

24. Colonic Motility • Right and transverse colon are major sites of solid stool storage • Remains in right colon for extended periods to allow for mixing • Gastrocolic reflex • Immediate increase in tonic contraction of proximal colon after a meal • Unknown mediator • CCK • Well know colonic stimulator • Increases colonic spike activity in a dose-dependent manner • Possible postprandial stimulator

25. Defecation • Process begins up to an hour before—a preexpulsive phase • Increased propagating and nonpropagating activity in the entire colon • May propel stool to distal colon and stimulate afferent nerves • 15 min before defecation, second phase increases sensation of the urge to defecate through propagating sequences • Associated with at least one high amplitude HAPC

26. Modulation of Visceral Sensation • Enteroenteric reflexes mediated by spinal cord • Alters smooth muscle tone, increasing or decreasing activation of nerve endings in gut or mesentery • Direct central modulation of pain • Through descending noradrenergic and serotonergic pathways from the brainstem • Referred pain • Overlap of input from visceral structures perceived as being from somatic structures • Same embryonic dermatome • Visceral sensation can relay via collaterals to reticular formation and thalamus • Changes in appetite, affect, pulse, blood pressure through autonomic, hypothalamic, and limbic systems

27. Constipation • Infrequent or hard to pass stools • Dietary, pharmacologic, systemic, or local causes • Seen more frequently in sedentary people • Idiopathic slow transit constipation • Altered colonic motor response to eating, impaired or decreased HAPCs • Reduced or absent propulsive activity • Not helped by fiber • IBS • 5-HT4 receptor agonists and CCK-1 agonists

28. Obstructed Defecation • Usually due to abnormalities in pelvic function • Failure of puborectalis to relax with defecation, rectocele, perineal descent, etc • Marker studies show collection in left colon • Associated with total colonic inertia • Sigmoidocele • Colonic source • Relieved and treated with sigmoid resection

29. Ogilvie’s Syndrome • Acute colonic pseudoobstruction • Parasympathetics have decreased function with increased sympathetic input • Cecum can become extremely dilated • Treatment is Gastrografin enema to R/O distal obstruction • Can also treat with neostigmine • Cholinesterase inhibitor • Allows more available acetylcholine for neurotransmission in parasympathetic system to promote contractility

30. Irritable Bowel Syndrome • Altered bowel habits associated with pain • constipation-predominant, diarrhea-predominant, or mixed type • Unclear pathophysiology • Men—diarrhea predominates • Antispasmodics (anticholinergics), low-dose TCAs, 5-HT3 antagonists