Colon ap 7-21-10

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Colon ap 7-21-10

  1. 1. Colon Anatomy and Physiology 7/21/2010
  2. 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. 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  Angulation plays a role in prevention of reflux
  4. 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. 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. 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. 7. Descending colon 25 cm Retroperitoneal Narrower and more dorsally situated than ascending colon
  8. 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. 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. 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 hemorrhoidal after crosses left common iliac Venous drainage follows arterial supply
  11. 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. 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 o Motor component through acetylcholine and tachykinins (substance P) o 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. 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 and absorptive functions Acetylcholine, opioids, norepinephrine, serotonin, somatostatin, cholecystokinin, substance P, VIP, neuropeptide Y, and nitric oxide are important neurotransmitters
  14. 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 right and proximal transverse colon Proteins are broken down into SCFAs, branched chain FAs, ammonia, amines, phenols, and indols  Become a nitrogen source for bacterial growth
  15. 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. 16. Salvage, Metabolism, and Storage Proximal colon  More saccular  Acts as a reservoir  Fluid moves through quickly, solid material slower  Principal site for SCFA production Distal colon  More tubular  Acts as a conduit  Protein degredation Haustral segmentation facilitates mixing, retention of luminal material, formation of solid stool
  17. 17. Transport of Electrolytes Presented 1-2 L of water/day  Absorbs 90%  Only 100-150 mL eliminated 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. 18. Transport of Electrolytes Chloride is exchanged for bicarb  Secreted into lumen to neutralize organic acids produced  Occurs at luminal border of mucosal cells Potassium movement is passive secondary to active absorption of sodium  Active secretion may occur in distal colon  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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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

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