7/21/2010

Colon Anatomy and Physiology

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

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

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

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

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

Descending colon

25 cm Retroperitoneal Narrower and more dorsally situated than ascending

colon

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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