Colonic Motility Flashcards
Functions of phasic contractions in large intestine
. Optimal absorption of H2O and electrolytes
. Net aboral movement of contents
. Storage and orderly evacuation of feces
Tenia Coli
. 3 bands of concentrated longitudinal smooth m. That begin at the cecum and end at the rectum
. At rectum the bands spread out and form a longitudinal m. Layer of uniform thickness
Haustra
. Bulges in colon wall where the longitudinal m. Layer is thin
. Periodic contractions of circulating m. Then pulls the bulges into pouches or haustra
. Not fixed, appear and disappear is different locations
. Inc. SA available for absorption
Innervation of colon
. CN X innervates enteric neurons in cecum, ascending colon, and transverse colon (parasym.)
. Pelvic nn. W/ cell bodies in sacral parasym. Nucleus in sacral spinal cord, innervate enteric neurons in descending colon, sigmoid colon, and rectum
. SNS innervate entire colon
. Enteric neurons in myenteric and submucosal plexuses innervate smooth m. And mucosa of entire colon
. Enteric neurons innervate internal anal sphincter, pelvic nn. Innervate internal sphincter via enteric neurons
. Pudendal n.: somatic motor to striated m. Of external anal sphincter
Haustral shuttling
. Individual haustra appear and disappear randomly bc contractions of tenia coli and circular m
. Luminal contents displaced in both directions
. No net movement of chyme
. Mixing movements thoroughly mix contents and aid in absorption of H2O and electrolytes
Haustral propulsion
. Few adjacent haustra contract sequentially
. contents in 1 haustra are displace aborally to an adjacent haustra
. Net movement 5-10 cm
Multi haustral propulsion
. Combined contraction of many haustra
. Net aboral movement of intestinal contents over a moderate distance
Haustral retropulsion
. Few adjacent haustra contract sequentially
. Contents of 1 haustra are displaced backward to an adjacent haustra
Mass movement
. Simultaneous, very strong contractions of circular m. And tenia coli
. Contractions move the colonic contents over long distance
. Mass movements are infrequent in healthy adults and occur 1-3 times per day after the ingestion of a meal as part of the gastrocolonic reflex
Phasic contractions in prox. Colon
. Haustral propulsion
. Multihaustral propulsion
. Frequent haustral retropulsion
. Not much haustral shuttling
Phasic contractions in colonic regions
. Haustral shuttling
. Frequent haustral retropulsion
. Not much haustral or multihaustral propulsion
. Slow transit in distal colon which minimizes. Challenges to continence while maximizing proximal colon mixing
Rectum
. Fecal material enters rectum during a mass movement
. Can temporarily store feces
. After initial accommodation, subsequent haustral retropulsion returns fecal material to sigmoid colon
Internal anal sphincter control
. Rectoanal inhibitory reflex (short and long neural reflex)
. Distension of rectal smooth m. Relaxes the internal anal sphincter
External anal sphincter control
. Sphincter has a high basal tone (pudendal n.)
. Voluntarily constricted or relaxed
Rapid component of gastrocolonic reflex
. Gastric distension causes long neural reflex through parasympathetic vagus (afferent) and pelvic (efferent) nn.
Slow component of gastrocolonic reflex
. Gastric distension inc. release of gastrin from G cells in gastric mucosa (short neural reflex)
. Hormone gastric inc. colonic motility
Gastrocolonic reflex
. Has rapid and slow components
. Accounts for urge to poo 30 min after eating
.mass movement after a meal are facilitated by this reflex
Pressure in rectum
. Inc. As result of fecal matter moving in from mass movement
. Inc. pressure in rectum Causes relaxation of internal anal sphincter (rectoanal inhibitory reflex)
. Fecal material enters the anal canal and its presence is consciously sensed by the individual
Outcomes of inc. rectal pressure
. Retention of fecal contents
. Elimination of fecal contents
Retention of fecal contents
. If circumstances are not appropriate for defecation the external anal sphincter is not voluntarily relaxed by the individual
. Eventually smooth m. Relaxes to accommodate fecal material
. As rectal pressure dec. the rectal anal inhibitory reflex is no longer activated and tone of internal anal sphincter returns to normal
. Haustral retropulsion resumes clearing rectum of fecal material
Elimination of fecal contents
. When circumstances are appropriate both short and long neural reflexes and pudendal n. Participate in defecation
Short neural reflex for defecation
. Enteric IPANS are stimulated by presence of fecal material
. Enteric motor neurons inc. haustral propulsion in descending colon. Sigmoid, and rectum
. Inc. intraluminal pressure in those areas
. Relax internal anal sphincter
Long neural reflex of defecation
. Parasympathetic afferent nn. Are activated by presence of fecal material
. PNS pelvic efferent nn. Innervating the ENS of the descending, sigmoid, and rectum inc. haustral propulsion and inc. intraluminal pressure against the fecal material
. Internal anal sphincter promote relaxation
Somatic nerve involvement in defecation
. Involved in taking a deep breath, closure of glottis, contraction of abdominal mm. To force fecal contents downward and relaxation of pelvic mm.
. Squatting and relaxation of puborectalis m. Dec. angulation of the anal canal facilitating defecation
Transit time in colon
. Ranges from 1.5-2 days
. Fiber can dec. transit time to under 30 hours
Functions of dietary fiber
. Inc. stool water content
. Inc. stool weight
. Stimulates motility by stretching colon wall
. Absorbs organic materials (bile acids, lipids, cholesterol)
Slow-transit constipation
. Slower than normal movement of contents from prox. To distal colon and rectum (inc. transit time)
. Dec. number of high amplitude contractions or weak contractions assoc. w/ mass movements
. Inc. and uncoordinated motor activity in distal Colon that resists normal aboral transit
Pelvic floor dysfunction
. Preferential storage of fecal material for long periods of time in rectum
. Muscular hypertonicity: incomplete relaxation or paradoxical contraction of external anal sphincter
. Muscular hypotonicity: megarectum and excessive pelvic floor descent
Opioid-induced constipation
. Reduce NT release from excitatory and inhibitory enteric motor neurons so contraction and relaxation of visceral smooth m. Are less robust
. W/ generation of a weaker pressure gradient, transit time inc., resulting in desiccation of stool
. Individuals w/ OIC are initially treated w/ 1st line laxative agent
. If laxatives don’t work the next option is antagonist at enteric opioid receptors (peripherally-acting opioid receptor antagonist)
Constipation treatments
. Appropriate treatment of constipation depends on the etiology
. First line laxative agents for treatment of constipation promote passive water secretion into lumen, activate IPANS to inc. motility via short neural reflexes, inc. H2O content, or lib the interior gut wall
. Agents that inc. epithelial cell electrolyte secretion
. Agents that inc. release of excitatory NTs from enteric motor neurons
Common causes of acute lower GI bleeding
. Anorectal disease (hemorrhoids)
. Colonic lesions (UC, colon cancer, infections)
. Diverticulosis: outpouches of colon wall
