Colonic Motility

Question 1

Functions of phasic contractions in large intestine

Answer 1

. Optimal absorption of H2O and electrolytes
. Net aboral movement of contents
. Storage and orderly evacuation of feces

Question 2

Tenia Coli

Answer 2

. 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

Question 3

Haustra

Answer 3

. 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

Question 4

Innervation of colon

Answer 4

. 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

Question 5

Haustral shuttling

Answer 5

. 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

Question 6

Haustral propulsion

Answer 6

. Few adjacent haustra contract sequentially
. contents in 1 haustra are displace aborally to an adjacent haustra
. Net movement 5-10 cm

Question 7

Multi haustral propulsion

Answer 7

. Combined contraction of many haustra

. Net aboral movement of intestinal contents over a moderate distance

Question 8

Haustral retropulsion

Answer 8

. Few adjacent haustra contract sequentially

. Contents of 1 haustra are displaced backward to an adjacent haustra

Question 9

Mass movement

Answer 9

. 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

Question 10

Phasic contractions in prox. Colon

Answer 10

. Haustral propulsion
. Multihaustral propulsion
. Frequent haustral retropulsion
. Not much haustral shuttling

Question 11

Phasic contractions in colonic regions

Answer 11

. 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

Question 12

Rectum

Answer 12

. Fecal material enters rectum during a mass movement
. Can temporarily store feces
. After initial accommodation, subsequent haustral retropulsion returns fecal material to sigmoid colon

Question 13

Internal anal sphincter control

Answer 13

. Rectoanal inhibitory reflex (short and long neural reflex)
. Distension of rectal smooth m. Relaxes the internal anal sphincter

Question 14

External anal sphincter control

Answer 14

. Sphincter has a high basal tone (pudendal n.)

. Voluntarily constricted or relaxed

Question 15

Rapid component of gastrocolonic reflex

Answer 15

. Gastric distension causes long neural reflex through parasympathetic vagus (afferent) and pelvic (efferent) nn.

Question 16

Slow component of gastrocolonic reflex

Answer 16

. Gastric distension inc. release of gastrin from G cells in gastric mucosa (short neural reflex)
. Hormone gastric inc. colonic motility

Question 17

Gastrocolonic reflex

Answer 17

. Has rapid and slow components
. Accounts for urge to poo 30 min after eating
.mass movement after a meal are facilitated by this reflex

Question 18

Pressure in rectum

Answer 18

. 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

Question 19

Outcomes of inc. rectal pressure

Answer 19

. Retention of fecal contents

. Elimination of fecal contents

Question 20

Retention of fecal contents

Answer 20

. 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

Question 21

Elimination of fecal contents

Answer 21

. When circumstances are appropriate both short and long neural reflexes and pudendal n. Participate in defecation

Question 22

Short neural reflex for defecation

Answer 22

. 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

Question 23

Long neural reflex of defecation

Answer 23

. 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

Question 24

Somatic nerve involvement in defecation

Answer 24

. 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

Question 25

Transit time in colon

Answer 25

. Ranges from 1.5-2 days

. Fiber can dec. transit time to under 30 hours

Question 26

Functions of dietary fiber

Answer 26

. Inc. stool water content
. Inc. stool weight
. Stimulates motility by stretching colon wall
. Absorbs organic materials (bile acids, lipids, cholesterol)

Question 27

Slow-transit constipation

Answer 27

. 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

Question 28

Pelvic floor dysfunction

Answer 28

. 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

Question 29

Opioid-induced constipation

Answer 29

. 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)

Question 30

Constipation treatments

Answer 30

. 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

Question 31

Common causes of acute lower GI bleeding

Answer 31

. Anorectal disease (hemorrhoids)
. Colonic lesions (UC, colon cancer, infections)
. Diverticulosis: outpouches of colon wall