Motility of small and large intestines

Question 1

Purpose of motility in small intestines

Answer 1

1.mixing- luminal contents and pancreatic enzymes and bile
2.luminal digestion (carbohydrates, fats, proteins)major absorption of GI tract
3.maximal exposure of digested nutrients to the mucosa of small intestine
4.rapid movement of osmotically active substances to areas of tighter epithelium (lower SI and colon)- either absorption or rapid movement along tighter epithelium (ilium and colon etc.)
*liquids move at more rapid rate!

Question 2

Movement in small intestine

Answer 2

-contraction and movement controlled by electric slow wave
-Cajal cells located in longitudinal muscle of the duodenum next to entry of the bile duct
-other Cajal pacemakers along small intestine to assure continued contraction but at lower rate (more proximal than distal)

Question 3

Slow wave generation in small intestine

Answer 3

-generated every 17-18 mins in dog and cat, 14-15mins in horse
-slow waves will decrease in frequency and amplitude in aboral direction

Question 4

Modification of small intestine motility

Answer 4

-Enteric and autonomic nervous systems can either increase or decrease rate

Question 5

Gastrin purpose in small intestine

Answer 5

-secreted by enteroendocrine cells of the pyloric stomach when full
-can stimulate small intestinal motility and potentially aid in emptying

Question 6

CCK purpose in small intestine

Answer 6

-secreted by enteroendocrine cells in duodenum
-sense fat or amino acids in the lumen and stimulates contractions

Question 7

Secretin purpose in small intestine

Answer 7

-produced by duodenal enteroendocrine S cells in response to decreased pH
-slows intestinal motility

Question 8

Two phases of small intestine motility

Answer 8

1.digestive
2.interdigestive

Question 9

Digestive phase motility patterns of small intestines

Answer 9

1.propulsive (peristalsis)
2.nonpropulsive (segmentation)

Question 10

Peristaltic reflex

Answer 10

-contraction of the smooth muscle is controlled/modulated by the enteric nervous system through excitatory and inhibitory neurons
-results in propulsion of contents

Question 11

Components of peristaltic reflex

Answer 11

1.contraction of the longitudinal muscle and inhibition of the circular muscle distal to the bolus induces shorting of the muscle and expand the lumen
2. relaxation of the longitudinal muscle and contraction of the circular muscle proximal to the bolus  reduces the lumen size and prevents backward flow
**results in coupling of neural circuits along intestine. Receiving segments convert propulsive segments as the next segment becomes receiving segment

Question 12

Segmentation reflex

Answer 12

-intermittent circular muscle contraction occurring at different sites along the segment
-controlled by intrinsic neuronal influence on the circular muscle
-not usually in one direction, this reflex used for mixing

Question 13

Physiologic ileus

Answer 13

-normal absence of motility and propulsion in the small and large intestines

Question 14

Interdigestive phase in small intestine

Answer 14

*occurs 6-8 hrs after meal
-MMC occurs to prevent material sitting in intestines AND bacterial proliferation
>Phase I- slow waves only
>phase II-intermittent action potentials on slow waves
>phase III-intense bursts of spikes
-results in strong peristaltic contractions and aboral propulsion
-pylorus does not close during these contractions

Question 15

Fed state motor activity control

Answer 15

-stomach-pattern is initiated by hormones
-intestines-pattern initiated by extrinsic innervation

Question 16

Fasted state motor activity control

Answer 16

-stomach-hormone motilin- induces MMC
-intestines- enteric nervous system- Ach, substance P, VIP

Question 17

Function of large intestines

Answer 17

-motility patterns of the colon are for mixing and retention
-capacity to reabsorb large quantities of fluid
-some anti-diarrheal drugs target motility and when motility is disrupted, fluid absorption will be enhanced in the large intestines

Question 18

Entry into colon

Answer 18

-enter through ileocolic sphincter (dog, cats, ruminants) into large intestines/colon
-enter through ileocecal sphincter (horse, pig, rabbit, rat, elephant) into cecum

Question 19

Ileocoli/ileocecal sphincter opening and closing

Answer 19

-sphincters close when cecum or large intestine filled
-sphincter relaxes with either distention of ileum OR gastrin release caused by food in stomach

Question 20

Movement within the large intestines

Answer 20

-both mixing and peristaltic contractions aborally and reverse in the cecum
>reverse peristaltic believed to help move contents into cecum for better digestion ,fermentation. The mixing believed to reduce transit time allowing for more fermentation/digestion and absorption

Question 21

Aboral mass movement or big sweep of colon

Answer 21

-also called migrating spike burst
-starts 3cm past ileocolic junction
-different than MMC in stomach

Question 22

Pacemakers of colon

Answer 22

1.one at start of colon- conducts aboral mass movement
2.one mid colon- results in slow waves and peristaltic contraction in both directions (towards cecum and rectum)

Question 23

Reason for pacemaker placements

Answer 23

-allows retention and mixing of contents for better fermentation/digestion
-regular contractions move contents back and forth for better fermentation, and then final aboral mass movement moves contents past mid colon pacemaker to distal colon

Question 24

Hindgut fermenters

Answer 24

-in horse, rabbit, koala, elephant

Question 25

Horse hindgut fermentation

Answer 25

-3 separate fermentation compartments: cecum, ventral colon, dorsal colon
-controlled by enteric nervous system and pelvic and vagus nerves

Question 26

Horse hindgut structure

Answer 26

-ileum empties into the base of cecum which is a blind sac that is filled by gravity
-orifice between cecum and colon called cecocolic orifice is a point of resistance that can restrict cecal emptying
-right dorsal colon narrows as it converges onto the transverse colon=another point of restriction

Question 27

Horse hindgut fermenter contractions

Answer 27

-Segmental contractions in cecum to mix ingesta
-peristaltic contractions near apex move material up and out of ventral colon, propels food forward
-mixing contractions occur throughout ventral and dorsal colons

Question 28

Haustrations in horses

Answer 28

-saccules give a segmented appearance
-located in descending colon that hold fecal material to extract more water

Question 29

Defecation

Answer 29

-temporary reflex that interrupts normal anal continence
>anal sphincters normally closed, rectum empty
>an animal can resist defecation during a rectal peristaltic wave but eventually pressure becomes too great

Question 30

Control of anal sphincters

Answer 30

-internal anal sphincter- non-voluntary control smooth muscle
-external anal sphincter- striated and has conscious control

Question 31

Defecation reflex steps

Answer 31

1.peristalsis waves- move fecal material into rectum
2.sensory afferent fibers sense contact of feces with internal anal sphincter smooth muscle
3. sacral parasympathetic fibers cause relaxation of internal anal sphincter
4. fecal material reaches external anal sphincter composed of striated muscle. Information passed back to the spinal cord and brain by sensory afferent fibers
5.animal will either consciously relax external anal sphincter and defecate of make conscious effort to delay defecation (**mediated by somatic motor neurons)

Question 32

GIT transit times

Answer 32

-extremely variable depending on species and individuals
-seems longer in larger animals