GI Motility and Regulation

Question 1

Types of motility in the GI tract

Answer 1

Segmentation (mixing) (predominates)

Peristalsis (propulsive movements)– receptive relaxation occurs due to pressure produced by proximal bolus
-requires myenteric plexus
(stomach– BERs coordinated w/ vagal input leads to peristalsis)
Aboral movement: contraction of circ SM behind, longitudinal musc ahead, circ muscles

–smooth muscle with actin (thin) and myosin as major contractile prot. Complex b/t Ca and calmodulin activates myosin light chain kinase which phosphorylates myosin and allows cross bridge formation. Reverse thru dephosphorylation via myosin light chain phosphatase, inactivation of MLCK

Question 2

BER and relation to smooth muscle contractile activity

Answer 2

Basic electrical rhythm or “slow waves”

• consist of upstroke depol, partial repolariz, and plateau potential that can last for several secs.
• no external stimulus required (~12/min; spont)
• BER is myogenic (muscle derived)
• each depolariz doesn’t cause contractions. Contrac only occurs when depolarization exceeds a specific membrane potential.

Stomach BER: 3/min
Duodenum BER: 12/min

Question 3

describe how symp/parasymp systems communicate with intestinal smooth muscle

Answer 3

NE/adrenaline stimulates sympathetic NS; activation of SNS inhibits digestion

Parasympathetic NS (ACh) does opposite: stimulates digestions

Question 4

Smooth muscle in GI tract

Answer 4

-unitary cell type (single unit)
held together by adherens junctions
communicates electrically via gap junctions
pacemaker cells with spontaneous activity
intriniscally produces BER and muscle tone without tension (myogenic properties)

Tension comes from NTs acting on muscle – role of ANS

Question 5

Mast cell activity

Answer 5

endocrine
paracrine
neurocrine

Question 6

Phases of Digestion

Answer 6

cephalic phase: neural control (gastric muscle contracts, body prepares itself for food)
gastric phase: neural early and hormonal
intestinal: mostly hormonal but some neural

Question 7

swallowing 3 steps

Answer 7

voluntary stage 1: voluntary (oral cavity then bolus pushed by tongue to oropharynx)
pharyngeal stage 2: involuntary (glottis covers trachea; UES relaxes)
esophageal stage 3: involuntary (esophageal peristalsis)

Swallowing becomes a reflex after nasopharynx closed off (pharyngeal phase)

Question 8

esophagus

Answer 8

transports and adjusts food temp

• has upper and lower esophageal sphincters
• muscular walls transition from skeletal to smooth muscle
• thick muscular walls produce strong peristaltic waves

Peristalsis:
LES relaxes just before bolus arrives
-peristalsis controlled by vagus n. (and myenteric complex)

Question 9

LES

Answer 9

• gatekeeper b/t esophagus and stomach
• failure to relax due to damage/loss of enteric nerves of LES wall is “achalasia” and can make swallowing difficult
• inapprop LES relaxation can cause acid reflux and damage to the inner lining of the esophagus.

Question 10

receptive relaxation of stomach and retropulsion

Answer 10

• in synchrony with relaxation of LES
• up to 1.5 L accomodated in stomach without marked increase in intra-gastric pressure
• regulated by vagus n
• contractions start (slow wave)
• push bolus toward antrum, but since pyloric opening is small, most content reflected back toward body of stomach (retropulsion)
• phase 3: contents

Question 11

Gastrin

Answer 11

-hormone secreted in response to the presence of food in the stomach.

Gastrin also increases peristaltic contraction and decreases pyloric tone.
-the combination of distension and gastrin increase the rate of gastric emptying.

Question 12

arrival of food in duodenum

Answer 12

• causes distention and irritation by acidity and solutions of high osmolarity
• leads to reflex inhibition of gastric peristalsis and increase in pyloric tone (duodenum controls delivery rate)
• liquification, pH adjustment, release of hormones, and addition of enzymes in response to food entering duodenum
• Key hormones: CCK, Secretin, GIP
• enzymatic digestion occurs in lumen and at enterocyte surface
• absorption of nutrients, water, and ions occurs via cellular/paracellular pathways
• both peristaltic and segmentation motility

Question 13

arrival of fats in duodenum

Answer 13

• leads to secretion of CCK by enteric endocrine cells

- CCK decreases gastric motility

Question 14

Rate of gastric emptying

Answer 14

• foods rich in carbs leave stomach in a few hrs
• protein rich foods leave more slowly
• emptying is slowest after meal containing fat

Question 15

Emesis

Answer 15

-centrally regulated by vomiting center in brain
Steps:
-salivation (HCO3-) and sensation of nausea
-reverse peristalsis from upper small intestine to stomach
-abdominal muscles contract and UES and LES relax
-gastric contents ejected

Question 16

Intestinal reflexes

Answer 16

-mediated by both ENS and external innervation:
Gastroileal reflex: stomach activity stimulates movement of chyme through the ileocecal sphincter
Gastrocolic reflex: food in stomach stimulates mass movement in colon

Mediated by the enteric nerves:
Localized perstaltic waves and segmentation mixing

Question 17

Migrating Motor Complex (MMC)

Answer 17

• occurs in the absence of feeding (during fasting); 90-100 mins with 3 phases starting from the stomach and propagating aborally to ileocecal valve.
• “Motilin” hormone intitiates but neural component too

Phase I: quiescence occurs for 40-60 % of the 90 minute duration
Phase II: motility increases but contractions irregular; fails to propel luminal content; Lasts 20-30% of MMC
Phase III: 5-10 mins of intense contractions; from body of stomach to pylorus to duodenum to ileocecal valve
Pylorus fully opens

Question 18

Ileocecal valve

Answer 18

• normally closed (to prevent reflux of bacteria from colon to ileum)
• opened by distention of end of ileum (local reflex)
• closed by distention of proximal colon (local reflex)

Question 19

Colon

Answer 19

• reservoir for undigested foodstuff
• main func: reabsorb water, ions
• elimination of waste controlld by two sphincters: internal (involuntary) and external (voluntary) anal sphincters

Question 20

Types of motility in large intestine/colon

Answer 20

haustration: muscles of the colon wall are contracted intermittently to divide the colon into functional segments known as haustra
mass movements: strong peristaltic waves 1-3x/d; wave of contraction that usually follows a meal and moves content over greater distance than regular peristalsis; colon remains contracted for a while
Overall movement is slow: 5-10cm/hr

Question 21

rectoanal inhibitory reflex

Answer 21

• filling of the rectum causes relaxation of the internal anal sphincter via release of VIP and NO from intrinsic nerves.
• simultaneously, EAS contracts

Question 22

evacutation

Answer 22

• defecation

- occurs when EAS is voluntarily relaxed and is enhanced by increased intra-abdominal pressure