GI Physiology

Question 1

explain:

i) motility
ii) secretion
iii) digestion
iv) absorption

Answer 1

i) muscular contractions that mix, move forward contents of GI tract
ii) glands located along GI tract secrete their contents into the tract, assisting in motility, digestion, absorption
iii) biochemical breakdown of large particles and molecules into smaller, absorbable particles
iv) small particles absorbed into blood/lymph

Question 2

differentiate mixing & propulsive movements in GI lumen (MOTILITY)

Answer 2

smooth muscle cells in wall of GI tract maintain constant level contraction at mid-length. allow for further contraction/relaxation.
tone maintains steady-state pressure on contents

MIXING: redistribute luminal contents locally. enhance exposure to digestive secretions. expose luminal contents to GI tract absorbing surfaces

PROPULSIVE: move luminal contents forward. rate of propulsion varies with specific function of region
esophagus = rapid
small intestine = slow, allows abs.

Question 3

how GI exocrine glands produce/secrete products (SECRETION)

Answer 3

digestive juices secreted into lumen by exocrine glands through ducts. consist of:
- water
- electrolytes
- organic substances (mucus, enzymes, bile salts)
perform specific functions within GI tract

Question 4

outline enzymes involved in, and products of, digestion of proteins, fats, carbs

Answer 4

nutrient-specific enzymes mediate addition of H2O molecule to bonds linking component molecules together causing molecules to split apart “hydrolysis”

carbs: amylase, sucrase, lactase, maltase
proteins: pepsin, trypsin, chymotrypsin, carboxypeptidase, aminopeptidases

lipids: lipase

Question 5

describe sites of nutrient absorption in GI tract

Answer 5

predominantly occurs across membrane of epithelial cells in small intestine
- then travels through epithelial cells into blood (carb/protein) OR lymphatic (fats)

water/vitamins absorbed somewhat in large intestine

Question 6

identify GI tract organs and accessory organs

Answer 6

GI tract:

• mouth
• pharynx
• esophagus
• stomach
• small intestine
• large intestine

accessory:

• salivary glands
• liver
• gallbladder
• pancreas

Question 7

describe 4 layers of wall

function

Answer 7

1. mucosa- lines luminal surface. subdivided into mucus membrane and lamina propria
2. submucosa- thick connective tissue, contains blood, lymph vessels, exocrine glands
   * *submucosal plexus- neurons control GI motility/secretion
3. muscularis externa- major smooth muscle layer
   - inner circular layer and outer longitudinal layer
   * myenteric plexus btw layers, coordinates contractions/motility
4. serosa- connective tissue outer covering
   - anchors GI tract w/in abdominal cavity
   - secretes lubricating fluid

Question 8

how intrinsic muscle excitability, ENS, ANS, hormones influence function

Answer 8

motility/secretion tightly regulated by 4 factors:
1. intrinsic electrical properties of SM cells- ICC cells= pacemaker, non-contractile, “slow-waves”. AP proportional to duration of slow wave above threshold, leads to SM contraction

2. ENS- 2 nerve plexi in GI wall (submucosal and myenteric). operate without external input “reflexive”. communicate via electrical and release of NT
   = sensory neurons, interneurons, secretomotor cells
3. ANS- influence ENS activity, affect SM glands, alter GI hormones
   - PSNS increases motility/secretions
   - SNS inhibits
4. GI hormones
   - Gastrin (stomach)
   - Secretin (duodenum)
   - CCK (duodenum)
   - GIP (duodenum/jejunum)