GI physiology 1: Upper GI tract

What are the 6 major functions of the mammalian GI tract?

• ingestion of food

• secretion of fluids and digestive enzymes

• mixing and movement of food and wastes through the body

• digestion of food into smaller pieces

• absorption of nutrients

• excretion of wastes

What direction does movement in the GI tract go in?

unilateral

What are the clinical terms used to describe the parts of the GI tract?

upper and lower

What are the embryological terms used to describe the parts of the GI tract?

upper/fore, mid, lower/hind

What animals does this relate to?

carnivores

What animals does this image relate to?

ruminants

What animals does this image relate to?

simple-stomached herbivores

What do herbivores utilise for digestion?

microbial fermentation

Why do herbivores utilise microbial fermentation?

structural carbohydrates not digested by mammalian enzymes

1

lamina propria

2

mucosa

3

submucosa

4

muscularis

5

serosa

6

epithelium

7

connective tissue

8

venule

9

nerve

10

arteriole

11

lymphatic

12

muscle layer

13

circular musculature

14

longitudinal musculature

15

peritoneum

16

villus

17

crypt

18

network of nerve cells

19

mesentery

20

nerve and blood vessels

Do the crypts secrete or absorb?

secrete

Do the villi secrete or absorb?

absorb

What does the circular muscle layer do to GI tract?

contract to decrease diameter

What does the longitudinal muscle layer do to the GI tract?

contract to shorten length

What are the 4 distinct functional layers of the GI tract?

mucosa, submucosa, muscularis propria and adventitia

1

muscularis mucosae

2

lamina propria

3

lumen

4

epithelium

5

adventitia

6

submucosal plexus

7

lymphoid aggregate

8

submucosa

9

myenteric plexus

10

muscularis propria: inner circular layer

11

muscularis propria: outer longitudinal layer

What 3 components make up the mucosa?

epithelium, lamina propria, muscularis mucosae

What does the muscularis mucosae do?

produce local movement and folding of mucosa

submucosa

layer of loose collagenous connective tissue supporting mucosa & containing larger blood vessels, lymphatics and nerves

muscularis propria

smooth muscle usually arranges as inner and outer longitudinal layer

What are the 2 layers of the muscularis propria the basis of?

peristaltic contraction

Adventitia

outer layer of loose supporting tissue conducts major vessels, nerves and contains variable adipose tissue

In the gut, what is the adventitia (serosa) lined with?

simple squamous epithelium

What increases the surface area in the GI epithelia?

• folding of mucosa

• villi

• microvilli

1

celiac artery

2

cranial mesenteric artery

3

caudal mesenteric artery

4

hepatic portal vein

5

hepatic vein

1a

hepatic artery

1b

splenic artery

What 3 things is the function of the GI tract coordinated by?

• nervous systems

• paracrine substances

• hormones

What are the 3 nervous systems that the function of the GI tract is coordinated by?

central, autonomic and enteric

Enteric NS

nervous tissue within wall of gut

What are the 2 types of reflexes involved in the nervous control of the GI tract?

long and short

Where do paracrine substances act?

locally

Where do hormones act?

secreted into blood to act systemically

What type of control/nervous system is involved with long reflexes?

CNS

What nervous system controls are involved with short reflexes?

enteric

What kind of tissue is involved in the motor activity of the GI tract?

contractile tissue (smoot muscle)

What does motility relate to?

function (mixing, propulsion, etc)

1

oscillations

2

action potentials

3

slow wave contractions

When do slow waves not reach the threshold?

if fasted

When are slow waves activated/stimulated?

after a meal

What happens after a meal that causes smooth muscle to be stimulated?

acetylcholine is released by nerve impulses, and gastrin release

What is motility regulated and coordinated by?

enteric NS, hormones, ANS

What waves are involved in stomach motility?

slow waves

What are the features of motility in the small intestine?

segmentation (minute rhythm) and peristalsis

In the stomach, how many small waves are there per minute?

2/4

What are the 3 main steps in stomach motility?

1. propagate from fundus to plyorus

2. breaks up food

3. moves material to duodenum

What factors stimulate the stomach emptying?

• distension of stomach

• peptides in stomach

What does distension of the stomach do that stimulates the stomach emptying?

1. increase activity of stretch-sensitive sensory cells

2. increase contraction of smooth muscle cells

3. emptying increases

What does peptides in the stomach do that stimulates the stomach emptying?

1. increase gastrin

2. increase contractions of smooth muscle cells

3. emptying increases

What factors inhibit the stomach emptying?

• high peptide conc

• high pressure

• high osmolarity

• low pH

• high fat content

What does high fat content do that inhibits the stomach emptying?

increase release of hormones from duodenal epithelium

What happens due to high peptide conc, high pressure, high osmolarity and low pH that inhibits the stomach emptying?

• increase activity of sensory cells in duodenum

• (CNS) increase sympathetic activity & decrease parasympathetic activity of nerve fibres to stomach

1

fundus

2

corpus

3

pylorus/antrum

4

pyloric sphincter

5

cardia

6

esophageal sphincter

7

lumen

8

gland entrances

9

mucin-producing cells

10

parietal cells

11

ECL cell (enterochromaffin-like cells)

12

chief cells

13

endocrine cells