GI Physiology of the GIT

Functions of the GIT

Digestion, transport of water, nutrients, and electrolytes, and defensive barrier

Digestive Process

Ingestion, motility (driving food along intestinal tract), mechanical breakdown (chewing etc.), chemical breakdown (enzymatic degradation), absorption/secretion and defecation

Intestine Structure

Lumen, mucosa, submucosa, circular muscle, longitudinal muscle, serosa

Mucosa

Innermost layer of the GIT, surrounding the lumen. It's made up of epithelium (innermost layer), lamina propria and muscularis mucosa

Submucosa

Connective tissue containing blood and lymphatic vessels, nerve fibers and lymphoid follicles

Muscularis (consists of circular and longitudinal muscle) function

Consists of smooth muscle cells which are responsible for controlling intestinal motility (segmentation + peristalsis)

Serosa

Consists of rough, connective tissue

4 types of epithelial cells in the intestine

1. Enterocytes

2. Goblet cells

3. Paneth cells

4. Enteroendocrine cells

Enterocytes function

Transport fluid, nutrients, and electrolytes. Also secrete beta-defensin in the colon.

Goblet cells function

Secrete mucus, protecting mucosa from abrasion

Paneth cells function

Secrete antimicrobial peptides (alpha-defensins) which kill pathogenic bacteria

Enteroendocrine cells function

Luminal sensors, secrete mediators, peptides, and hormones

Epithelium

Part of mucosa. Innermost layer of intestine which lines the lumen. It's characterised by tight junctions.

Epithelium structures

1. Stratified squamous

2. Simple columnar

stratified squamous epithelium function

- Protects against wear & tear, and secretes fluid and mucus

- Found in mouth, oesophagus, and anal canal

- No nutrient transporters

simple columnar epithelium function

- Absorption and secretion

- Found throughout rest of intestine

- Express proteins involved in fluid, nutrient, and electrolyte transport.

The epithelium poses a...

...physical phospholipid barrier in which only lipophilic molecules can directly diffuse across

Epithelium tight junctions function

Only allow small ions/molecules to pass through and prevent bacteria and toxin entry

Enteric Tear Mechanism

When toxins/pathogens are present in the lumen, the lower intestine can switch from being absorptive to secretory (e.g. diarrhoea)

Most prevalent epithelial cells in small intestine

Enterocytes

Where are immune cells found in the intestine?

In the lamina propria (part of the mucosa)

What do immune cells do?

Engulf and kill bacteria, and release cytokine which is responsible for lymphocyte-mediated-immunity

Where are smooth muscle cells found in the intestine?

In the muscularis layer (circular & longitudinal)

Segmentation (circular muscle)

Mixes and churns food bolus

Peristalsis (longitudinal muscle)

Propels food along GIT

What the enteric nervous system (ENS) is made up of and their functions?

200M neurons and:

1. submucosal plexus which regulates epithelial function

2. myenteric plexus which regulates smooth muscle contraction

Sensory neurons' role in GIT

Receive info from mucosa and activates muscle to stretch

Motor neurons' role in GIT

Control muscular contractions and epithelial secretion + absorption

Inter-neurons' role in GIT

Integrate information

What are enteroendocrine cells?

Epithelial cell type specialised for sensing changes in intestinal environment e.g. pH. They release mediators, peptides, and hormones

Types of enteroendocrine cells

G, S, I, L cells

G cells location and function

Stomach epithelium. Release gastrin in response to nutrients

S cells location and function

Duodenum epithelium. Release secretin in response to luminal HCl

I cells location and function

Duodenum/jejunum epithelium. Release cholecystokinin in response to nutrient and caloric intake

L cells location and function

Ileum/colon epithelium. Release GLP-1

Parts of small intestine

1. Duodenum (1st part)

2. Jejunum (2nd part)

3. Ileum (3rd part)

Where are enterocytes found?

Small intestine epithelium (most prevalent epithelial cell here!)

Where are Goblet cells found?

Small and large intestine epithelium

Where are Paneth cells found?

Small intestine epithelium

Where are enteroendocrine cells found?

All along GIT epithelium e.g. G, S, I, L cells found in various locations.