Week 12: GI Tract Regulation

Volume & Luminal contents

The contents and volume of the GI tract can trigger neural or hormonal regulation

Neural regulation — Enteric nervous system

1. Submucosal plexus – primarily influences secretory activity

2. Myenteric plexus – primarily influences motility

Can cause responses WITHOUT influence from the CNS.

Neural regulation — short vs long reflexes

Short reflex: Reflex takes a short period of time to take effect — goes to enteric nervous system

Long reflex: Reflex takes a long period of time to take effect — goes to CNS (parasympathetic/sympathetic nervous system — autonomic)

Hormonal regulation

Hormonal regulation: Gastrin

Produced by G cells

Stimulatory effects.

Stimulates parietal cells & ECL cells — influences release of gastric acid & histamine

Hormonal regulation: Somatostatin

Does opposite of gastrin

Inhibits:

• Inhibits parietal cells (↓ acid production)

• Inhibits ECL cells (↓ histamine = ↓ acid)

• Inhibits G cells (↓ gastrin = ↓ acid)

Prevents excessive acidity of the stomach

Hormonal regulation: CCK

Released in the small intestine, stimulated when chyme enters the small intestine:

Targets:

• Gallbladder (releases bile)

• Pancreas (secretes digestive enzymes)

• Stomach (↓ gastric emptying/secretion)

Hormonal regulation: Secretin

Stimulates bicarbonate release into the small intestine

Helps neutralise acids in the pancreas

Targets:

• Pancreas (secretes HCO3 - )

• Stomach (↓ gastric emptying/secretion)

Hormonal regulation: Motilin

Acts when no food is in the stomach.

Triggers the change from segmentation to MMC (peristaltic) contractions to clear up the intestines.

Hormonal regulation: GLP-1 & GIP

Stimulus:

Nutrients (particularly carbs) in duodenum

Targets:

Endocrine pancreas (‘feed-forward’ stimulation of insulin release)

Stomach (↓ gastric emptying/secretion)

Control nutrients coming from stomach and get body ready to deal with imminent surge in blood glucose, etc.

3 Phases of the GI system

1. Cephalic phase - Sight, smell, taste, thought of food, emotional state

2. Gastric phase - Distension, ↓/↑ acidity, peptides & amino acids in stomach

3. Intestinal phase - Distension, ↑ acidity, ↑ osmolarity, nutrients in duodenum

Can occur SIMULTANEOUSLY

Liver

Portal tried — Present in the converging corners of every hexagonal liver lobule

• Contains:

  • Hepatic artery

  • Hepatic vein - Comes from GI system, with all nutrients

    • Converges into the central vein, filtered blood that will go to the heart

  • Bile duct — travels in opposing flow direction of hepatic vein

Pancreas hormones: Insulin & Glucagon

Pancreas hormones: Insulin

2 cellular responses:

1. Increase in glucose uptake by cells

2. Increase in cellular metabolism

GLUCOSE UPTAKE INTO CELLS

Pancreas hormones: Glucagon

GLUCOSE CREATION AND RELEASE OUT OF CELLS AND INTO EXTRACELLULAR FLUID

diffuse DOWN the concentration gradient out of the cell via constant GLUT2 channel proteins in liver cells

Type 1 Diabetes

Autoimmune — Body attacks pancreas beta cells

• Pancrease cannot produce insulin

• Blood glucose cannot be uptaken

Type 2 Diabetes

Insulin can be produced. however body cells are LESS responsive to insulin.