Lecture 34 - Regulation of Digestion

What are the types of regulation of digestion?

Neural and hormonal

What is neural regulation of digestion?

Mechanical and chemical receptors respond to…

• Stretch

• Changes in osmolarity

• Changes in pH

• Presence of substrate and end products of digestion

What is hormonal regulation of digestion?

Hormone release is stimulated by mechanical stimuli and chemical stimuli

How are hormones released for hormonal regulation of digestion?

Released from cells in the stomach and small intestine stimulate target cells in same or different organs (hormones secreted into the bloodstream)

What do both regulatory mechanisms of digestion have in common?

• Activate or inhibit digestive secretions

• Control smooth muscle to move and mix lumen contents

What are the 3 phases of GI control?

1. Cephalic (head) phase

2. Gastric phase

3. Intestinal phase

What is the cephalic phase?

Mechanical and chemical stimuli conditioned reflexes triggered by smell, taste, sight, and thought

What do all 3 phases of GI control have in common?

Respond to mechanical and chemical stimuli

What phase of GI control can control the other phases?

Cephalic can control gastric and intestinal as well as itself

What is cephalic regulation?

Control of digestion by the head through…

• Stimulation of saliva secretion (no hormonal control)

• Parasympathetic nervous system

• Sympathetic nervous system

• Stimulation of GI tract and smooth muscle

What glands secrete during cephalic regulation?

• Parotid

• Submandibular

• Sublingual

How does the parasympathetic nervous system affect the body during cephalic regulation?

• Promotes secretion of watery saliva and enzymes for digestion

• Ingested food stimulates chemoreceptors or mechanoreceptors

How does the sympathetic nervous system affect the body during cephalic regulation?

Promotes mucus release to prevent drying of the mouth during increased respiration

What organs does cephalic regulation affect through secretions and motility?

• Stomach

• Small intestine

• Pancreas

• Liver

• Gallbladder

What stimulates the gastric phase neurally?

• Sensory stimuli (in cephalic phase)

• Food entering the stomach

How is the gastric phase stimulated by food entering the stomach?

• Mechanical: stretching of the stomach (expansion by food)

• Chemical: peptides and amino acids (breakdown of protein)

How does the para and sympathetic nervous system affect the gastric phase?

• Parasympathetic: Increases secretions and motility

• Sympathetic: Decreases secretions and motility

What stimulates the gastric phase hormonally?

Gastrin release by enteroendocrine G cells

What is the release of gastrin stimulated by?

• Stretching of the stomach (via nervous system)

• Parasympathetic stimulation (acetylcholine)

• Amino acid and peptides

• pH > 2 (food generally makes stomach less acidic)

What is the release of gastrin promote?

• HCl secretion (strongest effect)

• Gastric enzyme secretions

• Gastric motility and emptying (muscle)

• Intestinal muscle contraction

What is the directionality of peristalsis and gastric emptying in the stomach?

Propulsion → grinding → retropulsion

What is propulsion in the stomach?

Peristaltic waves move from the fundus toward the pylorus

What is grinding in the stomach?

The most vigorous peristalsis and mixing action occur close to the pylorus

What is retropulsion in the stomach?

The pyloric end of the stomach acts as a pump that delivers small amounts of chyme into the duodenum, simultaneously forcing most of its contained material backward into the stomach

How does the parasympathetic nervous system affect the mechanical, chemical, and hormonal regulation?

Increases stretch of the stomach (mechanical), hypotonic (chemical), and gastrin release (hormonal)

How does stomach contents exit through the esophagus?

Vomiting

What causes vomiting?

• Extreme stretching of the stomach

• Irritants (bacterial toxins, viruses, certain drugs, spicy food, and excessive alcohol)

What causes excessive vomiting?

• Dehydration

• Electrolyte an acid-base imbalances (alkalosis)

What do parietal cells release?

H+

What stimulates parietal cells to release H+?

• Gastrin (enteroendocrine G cells)

• Acetylcholine (parasympathetic neurotransmitter)

• Histamine (enteroendocrine cells in stomach)

What can turn off stomach acid production?

• Very low pH in stomach promotes secretion of somatostatin to reduce gastrin secretion

• Sympathetic nervous system (norepinephrine)

How does neural regulation affect the intestinal phase?

Mechanical: stretching of the duodenum prevents gastric emptying

Chemical:

• Acid from stomach stimulates enterogastric reflex to prevent movement of chyme into duodenum

• Inhibits parasympathetic stimulation

• Activates sympathetic fibers

  • Tightens the pyloric sphincter → no more food entry to small intestine

  • Decreases gastric activity → protects small intestine from excessive acidity

What is secreted with hormonal regulation in the intestinal phase?

• Intestinal gastrin

• Secretin

• Cholecystokinin

What stimulates intestinal gastrin?

Partially digested food

What stimulates secretin?

Acid

What stimulates cholecystokinin?

Fatty acids and amino acids

What does intestinal gastrin promote?

Gastric secretions and motility

What does secretin do?

Inhibit gastric acid secretions

What does cholecystokinin do?

Inhibit gastrin motility

What is gastric inhibitory peptide (GIP)?

Glucose-dependent insulinotropic peptide

What is GIP stimulated by?

Digestion products in the small intestine (glucose, fatty acids, amino acids)

What does GIP do?

Inhibits both gastric emptying and acid secretion

What does GIP stimulate?

Insulin release from pancreas

What stimulates glucagon-like peptide-1?

Mixed meal of carbohydrates or fats

What does glucagon-like peptide-1 stimulate?

Insulin release from pancreas

How does the parasympathetic nervous system affect the mechanical, chemical, and hormonal regulation?

Stretch of the duodenum (mechanical), 1. acid 2. fat 3. amino acids 4. glucose 5. hypertonic conditions (chemical), and secretin, cholecystokinin, and GIP release (hormonal)

What type cells secrete cholecystokinin and secretin?

Exocrine cells

What does cholecystokinin cause and induce?

Cause: hepatopancreatic sphincter to relax to release pancreatic juice and bile

Induces: secretion of enzyme-rich pancreatic juice by acinar cell

What does secretin cause?

Secretion of bicarbonate-rich pancreatic juice by duct cells

What is bile production stimulated by?

Bile salts in enterohepatic circulation (after larger meal, large amounts of bile salts are returned to the liver)

Where is bile released?

Into the small intestine with gallbladder contraction

What stimulates gallbladder contraction?

Cholecystokinin