GI Control and Digestion Notes

GI Function and Control

Nervous and Hormonal Control

• Stimuli to GI structures are controlled via:

  • Nervous control (primarily vagus nerve)

    • Vagus nerve uses acetylcholine.

  • Hormonal control (peptides)

    • Hormones exert endocrine effects (into the blood).

    • Paracrine effects (stimulate nearby cells in connective tissue).

    • Neurocrine effects (enteroendocrine).

Hormone Actions

• Classification:

  • Endocrine: Hormone enters systemic circulation and stimulates cells.

  • Paracrine: Hormone released into lamina propria, affecting adjacent glands (e.g., histamine from ECL cells stimulating parietal cells).

  • Neurocrine: Neuron releases neurotransmitter (e.g., vagus nerve releasing acetylcholine).

Muscle Types in the GI

• Skeletal Muscle:

  • Found in oral cavity, pharynx, upper esophagus, and external anal sphincter.

  • Controlled by the somatic nervous system.

• Smooth Muscle:

  • Myenteric (Auerbach's) plexus:

    • Located between longitudinal and circular muscle layers of the muscularis externa.

    • Primarily stimulates muscle movement.

  • Submucosal (Meissner's) plexus:

    • Contains interstitial cells of Cajal.

    • Stimulates secretory and endocrine cells; predominantly for glandular secretions.

  • Both plexuses can stimulate muscles and glands; if only one is an option, choose the available one.

Extrinsic Control

• Parasympathetic:

  • Uses acetylcholine.

  • Vagus nerve stimulates esophagus, stomach, pancreas, and upper part of the large intestine.

• Sympathetic:

  • Uses norepinephrine (noradrenaline), not epinephrine (adrenaline from the adrenal gland).

  • Stimulates smooth muscles, secretory cells, and endocrine cells of the GI.

Hormone Classification

• Endocrine:

  • Enters systemic circulation.

• Paracrine:

  • Released into lamina propria; example: histamine from ECL cells stimulating parietal cells.

• Neurocrine:

  • Released from neurons at a synapse; example: vagus nerve releasing acetylcholine.

• Examples:

  • Gastrin (hormone, endocrine).

  • Histamine (paracrine).

Enteroendocrine Cells and Hormones

• Enteroendocrine cells synthesize hormones.

• Hormones:

  • Gastrin, CCK, secretin, gastric inhibitory peptide (GIP).

• Neurocrine:

  • Secreted by enteric nervous system neurons (e.g., vasoactive intestinal peptide).

• Paracrine:

  • Somatostatin, histamine.

• Key Hormones and Locations:

  • Gastrin:

    • Antrum of stomach, extending into duodenum.

    • Tumors can occur in duodenum or pancreas.

  • CCK, secretin, GIP, motilin:

    • Small intestine only.

  • CCK:

    • Required for gallbladder and pancreatic enzyme secretion in the duodenum.

  • Secretin:

    • Stimulates bicarbonate secretion throughout the small intestine.

  • GIP and motilin:

    • Regulate motility based on food type (carbohydrate-rich increases motility, fat/protein-rich decreases motility).

  • Secretin:

    • Large intestine; required for bicarbonate secretion.

Carbohydrate Digestion

• Begins in the oral cavity with salivary amylase.

• Minimal digestion of sucrose, lactose in the oral cavity.

• Stomach: No carbohydrate digestion.

• Small Intestine:

  • Pancreatic amylase digests polysaccharides.

  • Brush border enzymes (lactase, sucrase, dextrinase, maltase) break down into monosaccharides (glucose, galactose, fructose).

  • Monosaccharides are absorbed actively and passively.

• Starch Digestion:

  • Salivary and pancreatic amylase convert starch to dextrins, maltose, and maltotriose.

• Key brush border enzymes:

  • Lactase, sucrase, maltase, dextrinase.

• Final Products:

  • Glucose, galactose, fructose.

Carbohydrate Transport

• Passive Diffusion:

  • Pentoses, aldosaccharides directly transported through apical and basal surfaces.

• Facilitated Diffusion:

  • In fasting state, glucose concentration gradient drives transport (glucose is low)

  • Fructose, galactose, glucose transported via GLUT5 transporter on the apical surface.

  • GLUT2 transporter on the basal surface transports glucose, galactose, and fructose into capillaries.

• Secondary Active Transport:

  • Glucose and galactose transported with sodium via SGLT1 (sodium-glucose cotransporter 1).

  • SGLT2 is present in the proximal convoluted tubule of the kidney.

  • Sodium-potassium pump regulates sodium concentration.

Lactose Intolerance

• Lactose (milk carbohydrate) is not digested due to lactase deficiency.

• Undigested lactose enters the large intestine.

• Fermented by commensal bacteria, producing water, lactic acid, reducing sugars, and gases.

• Increased water due to osmotic effect, increased gases due to synthesis, stretching due to excess content, leading to increased motility.

• Symptoms: Increased gas, bloating, flatulence, loose stools, diarrhea, cramps.

• Causes:

  • Congenital: Born with lactase deficiency.

  • Primary: Normal initially, loss of lactase over time.

  • Secondary: Transient loss due to infection or antibiotics, causing loss of glycocalyx.

Protein Digestion

• Begins in the stomach with pepsin (active in acidic pH).

  • Pepsinogen activated to pepsin by HCl.

  • Proteins broken down into proteoses, peptones, and polypeptides.

• Small Intestine:

  • Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase, proelastase, elastase) digest proteins.

  • Brush border enzymes (peptidases, aminopolypeptidases, dipeptidases) further digest polypeptides.

  • Proteoses, peptones, and polypeptides are converted into polypeptides and amino acids.

  • Brush border enzymes convert these to amino acids, dipeptides, and tripeptides.

  • Absorbed via apical surface.

• Enzyme Classification:

  • Endopeptidases (trypsin, chymotrypsin, elastase).

  • Exopeptidases.

Protein Absorption

• Amino acids, dipeptides, and tripeptides absorbed via the small intestine.

• Amino Acids:

  • Classified as basic, neutral, and acidic.

  • Transported via sodium symporters (active transport).

• Large Peptides:

  • Must be broken down further into dipeptides, tripeptides, and amino acids.

  • Cannot be transported directly.

• Dipeptides and Tripeptides:

  • Transported with carriers (sodium-dipeptide transporter) or via endocytosis.

Vitamin and Mineral Absorption

• Duodenum:

  • Iron (absorbed as free iron, binds to apoferritin; receptor-mediated endocytosis).

  • Calcium, zinc.

• Jejunum:

  • Folic acid.

  • Vitamin C, thiamine, riboflavin, pyridoxine.

• Ileum:

  • Vitamin B12.

  • Bile salt reabsorption, fat-soluble vitamins.

Fat Digestion

• Emulsification of fat begins in the small intestine with bile and lipase.

• Oral Cavity:

  • Lingual/ salivary lipase from Von Ebner’s glands.

• Duodenum:

  • Bile salts emulsify fats.

  • Pancreatic lipase activated.

• Lipid Types:

  • Triglycerides, cholesterol esters, phospholipids.

• Enzymes:

  • Lingual lipase.

  • Gastric lipase (infants, or absence of pancreatic lipase).

  • Pancreatic lipase.

  • Cholesterol ester hydrolase (pancreas).

  • Phospholipase A2 (pancreas).

• Process:

  • Large fat globule broken into smaller droplets by bile salts (emulsification).

  • Lecithin aids in emulsification.

  • Pancreatic lipase breaks down triglycerides into free fatty acids and monoglycerides.

Fat Absorption and Transport

• Free fatty acids and monoglycerides transported into enterocytes.

• Resynthesized into triglycerides in the smooth endoplasmic reticulum.

• Triglycerides packaged with cholesterol, lipoproteins, and other lipids in the Golgi apparatus to form chylomicrons.

• Chylomicrons transported via lacteals into cisterna chyli, thoracic duct, and left subclavian vein (lymphatic system).

Hormone Locations and Functions

• Gastrin:

  • G cells (antrum, duodenum)

  • Stimulates HCl secretion; can cause gastritis if excessive leading to proliferation of gastric epithelium.

  • Stimuli: peptides, amino acids.

  • Inhibition: acid, somatostatin.

• CCK:

  • I cells.

  • Gallbladder contraction and pancreatic enzyme secretion.

  • Stimuli: fatty acids, amino acids.

• Secretin:

  • S cells.

  • Bicarbonate secretion in GB and pancreatic ducts.

  • Stimuli: Acidic pH.

• Ghrelin:

  • P/D1 cells.

  • Stimulates hunger.

  • Stimuli: Fasting state.

• Motilin:

  • MO cells.

  • Peristalsis, stimulated by food in the small intestine. Does not stimulate interstitial cells of Cajal.

  • Stimuli: unclear, but associated with fasting

• Gastric Inhibitory Polypeptide (GIP):

  • K cells.

  • Inhibits gastric secretion and motility; stimulates insulin release.

  • Stimuli: Fat and carbohydrates.