Gastro-intestinal physiology Flashcards
Roles of the GI tract
Feeding - prehension (food in mouth), mastication (chewing), deglutition (swallowing)
Neurohormonal regulation of feeding and appetite
Digestion
Absorption (digestion + absorption = assimilation)
Elimination - gets rid of the things that are NOT assimilated
Neural regulation of the GI Tract
This is mainly under autonomic control - sympathetic and parasympathetic.
Most of the functions are independent of CNS and voluntary control. Parasympathetic and
Sympathetic nervous system is extrinsic.
Parasympathetic neurotransmitter is acetlycholine on muscarinic receptors and
Sympathetic neurotransmitter is noradrenaline on α and β2 receptors.
Enteric Nervous System
This is intrinsic and has two major networks: Myenteric Plexus (Auerbach’s Plexus) - controls smooth muscle tone (motility) Submucosal Plexus (Meissner’s Plexus) - control of secretion, blood flow and afferent sensory information (chemical and stretch)
Neurotransmitters of the Enteric Nervous System -
Excitatory - acetylcholine (M1, M2 receptors), serotonin (5HT receptors)
Inhibitory - enkephalins (reduce secretion - opioid receptors)
Acetylcholine of the parasympathetic nervous system is excitatory, increasing the membrane potential to threshold, which means that there is contraction in the GI tract causing motility, secretions etc. Noradrenaline of the sympathetic nervous system is inhibitory so membrane potential doesn’t reach a threshold and there are no contractions in the GI tract.
Secretions WITH digestive function
• Saliva (amylase, lipase) • Gastric Juice (HCl, pepsins) • Pancreatic Juice (proteases, lipase, amylase, nuclease) • Bile (emulsifying agents)
Salivary Secretion
This is a spontaneous secretion innervated by the autonomic nervous system.
• Parasympathetic post ganglionic fibres - acinar cells, myoepithelial cells, ductal cells
• Sympathetic stimulates secretion from arterioles and venules
Digestive enzymes include α-amylase (ptyalin) and acidic lipase (from the tongue)
Gastric Secretion
- HCl - from parietal cells
- Pepsinogen and acidic lipase - from peptic cells
- Intrinsic factor - from parietal cells (needed for absorption of vitamin B12)
- Alkaline mucus - from mucous cells
Pancreatic Enzymes
Secreted from acinar cells by exocytosis in response to neural and hormonal stimuli
Active enzymes- pancreatic lipase, a-amylase, deoxyrbonuclease
Inactive zymogens- trypsinogen, chymotrypsinogen, procarboxypeptidase, prophospholipase A
Primary bile acids
Cholic acid, chenodeoxycholic acid. Glycocholic acid and taurocholic
acid are derivatives of cholesterol conjugated with taurine and glycine to give water soluble
derivatives. They are emulsifiers of fats
Primary bile acids
Cholic acid, chenodeoxycholic acid. Glycocholic acid and taurocholic acid are derivatives of cholesterol conjugated with taurine and glycine to give water soluble
derivatives. They are emulsifiers of fats
Secondary bile acids
Deoxycholic acid, lithocholic acid, they are recycled primary bile acids by dehydroxylation
Absorption of Monosaccharides
Glucose, fructose and galactose. They are absorbed via:
- Passive Diffusion (slow and limited)
- Na+/glucose symport (secondary active transport) MOST IMPORTANT!
- Na+ independent facilitative transport
There is effective reabsorption of Sodium, particularly in the distal ileum.
Lipid Absorption
• Formation of mixed micelle(monoacylglycerol, free fatty acids, bile acids and cholesterol)
• Diffusion across apical membrane
• Binding to fatty acid binding protein in cytosol for transport to smooth ER
• Re-synthesis of triacylglycerol
• Packaging of triacylglycerol with cholesterol, apoproteins and phospholipids
(chylomicrons)
• Transport of chylomicrons across basal membrane into lacteals
