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Lecture

GI Summary.pdf

5 Pages
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Department
Physiology
Course Code
Physiology 2130
Professor
Sarah Mc Lean

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Description
Structure and organization of the digestive system - Absorption occurs in the small intestine and large intestine - Secretion occurs in the mouth, stomach, small intestine, and large intestine Salivary secretion - Saliva (almost pure water) lubricates food for travel through the digestive tract and also plays a small role in digestion (breakdown of carbohydrates by amylase) - - Saliva also contains bicarbonate ions (HCO )3 which help neutralize acidic foods - Three main salivary glands – submandibular, sublingual, and parotid - Primary functional unit of salivary glands is the salivon (diagram on page 552) - Saliva in its initial form (secreted by acinar cells) is isotonic (same osmolarity as plasma) + - As saliva moves from the acinus through the striated duct, Na ions are reabsorbed (taken out of the saliva) and K ions are secreted (added to the saliva) by an active transport mechanism. + + More Na is removed than K added, meaning that saliva moving through the striated duct becomes more and more hypotonic (lower osmolarity than plasma) - Autonomic regulation o Parasympathetic (PNS) signalling increases the volume of saliva produced – very dilute, and almost isotonic (not much time for active transport mechanism, since there is so much saliva flowing through the salivon) o Sympathetic (SNS) signalling increases protein secretion, but not much water – very dry, gummy feel - o PNS and SNS both increase HCO secr3tion from striated duct cells (neutralization) o In PNS-mediated saliva production, HCO conc3ntration is lower (each bicarbonate ion is dissolved in a greater volume of water: osmolarity = solute ÷ solvent) Gastric Secretion - Two main parts of the stomach: oxyntic gland area (also called Orad area; top 2/3 of exorcine stomach) and pyloric gland area (also called Caudad area; bottom 1/3 of exocrine stomach) - The oxyntic gland area has three important cell types: peptic cells (also called chief cells), oxyntic cells (also called parietal cells), and enterochromaffin-like (ECL) cells o Peptic (chief) cells secrete the enzyme pepsinogen (inactive form of pepsin; see section below on processing of proteins) o Oxyntic (parietal) cells secrete HCl (stomach acid) o Enterochromaffin-like cells (ECL) secrete histamine (histamine activates oxyntic cells) - The pyloric gland area has one important cell type: G cells o G cells secrete the hormone gastrin (increases release of acid from oxyntic cells) - Three phases of gastric acid secretion: cephalic, gastric, intestinal (first two are most important) - The cephalic phase relies on the autonomic nervous system – stomach is innervated by the vagus nerve. o Activity in this system is based on senses (sight, smell, taste of food) o Vagus nerve fibers release acetylcholine onto a number of different cell types in the stomach: oxyntic cells, chief cells, G cells, ECL cells o The effects of acetylcholine on cells in the stomach:  Oxyntic cells: increase acid secretion  Chief cells: increase release of pepsinogen  G cells: increase release of gastrin  ECL cells: increase release of histamine - The gastric phase primarily relies on the enteric nervous system (network of nerves within the stomach), but it can also activate the same nerves that are part of the cephalic phase o Activity in this system is based on mechanoreceptor activation in the walls of the stomach o Gastric phase increases acetylcholine release onto oxyntic cells, chief cells, G cells, and ECL cells (effects of acetylcholine on these cell types are listed above) - Acetylcholine activates oxyntic cells to release acid both directly (Ach binding to receptors on oxyntic cells) and indirectly (Ach promoting release of gastrin and histamine, both of which can bind to oxyntic cells) - The effects of these signalling molecules (Ach, gastrin, histamine) on oxyntic cells is synergistic (the overall measured effect is greater than the sum of its parts: 1 + 1 + 1 = a bajillion) - The overall effect of both cephalic and gastric phases is very similar: nerves release acetylcholine onto a number of different cell types in an effort to promote secretion of acid and pepsinogen. The major differences are related to how the two phases are activated (cephalic is as simple as seeing and smelling food, while gastric phase requires food to be in the stomach) - NOTE: G cells can also be stimulated to release gastrin by broken down amino acids and peptides in the stomach (specialized chemoreceptors). This will also result in elevated acid secretion (increased gastrin means increased activation of oxyntic cells) Pancreatic secretion - The primary functional unit of the pancreas is very similar to the salivon (composed of an acinus and a duct) - There are three main cell types in the functional unit of the pancreas o Acinar cells secrete digestive enzymes:  Pancreatic amylase helps break down carbohydrates in mono- and disaccharides  Pancreatic proteases help break down proteins in single amino acids, dipeptides, and tripeptides  Pancreatic lipase helps break down the primary lipid component of our diets (triglycerides) o Intralobar duct cells secrete bicarbonate ions (HCO ). Secretion by these cells is tonic, 3 meaning that they are always releasing small amounts of bicarbonate (without the need for a stimulus) o Extralobar duct cells secrete bicarbonate ions. Secretion by these cells is purely responsive, meaning that these cells only release bicarbonate when they are hormonally activated - Hormonal activation of the pancreas o Acinar cells are hormonally activated by cholecystokinin (CCK). This hormone is released st from cells in the duodenum (1 part of the small intestine). The primary stimulus for CCK release into the circulation is fat molecules in the small intestine. o Extralobar duct cells are hormonally activated by secretin. This hormone is also released from cells in the duodenum. The primary stimulus for secretin release into the circulation is acid in the small intestine. - Neuronal activation of the pancreas o Both acinar cells (digestive enzymes) and ductal cells (bicarbonate) can be activated by acetylcholine release from vagal or enteric nerve fibres. - Activation of acinar cells and ductal cells by acetylcholine, secretin, and CCK is synergistic Motility - Contractions throughout the gastrointestinal tract are caused by activation of enteric smooth muscle cells - Two important points about enteric smooth muscle cells: o Electrically-coupled by gap junctions, meaning you’ll have fast activation and well- coordinated activity o These muscle cells have an intrinsic electrical activity – a small fluctuation in the membrane potential of the cell (termed the slow wave) - Each peak in the slow wave bring
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