GI notes

• mucus producing cells,
• a solution of varying concentrations in different areas of the GI tract

produce digestive enzymes and juices released directly into the lumen

• produce hormones that enter the underlying vasculature
• some act locally, have paracrine effects and others can act systemically

• inner circular layer: fibers arranged around the circumference of GI tract
• when they contract, the diameter of the tube decreases

• longitudinal layer: fibers arranged lengthwise along tube
• when they contract, the tube shortens

• double fused layer that connects parietal with the visceral
• connects tube to the cavity wall and organs to tube
• also a conduit for blood and lymph vessels and nerves

• movement of motility, contraction of inner muscles that narrows diameter and helps to mix food in that area
• happens in seemingly random sections along GI tract

• primary propulsive movements designed to move food along GI tract
• involves contraction of both inner and outer muscle groups
• where bolus of food is: behind bolus will have inner muscles contract to move bolus and outer muscles in front of bolus will contract at same time to help move food forward

• start as poly or di- saccharides usually in the form of starch of glycogens
• long chains of monosaccharides ( glucose, galactose, fructose)
• need to be monosaccharides for us to digest

• start in form of triglycerides, 3 fatty acid chains held together in glycerol backbone
• broken down by lipase into free fatty acids

• amylase, mucus, lysozyme: all begin to breakdown cell walls or carbs in the mouth
• can break down some sublingual drugs

• happens in the stomach
• capacity of the stomach to hold food

digestive juices: HCL, pepsin, mucus, intrinsic factor ( necessary for us to absorb B12)

• excretion
• fluid and electrolyte balance- small intestine reabsorbs 9-11 liters/day
• immunity: Kupffer cells in the liver

• has intrinsic control of gut function
• branch of ANS, can still have GI function with a serious spinal injury
• uses post-ganglionic fibers of the Para NS

• governs and regulates all motility
• between inner and outer layer of muscles,
• can regulate muscle movement itself, or with outside stimulation
• sensory component can sense distention or stretch and result in muscle contraction

• direct control of secretion and indirect control of absorption and motility
• regulating secretion regulates how fast digestion and absorption occur
• can stimulate myenteric plexus to affect motility
• sensory neurons can sense osmolarity, nutrient levels, toxins, distension, pH

limits air trapped in GI tract, relaxes only when it needs to

protects esophagus from acidic juices of the stomach

portion of stomach above the level of the lower gastroesophageal sphincter

• bottom portion of stomach
• where muscle is the thickest, responsible for mixing food

refers to mucosal lining, segment of mucus membrane found in antrum of stomach

• deep folds of stomach
• allow for increased capacity of stomach during receptive relaxation, rugae flatten
• can expand up to 20 times its resting size of 50 ml

• in fundus of stomach
• rhythmically depolarizing cells, waves are sub-threshold with we are at rest
• when we anticipate food, waves begin to exceed threshold and they become peristalsis that moves down stomach wall to mix food into chyme
• happens at regular interval

• cells of gastric secretion, within gastric pit
• contain numerous cells types that secrete gastric juices

secrete thick alkaline mucus designed to create barrier to buffer walls from pH of stomach

• create pepsinogen, which is converted to pepsin when it hits acid in stomach
• pepsin breaks down proteins
• produced in inactive form to save chief cells from autodigestion by acidic juices

release HCL and intrinsic factor

release small amounts of histamine which enhances release of HCL and pepsinogen in the stomach

• release gastrin which stimulates release of pepsinogen and HCL,
• enhances gastric motility throughout rest of GI tract
• "on" switch for gastric secretion

• release somatostatin
• beginning of "off" switch for gastric secretion

• what happens btw periods of digestion
• governed by circadian rhythms- lowest in morning, highest at night

• cues that are all anticipatory of food entering the stomach
• stimulates gastric secretion and motility so that as soon as food arrives it can begin to be digested

• vagal enervation to enteric NS, will affect sub-mucosal plexus
• this causes secretion stimulation and indirectly affects myenteric plexus
• increased levels of pepsinogen, HCL, gastrin and histamine ( redundancy)

• governed by cues surrounding presence of food in the stomach
• once food hits stomach, motility and secretion are stimulated

• when we start to ramp down secretion and motility
• governed by food withdrawal from stomach and entry of food into duodenum
• cues are withdrawal of protein from stomach, increased acidity of stomach stimulates secretion of somatostatin which inhibits parietal, G and ECL cells

• presence of fat
• presence of acidity: intestine needs pancreatic secretions to protect itself from HCL
• hypertonicity: one big substance is becoming 20 smaller substances
• distention
• duodenal mucosal cells sense changes and can affect gastric fxn through ENS, reduction in hormone release ( G cells)

• released in duodenum, inhibits gastric emptying
• increases pancreatic and bile secretion
• acts on acinar cells in response to fat and protein
• release is shut off when proteins and fats are digested

• released in duodenum, inhibits gastric emptying,
• decreases HCL secretion, increases pancreatic secretion of Na bicarb to help neutralize acid
• acts on duct cells

• exocrine cells of the pancreas
• digestive enzymes are all packaged and released together, no matter the content of your meal
• stimulated by CCK

• release sodium bicarbonate to neutralize stomach acid in the intestine
• stimulated by secretin
• mixes with bile before it enters duodenum

• all releases in inactive forms
• trypsinogen: trypsin, actives the next two
• chymotrypsinogen: chymotrypsin
• Procarboxypeptidase: carboxypeptidase
• pancreatic amylase and lipase

• between bile duct and duodenum
• shut when we arent actively digesting fat
• relaxation is stimulated by CCK to allow bile to enter duodenum

• absorbable unit of fat in the body, what lipase works on
• droplet of fat that is surrounded by bile salts- has been emulsified
• increases surface area

• absence of fat in the duodenum means there is no longer stimulation for CCK
• CCK levels drop, Sphincter of Oddi constricts and no more bile gets into the duodenum

distention, presence of gastrin, extrinsic nerve activity- vagal input

• bound to brush border
• enterokinases: aid in protein digestion
• disaccharidases: break di into mono
• aminopeptidases: aid in protein digestion

• increase surface area by 600 times
• each is lined with cells that contain microvilli

• valleys between villi,
• contain constantly dividing stem cells that move up the villi to ensure we have the optimal absorptive cells on the top of the villi
• cells are constantly shedding

lymph vessel inside the villi where fat enters the systemic circulation

• haustra: tiny pouches in the colon
• similar to peristaltic wave, enhance water and electrolyte reabsorption
• mass movements that happen 3-4 times/day
• stimulated by release of gastrin, can drive feces the entire length of the colon

• when propulsive movement push feces into the rectum, distention stretch receptors sense stretch and causes receptive relaxation of the internal anal sphincter
• you voluntary relax the external sphincter when you are ready and this allows you to have a bowel movement
• lose reflex if you dont have a BM