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goblet cells
- mucus producing cells,
- a solution of varying concentrations in different areas of the GI tract
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exocrine cells
produce digestive enzymes and juices released directly into the lumen
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endocrine cells
- produce hormones that enter the underlying vasculature
- some act locally, have paracrine effects and others can act systemically
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inner layer of muscularis externa
- inner circular layer: fibers arranged around the circumference of GI tract
- when they contract, the diameter of the tube decreases
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outer layer of muscularis externa
- longitudinal layer: fibers arranged lengthwise along tube
- when they contract, the tube shortens
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Mesentery
- 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
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segmentation contraction
- 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
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Peristalsis
- 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
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digestion of carbs
- 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
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digestion of fats
- start in form of triglycerides, 3 fatty acid chains held together in glycerol backbone
- broken down by lipase into free fatty acids
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secretions in the mouth
- amylase, mucus, lysozyme: all begin to breakdown cell walls or carbs in the mouth
- can break down some sublingual drugs
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receptive relaxation
- happens in the stomach
- capacity of the stomach to hold food
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secretions in stomach
digestive juices: HCL, pepsin, mucus, intrinsic factor ( necessary for us to absorb B12)
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non-digestive functions of GI tract
- excretion
- fluid and electrolyte balance- small intestine reabsorbs 9-11 liters/day
- immunity: Kupffer cells in the liver
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Enteric Nervous system
- 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
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Myenteric Plexus
- 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
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Sub-Mucosal Plexus
- 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
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upper or pharyngoesophageal sphincter
limits air trapped in GI tract, relaxes only when it needs to
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lower or gastroesophageal sphincter
protects esophagus from acidic juices of the stomach
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fundus
portion of stomach above the level of the lower gastroesophageal sphincter
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antrum
- bottom portion of stomach
- where muscle is the thickest, responsible for mixing food
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pyloric gland area
refers to mucosal lining, segment of mucus membrane found in antrum of stomach
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rugae
- 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
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pace setter cells
- 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
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oxyntic mucosa
- cells of gastric secretion, within gastric pit
- contain numerous cells types that secrete gastric juices
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goblet cells in oxyntic mucosa
secrete thick alkaline mucus designed to create barrier to buffer walls from pH of stomach
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chief cells of oxyntic mucosa
- 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
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parietal cells in oxyntic mucosa
release HCL and intrinsic factor
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Enterochromaffin-like cells in oxyntic mucosa
release small amounts of histamine which enhances release of HCL and pepsinogen in the stomach
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G cells of pyloric gland area
- release gastrin which stimulates release of pepsinogen and HCL,
- enhances gastric motility throughout rest of GI tract
- "on" switch for gastric secretion
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D cells in pyloric gland area
- release somatostatin
- beginning of "off" switch for gastric secretion
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interdigestive phase of gastric secretion
- what happens btw periods of digestion
- governed by circadian rhythms- lowest in morning, highest at night
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cephalic phase of gastric secretion
- 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
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extrinsic influence of cephalic phase
- 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)
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gastric phase of gastric secretion
- governed by cues surrounding presence of food in the stomach
- once food hits stomach, motility and secretion are stimulated
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intestinal phase of gastric secretion
- 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
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signals of food entering duodenum
- 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)
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CCK
- 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
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Secretin
- released in duodenum, inhibits gastric emptying,
- decreases HCL secretion, increases pancreatic secretion of Na bicarb to help neutralize acid
- acts on duct cells
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acinar cells
- exocrine cells of the pancreas
- digestive enzymes are all packaged and released together, no matter the content of your meal
- stimulated by CCK
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duct cells
- release sodium bicarbonate to neutralize stomach acid in the intestine
- stimulated by secretin
- mixes with bile before it enters duodenum
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Pancreatic enzymes
- all releases in inactive forms
- trypsinogen: trypsin, actives the next two
- chymotrypsinogen: chymotrypsin
- Procarboxypeptidase: carboxypeptidase
- pancreatic amylase and lipase
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sphincter of Oddi
- between bile duct and duodenum
- shut when we arent actively digesting fat
- relaxation is stimulated by CCK to allow bile to enter duodenum
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micelle
- 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
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control of bile secretion
- 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
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cues that increase segmentation in jejunum and ileum
distention, presence of gastrin, extrinsic nerve activity- vagal input
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membrane bound enzymes in small intestine
- bound to brush border
- enterokinases: aid in protein digestion
- disaccharidases: break di into mono
- aminopeptidases: aid in protein digestion
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Intestinal Villi
- increase surface area by 600 times
- each is lined with cells that contain microvilli
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crypts of Lieberkuhn
- 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
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central lacteal
lymph vessel inside the villi where fat enters the systemic circulation
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haustral contractions
- 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
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defecation reflex
- 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
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