GI part I

• provide nutrients
• remove waste
• water/electrolyte homeostasis
• storage
• propulsion
• break up food material, mechanically or chemically
• immunity (barrier)

• process of breaking down nutrients into simple molecules
• carbohydrates=monosaccharides
• proteins=small peptides and amino acids
• lipids=monoglycerides and fatty acids

the process of transporting the end products of digestion across the gastrointestinal walls and into the bloodstream, lymphatic vessels and tissue cells. 

• carnivores: flesh-eating, inconspicuous cecum (dog, cat)
• herbivores: plant-eating, voluminous cecum (horse, cow)
• omnivores: both-eating (pig, human)

tube that runs from the mouth to the anus, containing esophagus, stomach, small intestine, large intestine

• Outside to in:
• serosa,
• 2 muscle layers (muscularis externa, circular and longitudinal, some have skeletal),
• submucosa,
• 3-layer mucosa (epithelial, lamina propria, muscularis mucosa)

• Gi is double-controlled, containing both regular organ (CNS and endocrine) control and a unique intrinsic NS and endocrine within GI organs.  If deinnervated, funcion near normally. 
• 2 systems of ganglia in "brain of gut": submucosal plexus and myenteric plexus

• parasympathetic: enhances digestion
• sympathetic: inhibits digestion

• Two systems of ganglia in enteric nervous system, extend from esophagus to anus, "Brain of Gut". 
• Submucosal includes secretion of epithelial and blood flow
• Myenteric is in between 2 muscular layers, controlls movement

secrete directly into blood, no duct.  Regulatory in GI, do not actually break down food.  Distriubuted through gut epithelium. 

secrete into interstitial fluid, moves by diffusion into cells.  More local-acting than endocrine.  Regulatory in GI, do not actually break down food.  Distriubuted through gut epithelium. 

• Cholecytokinin (CCK): stimulate pancreas and inhibit gastric emptying (not too fast)
• gastrin: stimulate acid secretion and gastric motility, growth of epithelium
• Secretin
• gastric inhibitory peptide
• Motilin

• Mandibular (submandibular, submaxillary): mixed, below parotid, below/at jaw below ear
• Sublingual: Mucous, below tongue
• Parotid: serous, just below ear
• zygomatic

• solubilizes food
• provides alkaline buffer and fluid
• remove waste
• lubricates and binds
• initiates starch digestion in omnivores (Amylase, pigs and humans, NOT c/d)
• Assists in oral hygiene (immunoglobulins)
• enables evaporative cooling

• continuously produce saliva
• food increases salivation (parasympathetic)
• sight, smell, sound, taste, thought of food stimulates production. 

• passes through diaphragm and enters cardia of stomach. 
• Inner circular muscle layer, outer longitudinal muscle layer
• dog and cattle have skeletal muscle throughout
• horse, cat, primate, smooth muscle throughout

• inferior end of esophagus, cranial portion of abdomen.  Monogastric or complex (ruminants).  Rugae (wrinkes that go away when full)
• 4 functions: store injesta, mechanical breakdown, disrupt chemical bonds (acids and enzymes), product intrinsic factor for vitamin B12 absorption in small intestine.

Cow, sheep, goat, deer, moose, elk, reindeer, caribou, antelope, giraffe, bison

• no omasum. 
• Camel, llama, alpaca, vicuna

• Rumen (Pouch, biggest, fermenter, forestomach)
• Reticulum (honeycomb, fermenter, forestomach)
• Omasum (book, leaves or pages, some absorption, connects fermenters to abomasum, forestomach)
• abomasum (true stomach)

• anaerobic bacteria, protozoa and fungi break down complex carbs (cellulose, hemicellulose, pectin) b/c mammals don't make enzymes. 
• Products are short-chain fatty acids (SCFAs) or volatile fatty acids (VFAs), methane gas, CO2, NH3

• 60-80% of dietary energy to ruminants
• absorbed in forestomach into hepatic portal circulation
• Proprionate (makes glucose and glycogen)
• Acetate (used for energy, makes fats)
• Butyrate (used for energy)

• microorganisms synthesize B vitamins except B12, produce ammonia
• ruminants absorb amino acids made by bacteria and digest microorganisms for protein. 

• where most digestion and absorption takes place (except for extensive fermenters).  3 parts
• Duodenum, jejunum, ileum
• large surface area with plicae (folds like rugae in stomach but permanent) and villi (fingers to increase surface area)

uses bile for fat absorption.  Comes in from gallbladder by common bile duct.

contains villi and peyer's patches (aggregates of lymph tissue in ileum)

permanent folds in intestines to increase surface area.  Villi project off plicae, microvilli project off villi

• groups of undifferentiated cells between adjacent villi, only cells that divide.  Migrate towards tips of villi as older cells are lost from top.
• In large and small intestine

• finger-like projections off plicae in small intestine to increase surface area. Microvilli off villi (brush border)
• Lacteal (white body inside villi)=lymphatics, blood vessel
• Not in large intestine

• embedded in microvilli of absorptive epithelial cells lining small intestine. 
• Enzymes digest carbs, proteins, nucleotides.

• Includes cecum and ascending, descending and transverse colon.  Ascending varies most between species
• Some fermentation occurs in LI in ALL SPECIES
• non-ruminant herbivores (horse, rabbit, pig, rat, guinea pig) have extensive hindgut fermentation.  Don't break down microbes (ferm after stomach)

• Cows ferment pre-stomach, (rumen, etc.), and digest microbes in stomach
• Horses ferment post-stomach in cecum and colon and cannot digest microbes. 

Horse

cecum, ascending colon, transverse colon, descending colon, rectum, anus

• accessory digestive organ, sits alongside stomach and duodenum.  2 portions
• Endocrine passes directly into blood, alpha cells (glucagon) increase blood glucose, beta cells (insulin) decrease blood glucose.  Hemostasis. 
• Exocrine passes through duct into duodenum.  Digestive enzymes like trypsin and chemotrypsin

• accessory digestive organ
• produces bile to aid in absorption of fats and fat-soluble vitamins

getting food into the mouth.  Prehensile organs include lips of horse, tongue of cow, snout of pig. 

• Voluntary but some reflex action.  Teeth, jaw and chew type vary with species/diet. 
• carnovores have simple food to tear, little grinding
• herbivores have some hypsodont teeth, upper jaw is wider than lower and chewing is thorough. 

• LOTS of saliva (cows), high pH, basic to neutralize acids.  Regulated parasympathetically. 
• saliva is water, electrolytes, mucus and enzymes
• lubricates and softens, lysozymes, and amylase for starch breakdown in omnivores (a little in horses, none in ruminants and carnivores)

• swallowing, act of conveying food from mouth to stomach.  Requires some degree of voluntary/consciousness
• 3 stages include through the mouth (voluntary), through the pharynx (reflex), through the esophagus (reflex)

Voluntary phase.  chewing/saliva, tongue forms bolus, pushes toward pharynx, soft palate is raised, caudal nares closed

• pharyngeal stage.  Involuntary, controlled by swallowing center in brainstem. Initiated when bolus stimulates pressure receptors in pharyngeal wall. 
• openings to pharynx other than esophagus close, contraction wave pushes bolus into esophagus, upper esophageal sphincter relaxes to accept. 

closes over trachea during swallowing to prevent entry of food

• esophageal stage.  Involuntary reflex controlled by swallowing center in brainstem, peristalsis stimulated by food. 
• luminal constriction preceded by luminal distension, propelling the bolus distally toward the stomach. 

• a moving wave of luminal constriction preceded by and area of luminal distension, which results in propelling the food bolus distally toward the stomach. 
• alternate contraction of longitudinal and circular muscle (dilation is relax of circular, contract of long)

• stores food
• reduces food, mechanically and enzymatically, into particles small enough to be digested in small intestine
• releases into small intestine at controlled rate 

• muscle activity is spontaneous and is modulated by autonomic nervous system. 
• Slow wave potential, fires action potential when reaches threshold (sin wave gradually rising)
• Parasympathetic and Sympathetic can raise or lower baseline to increase or decrease.  ACH spikes, noreph lowers
• 3 parts Fundus, Body, Antrum

• Fundus: receives and stores by adapting volume
• Body: mixing vat for saliva, food and gastric secretions
• Antrum: pump, regulates the propulsion of food past pyloric sphincter into duodenum

relaxation of the walls of proximal simple stomach in response to injestion of a large amount of food at one time

peristalsis is weak in fundus and body, strong in antrum, grow as preceed (distally) towards pylorus

• Fragment food into smaller particles
• empty gastric contents into duodenum at a controlled rate (pyloric sphincter constricts as peristalsis approaches, allowing only very small molecules (chyme) through.  large are retropulsed or sent back. 

vigorous backward motion of gastric contents from pylorus to antrum, to mix and grind the contents further (for larger molecules)

Neurohumoral. 

• Neuro: fibers from vagus to gastric myenteric plexus.  High degree of control over motility. 
• Proximal: vagus suppresses contractions (adaptive relaxation, fundus relaxes for more food in)
• Distal: vagus stimulates lots of peristaltic activity.

• Max # of contraction controlled by slow waves(undulating baseline resting potential)--not all result in contraction (spikes on top)
• Ach of Vagus can bring membrane potential closer to threshold (stimulated by CNS, stomach and intestine.  Thought of food or stomach distension)

• Gastrin: secreted by cells in gastric antrum, ENHANCES gastric motility
• Cholecystokinin (CCK) and Gastric inhibitory peptide: SUPPRESS gastric motility (dog)

• 2 mechanisms
• Enterogastric reflex: sensory receptors in duodenal wall slow gastric emptying
• Hormonal reflex: cholecystokinin (CCK) and Gastric Inhibitory Polypeptide (GIP) delay gastric emptying
• Prevent too much water to leave the body if chyme is hypertonic, and allows proper reabsorption etc to occur

• Sensory receptors in duodenal wall slow rate of gastric emptying by slowing and decreasing force of gastric contractions, increase contraction of pyloric sphincter
• Stimulated by too much chyme, low chyme pH, high amino acids or fatty acids in chyme, hyper or hypotonic chyme.

• cholecystokinin secreted when chyme has too much amino acid or fatty acids, or low pH. 
• Secreted by duodenal mucosa into blood stream. 
• slows gastric emptying by further constriction of pyloric sphincter (slows chyme for normal digestion)

• Gastric inhibitory polypeptide
• secreted by jejunal mucosa in response to presence of lipids and carbohydrates
• delays gastric emptying.

• expulsion of contents of stomach and proximal duodenum through mouth. 
• Controlled by emetic center in brain (brainstem--medulla?)
• Series of reflexes initiates and closes glottis and nose. 
• Easy for pigs, d/c.  Ruminant vomits abomasal into forestomach. 
• Horses can't/shouldn't.  Terminal.  Stomach rupture

• 2-3 per minute.  2 parts
• Primary is mixing and separating large and small molecules
• Secondary is eructations, contractions moving liquid down so gasses can go up. 

• Process of bringing food material back from the ruminant stomach to the mouth for further mastication. 
• Allows for rapid food intake during grazing, chew later.  Spread out evenly throughout day.  Different hours with diet (8 for hay)
• 4 phases include regurgitation, remastication, reinsalivation, reswallowing (redeglutition). 

• step one in rumination. 
• Reticulum contracts, bringing material from dorsal to cardia.  Cardiac sphincter relaxes, animal takes breath against closed glottis, negative pressure in thorax, dilates esophagus and cardia, rumen contracts, pressure in rumen rises, bolus goes to esophagus for lower pressure, reverse peristalsis, bolus in mouth. 

• steps 2-3 in rumination.  Occur together
• Immediatly after bolus in mouth, liquid squeezed out and swallowed. 
• Thorough and deliberate remastication, # based on diet, roughage chewed more than 100 times. 
• 100-200 L (25-50 gallons) saliva in 24 hours. 

• last step in rumination.  Reswallowing of bolus. 
• Occurs when food is ready.  Next cycle begins in 5 seconds. 

• Process by which gas from forestomach is removed by esophagus and pharynx
• Gasses produced during fermentation are mainly CO2 and methane. 
• CO2 from fermentation of carbs and deamination of amino acids (60-70%)
• Methane is reduced from CO2 by bacteria (30-40%)

• Gas bubble moves cranially and ventrally towards cardia with contractions/relaxation, making sure there is no injesta.  (like belch but no sound)
• Nasopharyngeal sphincter contracts, directing part of gas into trachea, where it is moved into lungs on inspiration. 

• peristalsis
• segmentation

• Stretching of intestinal wall trigters ring of circular muscles to contract.  As one area relaxes, another contracts. 
• Mix digesta with enzymes/juices and increase contact with membrane for absorption. 

• absorption of water and electrolytes
• storage of feces
• in non-ruminant herbivores, fermentative digestion and absorption of nutrients

• segmentation - like small intestine, localized circular contractions to make sacs
• antiperistalsis - to mix and bring near membrane
• mass movement - moves entire contents nearer rectum. 

modified form of segmentation in which intense local contractions of circular muscle cause large intestine to appear to bulge into sacs. 

• Activity of colon is for delay of transit and filling of parts (reservoir)
• Increased activity = constipation
• decreased activity = diarrhea

• Period of intense propulsive activity that moves entire contents of colon distally towards rectum. 
• Colon, sometimes equine cecum. 

• Complex reflex act in which feces are evacuated from terminal colon and rectum. 
• Frequency depends on species/animal (5-10/day horses; 10-20 cows; 2-3 carnivores)

• Amount of time it takes for material to travel from one part of the gut (or whole gut) to another.  (fast = decreased intestinal transit time)
• Varies with species (Pig 48h, horse 24-48h, cattle see in 12-24, 80% in 3-4d, all in 7-10d).