Alimentary System

Alimentary System

1. Introduction to GI tract

   • Headgut
     • Function:- prehension and mastication
     • Lips, tongue, teeth and oesophagus
     • Comparision- Dog has a wide mouth with sharp serrated teeth. good for holding prey still and cutting meat so matches diet
     • Comparision- horse has v small mouth with very flat teeth to grip and tear grass
     • GLANDS- Parotid, mandibular and sublingual salivary glands
     • oesophagus- layers: mucosa, submucosa, tunica muscularis and serosa. Glands found in submucosa
       • tunica muscularis responsible for peristalsis 
       • take ingested material to stomach, stratified squamous epithelium
   • Foregut
     • Stomach, liver and pancreas
     • Stomach
       • Gastric pits found in the submucosa and lamina muscularis contracts to empty the glands
       • Ruminant stomach
         • Rumen + reticulum- fermentation by microbes
         • omasum- water reabsorption
         • abomasum- similar to normal stomach- glandular secretes HCL and pepsinogen- acidic
           • duodenum branches from here 
     • Liver
       • all blood from the gut goes through the liver before systemic circulation
       • gall bladder found between the lobes- stores bile before secretion- NOT IN HORSES
       • liver produces bile
         • also lots of other functions but just concentrate on this for the time being
     • pancreas- alongside duodenum- secrete pancreatic juice
   • Midgut
     • Duodenum- fixed to right and side wall. secretes mucous, bile duct and pancreatic duct attach here. Caudally there are villi
     • Jejunum- much longer and more mobile
     • ilum- v short also more mobile than duodenum
     • more proximally focusses on digestion, more distally is absorption
   • Hindgut
     • caecum- blind ended sack between SI and LI important in hindgut fermenters like horses
     • Colon- split into ascending, transverse and descending. absorb water and compact undigested material
     • faeces expelled via rectum and anus
     • folded but no villi

2. Regulation of Gut Motility

   • Mechanisms
     • Peristalsis- two layers of muscle in tunica muscularis. Inner circular layer contract behind bolus, longidudinal layer contract in front
     • mechanical digestion- in stomach- tunica muscularis and lamina muscularis
       • proximal region more important to store material, more elastic thinner tunica musuclaris
     • rumen- figure of 8 motion to maintain pH and environment for microbes and pass ingested material pass exit regularly
   • MOtility types in the intestines
     • Small intestine
       • peristalsis
       • segmental motility- mixing using rhythmic contraction of circular muscle. like peristalsis but moving one direction then the other
     • Large Intestine
       • peristalsis (including mass movements)
         • mass movement is peristalsis in longer phases to clear out intestine
       • haustration- in the large intestine longitudinal muscle is concentrated into bands, when it contracts the bands shorten but the sparse areas form sacs
         • acts to mix the contents of the intestine, exposing it all to intestinal lining so water and ions can be absorbed
   • Intrinsic control
     • enteric nervous system
     • myogenic muscle- exhibits basic electrical rhythm- when it exceeds threshold value it depolarises
     • intestinal cells of cajal can regulate threshold value so can be influenced by external factors
   • endocrine/ paracrine control
     • endocrine- the hormone must travel in the blood e.g. gastrin is secreted by the stomach in response to stretch- increase HCL and pepsinogen secretion and gastric motility
     • Cholecystokinin (CCK) is released by the SI and increases bile secretion and pancreatic secretion while increasing SI motility
       • also endocrine
     • paracrine agents act locally- do not travel in blood, e.g. SOMATOSTATIN inhibits HCL secretion AND GUT MOTILITY
   • Nervous Control
     • GIT has both intrinsic and autonomic control
     • myenteric plexus is found in the tunica muscularis
     • submucosal plexus is in the submucsoa along with sensory fibres
     • all 3 feed back to the CNS for coordinated action e.g. vomiting

3. Protein sorting and secretion

   • The final destination of a protein
     • all ribosomes start free, proteins from free ribosomes are translated directly into the cytosol
     • most free proteins stay in the cytosol, if they have a sorting signal they will be sorted into other compartments
     • if a protein is meant for the ER the ribosome becomes fixed to the ER and the protein is translated directly into it. this is called co-translation translocation
   • ER
     • proteins here confined to SECRETORY ENDOCYTIC PATHWAY
       • makes phospholipids for cell membrane, stores calcium and synthesises cell surface membranes.
         • proteins moved to golgi by vesicle traffic
   • Golgi
     • most protein sorting occurs in trans golgi
       • lots of proteins leave golgi and exit cell by default exocytosis
         • if protein has a specialised modification it can be sent to lysosome
           • other signals will mean a protein is secreted by MEDIATED exocytosis e.g. hormones
   • lysosome extras (how do things enter?)
     • augophagy- old cell material taken in by lysosome to be digested
     • protein translocation from cytosol
       • endocytosis (can be phagocytosis or pinocytosis)

4. GIT Secretions

   • 3 phases of secretion
     • CEPHALIC- conditioned reflex in response to sight smell or taste of food
       • via vagus innervation (PNS) increase gastrin secretion and histamine
         • this in turn increases HCL secretion, pepsinogen and increases gut motility
     • GASTRIC- chemo and mechano receptors in stomach sense food and increase gastrin + histamine secretion
       • local negative feedback loop if pH drops below 3. secretion of gastrin incresaes HCL secretion and pepsinogen along with increasing stomach motility
     • INTESTINal- chyme in duodenum stimulates secretin, gastric inhibitory peptide and CCK
       • SECRETIN- increase pancreas secretion, inhibit gastrin
       • GIP just inhibits HCL and pepsinogen secretion
       • CKK- increase pancreas secretion and increase secretion from gall bladder
         • gall bladder also secretes bicarbonate to neutralise chyme entering SI
   • 1-Saliva
     • amylase
     • bicarbonate buffer
     • pH 6.5
     • function- lubrication and digestion, under AUTONOMIC CONTROL
   • 2-stomach
     • PARIETAL CELLS- HCL SECRETION-
       • break down of sodium chloride and carbonic acid, HCL secreted into the stomach, sodium bicarbonate secreted into the blood
     • CHIEF cells- pepsinogen
     • goblet cells- mucous secretion and production
     • stimulated by gastrin, acetylcholine and histamine
   • 3- Small intestine
     • secretes 2 fluids
     • Succus Entericus
       • lots of digestive enzymes
     • pH 7-8 more alkaline
     • Alkaline mucoid fluid (no enzymes)
       • CKK increases mucous
     • Glycocalyx- surround tips of microvilli and trap digestive material close to cell surface
   • 4- liver/ gall bladder
     • bile produced from cholesterol in smooth ER of hepatocytes is a detergent soemulsifies fat bile pigments are also excreted from blood
     • Liver secretes fatty acids, bile salts and cholesterol
     • pH 7-8
     • secretion of bile controlled by return of bile salts to liver, presence of food in the duodenum and CKK
     • bile secreted in canaliculi as sodium salts so water follows via osmosis, bile duct secretes bicarbonate and concentrates bile
   • 5-Pancreas
     • digestive enzymes produced by acinar cells
     • duct secretes sodium bicarbonate
     • stimulated by acetylcholine, secretin and CKK
       • CKK acts more on the acinar cells, secretin acts more on the duct cells

5. Digestion and Absorption Summary

   • DIgestion- mechanical, chemical and microbial breakdown of large insoluble compounds to simple absorbable compounds
   • absorption- simple compounds taken across intestinal membranes into blood
   • Carbohydrates
     • carbohydrates converted to di, tri and oligosaccharides by the action of alpha amylases (secreted by salivary glands and pancreas) which attacks the 1,4 glycosidic bonds in the middle of the carbohydrate chain
     • further breakdown in SI by isomaltase and disaccharidases, the main product being glucose and some fructose. glucose absorbed by sodium-glucose co transport
   • Proteins
     • proteases attack peptide bonds, they are secreted as inactive zymogens
     • pepsinogen secreted by stomach and active at low pH. will not digest keratin or mucin. secreted by chief cells and can catalyse own activation
     • Trypsin, chymotrypsin and elastase are pancreatic endopeptidases
       • secreted as zymogens and activated in SI
     • exopeptidases can be either carboxypeptidases or aminopeptidases
       • aminopeptidases split the amino end made by SI glands
       • carboxypeptidases break at carboxyl end secreted by pancreas
     • amino acids absorbed by 4 types of sodium dependant protein channel
   • Fat Digestion
     • Most fat in diet triacylglycerols
     • lipases remove fatty acids from TAGs
     • lingual lipases secreted by mouth and stomach
     • pancreatic lipase active in SI in the presence of bile
     • products- free fatty acids, 1-monoacylglycerol, 2-monoacylglycerol and plain ol glycerol
     • bile emulsifies to produce micelles which carry products of fat digestion to brush boarder where they are pssibley absorbed