Oral Digestion PDF

Section 3: Oral Digestion I. Prehension: Seizing and conveying of food into the mouth. Organs of prehension: Herbivore/Omnivores - Cattle: Tongue. Sheep: Tongue/Cleft upper lip. Horse: Mobile upper lip. Pig: Pointe...

Section 3: Oral Digestion I. Prehension: Seizing and conveying of food into the mouth. Organs of prehension: Herbivore/Omnivores - Cattle: Tongue. Sheep: Tongue/Cleft upper lip. Horse: Mobile upper lip. Pig: Pointed lower jaw. *Drink via suction. Carnivores - Tearing movements involving teeth, jaws and head. *Fluid intake via lapping. II. Mastication A. Functions: 1. Reduces size of foodstuff 2. Increase of surface area 3. Breaks off cellulose outer covering 4. Mixes food with saliva B. Teeth 1. Herbivores: Thorough grinding of plant matter (increase surface area). Ruminants - both initial intake and during rumination. Cheek Teeth: 1) Rough table surface of teeth. 2) Lateral and vertical (main) movements of jaw. 3) Continuously growing 2. Carnivores: Masticate only to reduce food to swallowing size. 3. Omnivores: 1) Thorough mastication on plant diet 2) Gulping on carnivorous diet 18 19 Step Mouth Step Mouth Wave Mouth 20  C. Tooth Decay 1. Formation of plaque on teeth (plaque = precipitate of food and saliva) 2. Oral bacterial colonization of plaque. 3. Lactate and protease production by bacteria. 4. Demineralizes enamel and attacks protein matrix. Fluoride - Replaces OH- groups on hydroxyapatite; bacteriocidal D. Chew Reflex Mastication is mainly voluntary but has an involuntary component. Stimulus: Presence of food in mouth. Reflex: Inhibition of motor neurons to jaw musclature→ Jaw drops → Stretches jaw musculature → Reflex contraction III. Salivation A. Salivary Glands: Saliva = Combined secretions of paired salivary glands - parotid, sublingual, submandibular Secretory cell types of glands: Parotid - Primarily serous. Sublingual, submandibular - Mixture of mucus and serous. B. Saliva flow rates: Interdigestive periods (between meals): Small tonic activity in most species; ruminant has high output because used to buffer rumen. Meal intake: Large increase in salivation with feeding (and with rumination). 21 Amount of salivary output: Dependent on animal’s dietary niche. Horse 10 gal/day Cattle 40 gal/day Dog 0.5 qt/day Man 1.5 qt/day C. Inorganic composition of saliva Non-ruminants: 1. Hypotonic to plasma at low salivary flow rates. 2. Na+, Cl-, HCO3-, K+ are main ions. 3. Slightly acidic (at low flow rates). 4. Many things in blood will get into saliva. Fig. 12 Concentration of major electrolytes in saliva of dog (non-ruminant) or sheep (ruminant) as the salivary flow rate increases. Modified from Thaysen et al. Am. J. Physiol. 178:155 andWest et al. Handbood of Physiology. Alimentary Canal Vol 2: Secretion. Secretory process - Applies mainly to non-ruminant 1. Primary secretion at the acinus of the salivary gland (serous) is similar in composition to ECF without the protein (Na, Cl, HCO3, K and water) 2. Modification in ducts 22  At low flow rates: As saliva passes down collecting ducts, it is modified by NaCl absorption (coupled Na+/H+ and Cl-/HCO3- exchangers), proton excretion (uncoupled Na+/H+ exchange), and K+ secretion. Duct epithelium is very tight and osmotic equilibration does not occur, i.e., ions absorbed but not water. At high flow rates: The salivary composition resembles the primary acinar secretion because there is less time for ion transport modifications in the ducts. Fig.13. Ruminants: 1. Isotonic at all flow rates. 2. PO4- important buffer at lower flow rates and reciprocates with HCO3- at higher flow rates. 3. Alkaline saliva, pH = 8.0. - Buffers rumen pH (VFA) 23  D. Organic constituents of saliva 1. Mucin 2. Ptyalin - Salivary amylase (pig, human) Hydrolyzes starches to maltose Optimal pH = 7.0 3. Urea - Passively moves from blood into salivary duct. Important in the urea recycling of the ruminant. 4. Bacteriocidal compounds a) Lysozymes b) Secretory antibody E. Control of salivation: 1. Autonomic nervous system control: Control of salivation is mainly neural. a) Afferent input to salivary nuclei of the CNS 1) Conditioned reflexes: Assoc. with preparation for eating. Visual input Auditory input Olfactory input 24  2) Unconditioned reflexes - Oral portion: Stimuli = Chemical and mechanical presence of food in mouth Tactile receptors Taste receptors 3) Unconditioned reflexes - Abdominal portion: Esophageal, gastric and intestinal receptors and afferents Ruminant: Fibrous material about cardia stimulates salivation. b) Efferent output by ANS *All glands receive PS and S nerves from CNS (Salivary Nuclei) Daily efferent output is usually a mixture of parasympathetic and sympathetic control. Parasympathetic control dominates the fluid content whereas sympathetic control has a role in releasing salivary enzymes and mucus. 1) Isolated effect of Parasympathetic NS: i. Yields a copious watery saliva due to direct action at acinus ii. Also causes an indirect vasodilation of perfusing vasculature (via bradykinin). 2) Isolated effect of Sympathetic NS: i. Yields a thick, mucus saliva due to direct action at acinus ii. Vasoconstriction -  adrenergic effect. 2. Systemic hormonal influence - Aldosterone Increases Na+ reabsorption and increases K secretion in distal salivary duct Especially accentuated in ruminant. Reduces blood flow to salivary gland End result is a large (up to 80%) decrease in salivary output 25  F. Functions of saliva: 1. All species: a) Softens and lubricates foodstuffs b) Oral hygiene. c) Promotes taste by dissolving chemical constituents. d) Thirst sensation (role in thirst mechanism). 2. Pigs, humans: Digestion of starch by ptyalin. Not in carnivores, cow or horse. 3. Dog and Cat: Evaporative cooling 4. Ruminants: a) Buffer VFA produced in rumen (healthy rumen pH 6.5 - 7.0). b) Recycles urea to rumen. IV. Taste A. Taste bud 1. Anatomy: Ovoid bodies composed of supporting cells, 5 - 18 hair cells (taste receptors), and unmyelinated sensory nerve endings. Fig. 14. Taste bud. Fig. 15. Thresholds for taste (human) 2. Distribution: Epiglottis, palate, pharynx and tongue 26 3. Basic taste modalities: Salt (Na+ channel, ENaC) – Na+ Sour (organic acids, eg, acetic acid) - Protons Sweet (hexose) – Tas1r2 Tas1r3 -G-protein coupled receptors (release ATP to afferent n.) – Loss of function mutation in carnivores Bitter (organic cation, eg, quinine) - Release ATP to afferent nerves Savory/Umami (glutamate receptor)- Release ATP to afferent nerves B. Taste acuity Does not closely correlate with the number of taste receptors (buds). Has several influences: 1. Stimulation of taste receptors 2. Smell 3. Temperature of food. C. Nutritional wisdom: 1. Regulation of energy intake. 2. Water taste. 3. Salt appetite. 4. Other nutrients. Objectives: 1. Define prehension and how it is accomplished in domestic animals. 2. Know the functions of mastication. 3. Describe the process of tooth decay. 4. Know the stimulus-response of the chew reflex. 5. Relate the salivary flow rate of a species to its dietary niche. 6. Know the inorganic composition of saliva in ruminants and non-ruminants. 7. Why is the buffering function of ruminant saliva important? 8. Know the process of saliva formation in the acinus and duct of salivary glands. 9. Know the organic constituents of saliva. 10. Know the effect of the ANS on control of salivation. 11. What are the afferent inputs to the salivary nuclei of the CNS? 27 12. What is the effect of hyperaldosteronemia on saliva output and composition? 13. Know the functions of saliva in the various species of animals. 14. Know the anatomy of the taste bud. Related questions: 1. How do you 'float' the teeth of a horse? 2. How would you feed a horse with severe 'step mouth'? 3. What would be the effect of rapidly consuming a large, concentrated meal on the extracellular fluid volume of a horse? 28