Saliva PDF

II SALIVA 2.1 Introduction The digestive system consists of a gastrointestinal tract and accessory glands. Its roles40 are: 1. Secretion of digestive juices 2. Digestion of food 3. Movement of food 4. Absorption of digestive products 5. Excr...

II SALIVA 2.1 Introduction The digestive system consists of a gastrointestinal tract and accessory glands. Its roles40 are: 1. Secretion of digestive juices 2. Digestion of food 3. Movement of food 4. Absorption of digestive products 5. Excretion 6. Endocrine function41 The anatomical order of the digestive tract: 1. Mouth (oral cavity)42 2. Pharynx 3. Esophagus 4. Stomach 5. Small intestine (bowel) a. Duodenum b. Jejunum c. Ileum 6. Large intestine (bowel) a. Cecum b. Colon i. Ascendant ii. Transverse iii. Descendent 7. Sigmoid Colon43 8. Rectum a. Anal sphincter Accessory glands: 1. Salivary gland 2. Pancreas 3. Liver 40 One could argue there is an “accessory” role of information: it tells the body what is or isn’t good to eat. 41 The endocrine system refers to the collection of glands of an organism that secrete hormones directly into the circulatory system to be carried towards a distant target organ. Conversely, exocrine refers to any gland or cell releasing substances into a tube, a cavity or outside the body. 42 Oral or buccal are adjectives relating to the mouth. 43 Some medical sources consider the sigmoid colon, the rectum and the anal canal to be part of the large intestine. On Dr. Papacocea’s slides, however, they were presented as separate parts. 16 2.1.1 Digestive tube layers From outside to inside: 1. Serosa 2. Longtitudinal muscle layer 3. Circular muscle layer 4. Submucosa 5. Mucosa The submucosa contains the so-called Meissner plexus: thousands of networked neurons comprising the local nervous plexus. 17 The muscularis also contains the Auerbach plexus (not pictured) which controls muscle contraction (for example to move the food down the tract). 2.1.2 Digestive secretions Secretions occur in most places along the digestive tract. These secretions consist of mixtures of varying content and concentrations. Generally, enzymes, mucus, ions and other substances can be found. Regulation of secretion is normally both nervous and humoral.44 It is mainly parasympathetic45, but can sometimes also be conditioned.46 Secretion can depend on certain variables: quantity (eg. more saliva is secreted when eating), quality (the type of food will dictate the content of secretion) and biorhythm (persons with a strict regular diet can get conditioned to secrete in anticipation even if no food is yet present). 44 From, with or by hormones 45 Unconscious, “rest-and-digest”. The opposite is sympathetic, together they form the autonomous nervous system. 46 See Pavlov’s dog. 18 2.2 Saliva The target cell in salivary secretion is an exocrine cell, located in one of the salivary glands. The salivary glands are the parotid, submandibular and sublingual glands, as well as some smaller buccal glands.47 The secretory or exocrine cells in such gland consist of acini48 which contain both serous49 cells and mucus cells. The secretion is then excreted in the oral cavity through the ducts, which are made from striated cells and are myoepithelial.50 Depending on the gland, acini are serous in the parotid gland, mucous in the sublingual gland and both in the submandibular gland. 47 These secrete mostly mucus. 48 Clustered cells resembling a raspberry, sg. acinus 49 Secreting a body fluid that is not mucus 50 Cells usually found in glandular epithelium as a thin layer above the basement membrane but generally beneath the cells in the lumen. 19 20 The exocrine cells pictured above are typical cells found in salivary glands. They have an apical pole (at the level of secretion), rich in granules containing enzymes, and a basal pole which is in contact with the blood vessel. 2.2.1 Primary saliva Saliva secretion actually happens in two steps. Primary saliva is produced in the acini by an active secretion done by ionic pumps. This primary secretion contains ptyalin and mucin in a solution of ions in concentrations not too different from plasma – it is (nearly) isotonic. 2.2.2 Secondary or Final saliva Primary saliva is not left untouched by the gland: in the salivary ducts, it is adapted and reformed: the final composition of saliva is altered. The striate cells of the duct actively reabsorb Na and secrete K instead.51 This process is regulated by aldosterone. Also, Cl is reabsorbed (passively) and exchanged for HCO3 (bicarbonate). The result is that the final saliva is hypotonic52 and has a pH that is very slightly acidic (6.8-7).53 In conclusion: the first stage involves the acini, and the second, the ducts. 51 Antiport mechanism 52 Up to 10 times less Na and 7 times more K concentration than plasma. 53 Enables ptyalin to do its work in optimal conditions 21 Daily secretion of saliva is about 1000 to 1500 ml per day, but this can vary: Increased flow with: - Pregnancy - Psychological disorders (eg schizophrenia) - Nervous diseases (eg Parkinson’s) Decreased flow with: - Age - Dehydration - Sleep - Stress 2.3 Saliva composition You now know that final saliva will contain water, mucus and various ions. There is, of course, more. Thanks to Burger and Emmelin and their experiment, we know that saliva also contains some other organic substances such as urea54 and ptyalin.55 54 CO(NH2)2 -- Urea serves an important role in the metabolism of nitrogen-containing compounds by animals and is the main nitrogen-containing substance in the urine of mammals. 55 Salivary amylase 22 2.4 Roles of saliva Saliva has digestive and non-digestive roles. Digestive roles: 1. Moistens dry food and so increases solubilisation of food particles 2. Allows mastication, swallowing, helps stimulate taste buds and olfactory receptors56 3. Hydrolyses starch with ptyalin, generating dextrines and finally maltose 4. Lubricates and washes buccal cavity 5. Protects against mechanical injuries Non-digestive roles: 1. Talking 2. Excretion of viruses, heavy metals, ions 3. Promotes oral hygiene by excreting bacteria 4. Antiseptic role through thiocyanate57 ions, lysosime, IgA58 5. thermoregulation59 6. endocrine role: secretes EGF60 and NGF61 56 Olfactory receptors are responsible for the detection of odor molecules. 57 Technically an anion. Thiocyanate is an important part in the biosynthesis of hypothiocyanite by a lactoperoxidase. Thus the absence of thiocyanate in the human body, (e.g., cystic fibrosis) is damaging to the human host defense system. 58 Immunoglobuline A (antibody) – generally protects mucosa (also in the nasal cavity) 59 Not in humans, not really, but in dogs and rats – fun to know. 60 Epidermal growth factor or EGF is a growth factor that stimulates cell growth, proliferation, and differentiation: involved in wound healing. 61 Nerve growth factor (NGF) is a small secreted protein that is important for the growth, maintenance, and survival of certain target neurons (nerve cells). 23 2.5 Regulation In the previous chapter on body water we talked about the reflex arc. Salivary regulation works much in the same way as it is a reflex that is regulated by both nervous and humoral stimuli. To recap, the reflex arc consists of: - receptors - sensory fibers (afferent) - nervous center - motor fibers (efferent) - effectors 2.5.1 Nervous regulation In the case of saliva, the nervous regulation consists of both parasympathetic and sympathetic reflexes. Parasympathetic reflexes (picked up by receptors 5, 7, 9 and 10) stimulate a very watery salivary secretion that is low in enzymes and mucus. The sympathetic reflex, controlled by fibers originating in the cervical ganglia, stimulate62 the secretion of a saliva rich in mucus and ptyalin. There is a conditioned part to this reflex, too. A smell, sight, remembrance or even thought of a favorite food can stimulate the appetite area located in the anterior part of the hypothalamus. This in turn stimulates salivary centers as if food were actually ingested. We will discuss this mechanism in more detail in the next chapter on gastric secretion. 62 Via sympathetic nerves. 24 2.5.2 Humoral regulation The hormones that affect salivary secretion are: Increase secretion: 63 1. Bradikynin 2. VIP64 Decrease secretion:65 1. Angiotensin II 2. Substance P 3. Oxytocin *** 63 These are generally vasodilators, so they widen blood vessels and increase blood flow 64 Vasoactive intestinal peptide 65 Vasoconstrictors – decrease blood flow 25