Saliva - General Physiology PDF

Saliva General physiology By Dr. Shaymaa jasim Ph.D. Neurophysiology Composition of Saliva Mixed saliva contains 99.5% water and 0.5% solids. Properties of Saliva 1. Volume: 1000 to 1500 mL of saliva is secreted per day and, it is approximately about 1 mL/ minute. Contribution by ea...

Saliva General physiology By Dr. Shaymaa jasim Ph.D. Neurophysiology Composition of Saliva Mixed saliva contains 99.5% water and 0.5% solids. Properties of Saliva 1. Volume: 1000 to 1500 mL of saliva is secreted per day and, it is approximately about 1 mL/ minute. Contribution by each major salivary gland is: i. Parotid glands: 25% ii. Submaxillary glands: 70% iii. Sublingual glands: 5%. 2. Reaction: Mixed saliva from all the glands is slightly acidic with pH of 6.35 to 6.85. 3. Specific gravity: It ranges between 1.002 and 1.012. 4. Tonicity: Saliva is hypotonic to plasma. Functions of Saliva Saliva is a very essential digestive juice. Since it has many functions, its absence leads to many problems. 1. Preparation of food for swallowing When food is taken into the mouth, it is moistened and dissolved by saliva. The mucous membrane of mouth is also moistened by saliva. It facilitates chewing. By the movement of the tongue, the moistened and masticated food is rolled into a bolus. The mucin of saliva lubricates the bolus and facilitates the swallowing. 2. Appreciation of taste Taste is a chemical sensation. Saliva by its solvent action dissolves the solid food substances, so that the dissolved substances can stimulate the taste buds. The stimulated taste buds recognize the taste. 3. Digestive function Saliva has three digestive enzymes namely, salivary amylase, maltase and lingual lipase. - Salivary Amylase Salivary amylase is a carbohydrate digesting (amylolytic) enzyme. It acts on cooked or boiled starch and converts it into dextrin and maltose. Though starch digestion starts in the mouth, major part of it occurs in the stomach because, food stays only for a short time in the mouth. The optimum pH necessary for the activation of salivary amylase is 6. The salivary amylase cannot act on cellulose. Maltase:The enzyme maltase is present only in traces in human saliva. It converts maltose into glucose. Lingual Lipase: Lingual lipase is lipid digesting (lipolytic) enzymes. It digests milk fats (pre-emulsified fats). It hydrolyzes triglycerides into fatty acids and diacylglycerol Enzyme Source of secretion Activator Action 1. Salivary amylase All salivary glands Acid medium Converts cooked starch into maltose 2. Maltase Major salivary glands Acid medium Converts maltose into glucose 3. Lingual lipase Lingual glands Acid medium Converts triglycerides of milk fat into fatty acids and diacylglycerol 4. Cleansing and protective functions i. Due to the constant secretion of saliva, the mouth and teeth are rinsed and kept free off food debris, shed epithelial cells and foreign particles. In this way, saliva prevents bacterial growth by removing materials, which may serve as culture media for the bacterial growth. ii. The enzyme lysozyme of saliva kills some bacteria such as staphylococcus, streptococcus, and brucella. iii. The proline-rich proteins and lactoferrin present in saliva possess antimicrobial property. These proteins also protect the teeth by stimulating enamel formation. iv. Saliva also contains secretory immunoglobulin IgA which has antibacterial and antiviral actions. v. Mucin present in the saliva protects the mouth by lubricating the mucous membrane of the mouth. 5. Role in speech By moistening and lubricating soft parts of mouth and lips, saliva helps in speech. If the mouth becomes dry, articulation and pronunciation become difficult. 6. Excretory function Many substances, both organic and inorganic, are excreted in saliva. It excretes substances like mercury, potassium iodide, lead, and thiocyanate. Saliva also excretes some viruses such as those causing rabies and mumps. In some pathological conditions, saliva excretes certain substances, which are not found in saliva under normal conditions such as glucose in diabetes mellitus. In certain conditions, some of the normal constituents of saliva are excreted in large quantities. For example, excess urea is excreted in saliva during nephritis, and excess calcium is excreted during hyperparathyroidism. 7. Regulation of body temperature In dogs and cattle, excessive dripping of saliva during panting helps in loss of heat and regulation of body temperature. However, in human being sweat glands play major role in temperature regulation and saliva does not play any role in this function. 8. Regulation of water balance When the body water content decreases, salivary secretion also decreases. This causes dryness of the mouth and induces thirst. When the water is taken, it reduces the thirst and restores the body water content. Regulation of salivary secretion Salivary secretion is regulated only by nervous mechanism. Autonomic nervous system is involved in the regulatory function. Nerve supply to salivary glands Salivary glands are supplied by parasympathetic and sympathetic divisions of autonomic nervous system. Parasympathetic fibers Parasympathetic Fibers to Submandibular and Sublingual Glands The parasympathetic preganglionic fibers to submandibular and sublingual glands arise from the superior salivatory nucleus situated in pons. After taking origin from this nucleus, the preganglionic fibers run through nervous intermedius of Wrisberg, geniculate ganglion, the motor fibers of facial nerve, chorda tympani branch of facial nerve and lingual branch of trigeminal nerve and finally reach the submaxillary ganglion. The postganglionic fibers arise from this ganglion and supply the submaxillary and sublingual glands Parasympathetic nerve supply to submaxillary and sublingual glands Parasympathetic Fibers to Parotid Gland The parasympathetic preganglionic fibers to parotid gland arise from inferior salivatory nucleus situated in the upper part of medulla oblongata. From here, the fibers pass through the tympanic branch of glossopharyngeal nerve, tympanic plexus and lesser petrosal nerve and end in otic ganglion. The postganglionic fibers arise from otic ganglion and reach the parotid gland by passing through the auriculotemporal branch in mandibular division of trigeminal nerve. Parasympathetic nerve supply to parotid gland Function of Parasympathetic Fibers When the parasympathetic fibers of salivary glands are stimulated, a large quantity of watery saliva is secreted with less amount of organic constituents. It is because the parasympathetic fibers activate the acinar cells and dilate the blood vessels of salivary glands. The neurotransmitter is acetylcholine. Sympathetic fibers The sympathetic preganglionic fibers to salivary glands arise from the lateral horns of first and second thoracic segments of spinal cord. The fibers leave the cord through the anterior nerve roots and end in superior cervical ganglion of the sympathetic chain. The postganglionic fibers from this ganglion are distributed to the salivary glands along the nerve plexus around the arteries supplying the glands. Function of Sympathetic Fibers The stimulation of sympathetic fibers causes less secretion of saliva, which is thick and rich in mucus. It is because these fibers activate the acinar cells and cause vasoconstriction by secreting noradrenaline. Reflex regulation of salivary secretion Salivary secretion is regulated by nervous mechanism through reflex action. Salivary reflexes are of two types: 1. Unconditioned reflex 2. Conditioned reflex. 1. Unconditioned Reflex Unconditioned reflex is the inborn reflex that is present since birth. It does not need any previous experience. This reflex induces salivary secretion when any substance is placed in the mouth. It is due to the stimulation of nerve endings in the mucous membrane of the oral cavity. Examples: i. When food is taken ii. When any unpleasant or unpalatable substance enters the mouth iii. When the oral cavity is handled with instruments by dentists. 2. Conditioned Reflex Conditioned reflex is the one that is acquired by experience and it needs previous experience. Presence of food in the mouth is not necessary to elicit this reflex. The stimulus for this reflex is the sight, smell, hearing or thought of food. It is due to the impulses arising from eyes, nose, ear, etc. Effect of drugs and chemicals on salivary secretion Substances which Increase the Salivary Secretion 1. Sympathomimetic drugs like adrenaline and ephedrine 2. Parasympathomimetic drugs like acetylcholine, pilocarpine, muscarine and physostigmine 3. Histamine. Substances which Decrease the Salivary Secretion 1. Sympathetic depressants like ergotamine and dibenamine 2. Parasympathetic depressants like atropine, and scopolamine. Applied physiology Hyposalivation The reduction in the secretion of saliva is called hyposalivation. It is of two types, namely, the temporary hyposalivation and the permanent hyposalivation. 1. Temporary hyposalivation occurs in: i. Emotional conditions like fear ii. Fever iii. Dehydration. 2. Permanent hyposalivation occurs in: i. Obstruction of salivary duct (sialolithiasis) ii. Congenital absence or hypoplasia of salivary glands iii. Paralysis of facial nerve (Bell’s palsy). Hypersalivation The excess secretion of saliva is known as hypersalivation. The physiological condition when hypersalivation occurs is pregnancy. Hypersalivation in pathological conditions is called ptyalism, sialorrhea, sialism or sialosis. Hypersalivation occurs in the following conditions: 1. Decay of tooth or neoplasm (abnormal new growth or tumor) in mouth or tongue – due to continuous irritation of nerve endings in the mouth. 2. Disease of esophagus, stomach and intestine. 3. Neurological disorders such as mental retardation, cerebral stroke and parkinsonism. 4. Some psychological and psychiatric conditions. 5. Nausea and vomiting. Other disorders In addition to hyposalivation and hypersalivation, salivary secretion is affected by other disorders also which include: 1. Xerostomia Xerostomia means dry mouth. It is also called pasties or cottonmouth. It is due to hyposalivation or absence of salivary secretion (aptyalism). Xerostomia causes difficulties in mastication, swallowing and speech. It also causes halitosis (bad breath). The causes of this disease (Xerostomia) are: i. Dehydration or renal failure ii. Sjögren’s syndrome iii. Radiotherapy iv. Trauma to salivary gland or their ducts v. Side effect of drugs like antihistamines, antidepressants, and, antiparkinsonian drugs vi. Shock vii. After smoking marijuana (psychoactive compound from the plant cannabis). 2. Drooling Uncontrolled flow of saliva outside the mouth is called drooling. It is often called ptyalism. Drooling occurs because of excess production of saliva in association with inability to retain saliva within the mouth. Drooling occurs in the following conditions: i. During teeth eruption in children ii. Upper respiratory tract infection or nasal allergies in children iii. Difficulty in swallowing iv. Tonsillitis v. Peritonsillar abscess. 3. Chorda Tympani Syndrome Chorda tympani syndrome is the condition characterized by sweating while eating. During trauma or surgical procedure some of the parasympathetic nerve fibers to salivary glands may be severed. And, during the regeneration some of these nerve fibers, which run along with chorda tympani branch of facial nerve may deviate and join with the nerve fibers supplying sweat glands. When the food is placed in the mouth, salivary secretion is associated with sweat secretion. 4. Mumps Mumps is the acute viral infection affecting the parotid glands. The virus causing this disease is paramyxovirus. It is common in children who are not immunized. It occurs in adults also. 5. Sjögren’s Syndrome It is an autoimmune disorder in which the immune cells destroy exocrine glands such as lacrimal glands and salivary glands. Common symptoms of this syndrome are dryness of the mouth due to lack of saliva (xerostomia), persistent cough and dryness of eyes. In severe conditions the organs like kidneys, lungs, liver, pancreas, thyroid, blood vessels and brain are affected