Digestion in the Mouth Lecture Notes PDF

Digestion in the mouth Oleg Osadchiy 1 Learning objectives ✓ Explain the role of saliva in digestion in the mouth. ✓ Describe the composition and physico-chemical characteristics of saliva. ✓ Compare and contrast characteristics of saliva produced by parotid, submaxillary, and sublingual glands. ✓ E...

Digestion in the mouth Oleg Osadchiy 1 Learning objectives ✓ Explain the role of saliva in digestion in the mouth. ✓ Describe the composition and physico-chemical characteristics of saliva. ✓ Compare and contrast characteristics of saliva produced by parotid, submaxillary, and sublingual glands. ✓ Explain the mechanism of saliva secretion by the salivary glands. ✓ Explain the control of the salivary secretion by the autonomic nervous system. 2 Learning objectives ✓ Describe the process of chewing (mastication). ✓ Explain the mechanism of swallowing. 3 Components of the oral cavity. Digestion in oral cavity (i) Chewing (mechanical decomposition of food) (ii) Saliva secretion by salivary glands (chemical digestion). (iii) Swallowing is initiated. 5 Saliva Salivary glands. Salivary glands ✓ Salivary glands are accessory organs that lie outside of the mouth and release their secretions into the ducts that empty into the oral cavity. ✓ There are three pairs of salivary glands – (i) Parotid glands (ii) Submandibular glands (iii) Sublingual glands 8 Functions of saliva in oral digestion. Helps buffer acidic foods Constituens of saliva and their functions. Composition of saliva ✓ The fluid secreted by the salivary glands is called saliva. ✓ Saliva contains water (99.5%) and solutes (0.5%). ✓ The solutes include (i) ions, (ii) mucus, (iii) lysozyme, and (iv) salivary amylase. ✓ The water in saliva helps dissolve foods, so they can be tasted and digestive reactions can begin (enzymes can react with food molecles in a liquid medium only). ✓ The bicarbonate and phosphate ions buffer acidic foods. 11 Composition of saliva ✓ Mucus lubricates food, so it can be moved around easily in the mouth, formed into a ball, and swallowed. ✓ Lysozyme is an enzyme that kills bacteria. ✓ Salivary amylase is an enzyme that begins the digestion of carbohydrates. 12 Characteristics of saliva ✓ About 1 liter of saliva is secreted daily. This amount is variable, being greater with dry foods, and being lesser with watery foods. ✓ pH 6.2 – 7.6 (average 6.7). ✓ Hypotonic (saliva tonicity is lower than that of blood plasma). ✓ Na+ and Cl- concentrations in saliva are lower than in blood plasma. ✓ K+ and HCO3- concentrations are higher than in blood plasma. 13 To oral cavity Structure of the salivary gland. Two types of cells in the acini of the salivary gland. Characteristics of saliva secreted by different salivary glands. Isotonic. Ionic composition is similar to plasma. Salt reabsorption without water reabsorption make saliva hypotonic. Changes in composition of saliva when it is moving through the duct. Ionic fluxes in the salivary duct cell. Note no absorption of water. Overview of a tight junction. Salivary flow through the ducts can be changed by multiple factors. Ionic composition of saliva (compared to blood plasma) at variable salivary secretion rates. Changes in composition of saliva when it is moving through the duct. Composition of saliva at different flow rates ✓ At the highest flow rates, the final saliva most closely resembles blood plasma and the initial saliva produced in the acinar cells. ✓ This is because when the transit of saliva through the ducts is fast, less time is available for completing the ion absorption and secretion by the ductal cells. ✓ Consequently, there is only a small change in the concentrations of Na+, Cl-, K+, and bicarbonate, once the saliva has moved from the acinus through the ducts. 23 Composition of saliva at different flow rates ✓ At the lowest flow rates, the final saliva shows a greatest difference in ionic composition compared to the blood plasma and the initial saliva produced in the acinar cells. ✓ This is because when the transit of saliva through the ducts is slow, more time is available for completing the ion absorption and secretion by the ductal cells. ✓ Consequently, there is a significant change in concentrations of Na+, Cl-, K+, and bicarbonate, once the saliva has moved from the acinus through the ducts. 24 Changes in composition of saliva when it is moving through the duct. Ionic composition of saliva (compared to blood plasma) at variable salivary secretion rates. Composition of saliva at different flow rates ✓ The only electrolyte which is not described by this “contact time” explanation is bicarbonate. ✓ As bicarbonate is secreted by ductal cells into the saliva, its concentration in the saliva can be expected to be the highest at the slow saliva flow rates, and smallest at the higher flow rates. ✓ This is opposite to what is seen in reality – the concentration of bicarbonate in saliva is actually increasing upon an increase in saliva flow rates. 27 Composition of saliva at different flow rates ✓ Hence when the flow of saliva increases, the bicarbonate concentration in saliva increases as well. ✓ This is explained by the fact that production of bicarbonate is selectively stimulated when saliva production is stimulated (e.g. by parasympathetic nerves). 28 Three types of mechanisms that control GIT function. Autonomic control of saliva secretion ✓ Secretion of saliva (salivation) is controlled by the ANS. ✓ In contrast to other GIT glands, hormonal control and paracrine control play no role in regulation of secretion by the salivary glands. 30 Autonomic control of saliva secretion ✓ Both parasympathetic and sympathetic stimulation increases secretion of saliva, but the sympathetic effects are much smaller. ✓ In addition, the composition of saliva is different – it is watery upon parasympathetic stimulation, but viscous (rich in mucin) upon sympathetic stimulation. ✓ This is partly related to the increased blood flow to the salivary glands upon parasympathetic stimulation, and vasoconstriction induced by sympathetic stimulation. 31 Autonomic control of salivary secretion. 32 Parasympathetic regulation of the salivary secretion. Taste and tactile receptors of the tongue. Sight and smell of the food. Preganglionic fibers Postganglionic fibers Submandibular and sublingual glands Increased salivary secretion Parasympathetic control of salivary secretion. Parasympathetic control of saliva secretion ✓ The saliva is secreted in response to stimulation of the taste buds in the tongue by chemicals present in the foods. ✓ The signals from the taste buds are sent to the salivary nuclei in the brain stem (superior and inferior salivatory nuclei). ✓ The salivatory nuclei then send a signal to the salivatory glands along parasympathetic fibers of the facial (VII) and glossopharyngeal (IX) nerves, which stimulate secretion of saliva. ✓ The sight, smell, or thought of food may also stimulate secretion of saliva. 35 Parasympathetic innervation of the salivary glands. Parasympathetic stimulation increases blood flow to the salivary glands through the formation of bradykinin. Sympathetic regulation of the salivary secretion. Sympathetic innervation of GIT. Sympathetic regulation of saliva secretion ✓ The sympathetic nerves that innervate the salivary glands originate in the Th1-Th3 segments of the spinal cord. ✓ They switch to the postganglionic neuron in the superior cervical ganglion. ✓ Postganglionic fibers release noradrenaline, which interacts with beta-adrenergic receptors on the acinar and ductal cells, and alpha-adrenergic receptors in the vascular smooth muscle cells. 40 Sympathetic regulation of saliva secretion ✓ Beta-AR stimulation increases intracellular concentration of cAMP, which increases saliva secretion. ✓ This effect is attenuated due to concurrent stimulation of alpha-AR, which induces vasoconstriction, thus decreasing the blood flow to the salivary glands. 41 Chemical digestion of food in the mouth. Chemical digestion in the mouth ✓ Chemical digestion in the oral cavity is accomplished by two enzymes present in saliva – (i) Amylase (ii) Lingual lipase 43 Digestion of starch by the salivary amylase. Salivary amylase ✓ Salivary amylase initiates breakdown of starch, which forms most of carbohydrate polysaccharides we eat. ✓ The starch is broken down into smaller molecules such as the disaccharide maltose, the trisaccharide maltotriose, and short-chain glucose polymers – oligosaccharides (aka alpha-dextrins). 45 Salivary amylase ✓ The food is usually swallowed quickly, so the time available for amylase to break down starch in the mouth is very short. ✓ However, the salivary amylase continues to act on the starch for about an hour after swallowing, until it is inactivated by the acid secreted in stomach. 46 Different GIT enzymes exhibit maximum activity at different pH levels. Lingual lipase ✓ Saliva also contains an enzyme lingual lipase, which is secreted by lingual glands in the tongue. ✓ This enzyme becomes activated in the acidic environment of the stomach, and starts to work after the food is swallowed. ✓ It breaks down dietary lipids into fatty acids and glycerol. 48 Mechanical digestion of food in the mouth. Mechanical digestion in the mouth ✓ Chewing, or mastication, allows mechanical digestion of the food in the mouth. ✓ Chewing has three functions – (i) It mixes food with saliva, lubricating it to facilitate swallowing. (ii) It reduces the size of food particles, facilitating swallowing. (iii) Once the food particles become smaller, they are faster digested by GIT enzymes (increased surface area of the food exposed to enzymes accelerates digestion). 50 Chewing (mastication) ✓ Chewing is accomplished through – (i) The action of teeth. (ii) The contraction of chewing muscles. 51 Different types of teeth. Functions of teeth ✓Incisors cut the food. ✓Cuspids (canines) tear and shred food. ✓Premolars crush and grind food. ✓Molars crush and grind food. 53 Chewing muscles ✓ The chewing muscles are – (i) m. temporalis (ii) m. pterygoid (medial and lateral) (iii) m. masseter. ✓ These muscles are attached to the rami of mandible, and function to move the jaw. ✓ The chewing muscles are innervated by the Vth cranial nerve (n.trigeminus), which nucleus is in the medulla. 54 Chewing muscles (innervated by the Vth cranial nerve, n. trigeminus). 55 Chewing reflex ✓ The presence of food in the mouth at first initiates reflex inhibition of the chewing muscles, which allows the lower jaw to drop. ✓This drop in turn initiates a stretch reflex of the jaw muscles causing them to contract. ✓Contraction of the chewing muscles causes closure of the teeth, and the bolus of food is compressed against the linings of the mouth. ✓This inhibits the jaw muscles again, causing the jaw to drop, and then contract. The cycle is repeated again and again. 56 Mechanical digestion in the mouth ✓ Chewing has both voluntary and involuntary components. ✓ Involuntary component involves reflexes initiated upon stimulation of taste receptors in the mouth by food. Information from these receptors is sent to the brain stem, which in turn sends a signal to the muscles involved in chewing. ✓ Voluntary component is controlled by the cerebral cortex. 57 Swallowing Swallowing is the movement of food from the mouth into the stomach. This involves several anatomical structures. Swallowing ✓ Swallowing occurs in three stages – (i) Oral stage (voluntary; the bolus is passed from the oral cavity into the pharynx). (ii) Pharyngeal stage (involuntary passage of bolus through the pharynx into esophagus). (iii) Esophageal stage (involuntary passage of bolus through the esophagus into the stomach). 60 The voluntary and pharyngeal stages of the swallowing reflex. Swallowing ✓ Swallowing starts when the bolus is forced to the back of the oral cavity and then into the pharynx by movement of the tongue (voluntary stage). ✓ When bolus is in the pharynx, it stimulates its receptors, which send a signal to the swallowing center in the medulla oblongata and lower pons. ✓ The swallowing center send a signal to the soft palate, in order to elevate it and to close off the upper part of the pharynx. This prevents swallowed food and liquids from entering the nasal cavity. 62 Swallowing ✓ In addition, the epiglottis closes off the opening to the larynx, which prevents the bolus from entering the respiratory tract. ✓ The bolus then moves through the remainder of the pharynx. ✓ Once the upper esophageal sphincter relaxes, the bolus moves into the esophagus. 63 Esophageal stage of the swallowing. Lower esophageal sphincter opens Esophageal stage of the swallowing. Swallowing ✓ During esophageal stage, the bolus is propeled toward the stomach due to peristaltic contraction of the esophageal muscle. ✓ Esophagus exhibits two types of peristaltic contractions – primary peristalsis and secondary peristalsis. ✓ Primary peristalsis is a continuation of the peristaltic wave that begins in the pharynx and spreads to esophagus. If this wave fails to move into the stomach all the food that has entered the esophagus, secondary peristaltic wave results from distension of the esophagus itself. ✓ The lower esophageal sphincter relaxes, and the bolus moves into the stomach. 66 Esophageal sphincters ✓ At each end of the esophagus, the sphincter is present. ✓ The upper sphincter regulates the movement of food from the pharynx into the esophagus. The lower sphincter regulates the movement of food from the esophagus into the stomach. ✓ The upper esophageal sphincter consists of striated muscle, whereas the lower esophageal sphincter consists of the smooth muscle. 67 Innervation of the upper and lower esophageal sphincters. Digestion in oral cavity (i) Saliva secretion by salivary glands. (ii) Chewing (mastication). (iii) Swallowing. 69