Digestion and Swallowing in the Mouth PDF

Digestion and swallowing in the mouth YÜKSEK IHTISAS UNIVERSITY Faculty of Medicine Department of Physiology Dr. Sümeyye RAMAZANOĞLU  The mouth  It also referred to as the oral or buccal cavity (bucca= cheeks), is formed by the cheeks, hard and soft palates,...

Digestion and swallowing in the mouth YÜKSEK IHTISAS UNIVERSITY Faculty of Medicine Department of Physiology Dr. Sümeyye RAMAZANOĞLU  The mouth  It also referred to as the oral or buccal cavity (bucca= cheeks), is formed by the cheeks, hard and soft palates, and tongue.  The cheeks form the lateral walls of the oral cavity.  They are covered externally by skin and internally by a mucous membrane, which consists of nonkeratinized stratified squamous epithelium.  Tongue  It is an accessory digestive organ composed of skeletal muscle covered with mucous membrane.  The extrinsic muscles of the tongue, include: I. Hyoglossus muscle. II. Genioglossus muscle. III. Styloglossus muscle.  The extrinsic muscles:  Move the tongue from side to side and in and out to maneuver food for chewing.  Shape the food into a rounded mass.  Force the food to the back of the mouth for swallowing.  They also form the floor of the mouth and hold the tongue in position.  The intrinsic muscles alter the shape and size of the tongue for speech and swallowing.  They include:  Longitudinalis superior.  Longitudinalis inferior.  Transversus linguae.  Verticalis linguae muscles.  The upper and lateral surfaces of the tongue are covered with papillae, projections of the lamina propria covered with stratified squamous epithelium.  Many papillae contain taste buds, the receptors for gustation.  Some papillae lack taste buds, but they contain receptors for touch and increase friction between the tongue and food, making it easier for the tongue to move food in the oral cavity. Mastication (Chewing)  The teeth are admirably designed for chewing. Teeth  The incisors, (central or lateral incisors) which are closest to the midline, are adapted for cutting into food.  Next to the incisors, moving posteriorly, are the cuspids (canines), which have a pointed surface called a cusp.  Cuspids are used to tear and shred food.  Posterior to the cuspids lie the first and second molars. They crush and grind food to prepare it for swallowing.  Most of the muscles of chewing are innervated by the motor branch of the fifth cranial nerve (the trigeminal nerve).  The chewing process is controlled by nuclei in the brain stem.  Stimulation of specific reticular areas in the brain stem taste centers will cause rhythmical chewing movements.  Also, stimulation of areas in the hypothalamus, amygdala, and even the cerebral cortex near the sensory areas for taste and smell can often cause chewing. Chewing reflex 1. The presence of a bolus of food in the mouth at first initiates reflex inhibition of the muscles of mastication, which allows the lower jaw to drop. 2. The drop in turn initiates a stretch reflex of the jaw muscles that leads to rebound contraction. 3. This raises the jaw to cause closure of the teeth, but it also compresses the bolus again against the linings of the mouth, which inhibits the jaw muscles once again, allowing the jaw to drop and rebound another time; this is repeated again and again.  Importance of chewing  Chewing is especially important for most fruits and raw vegetables because these have indigestible cellulose membranes around their nutrient portions that must be broken before the food can be digested.  Also, digestive enzymes act only on the surfaces of food particles; therefore, the rate of digestion is absolutely dependent on the total surface area exposed to the digestive secretions.  Grinding the food to a very fine particulate consistency prevents excoriation of the gastrointestinal tract and increases the ease with which food is emptied from the stomach into the small intestine, then into all succeeding segments of the gut. Mechanical and chemical digestion in the mouth  Mechanical digestion results from chewing (mastication), in which food is:  Manipulated by the tongue.  Grind by the teeth.  Mixed with saliva.  As a result, the food is reduced to a soft, flexible, easily swallowed mass called a bolus.  Food molecules begin to dissolve in the water in saliva, so that they can be tasted by gustatory receptors and so that digestive reactions can begin, because enzymes can react with food molecules in a liquid medium only. Salivary Glands  A salivary gland is a gland that releases a saliva into the oral cavity.  Saliva is secreted to keep the mucous membranes of the mouth and pharynx moist and to cleanse the mouth and teeth.  When food enters the mouth, secretion of saliva increases, and it  Lubricates the food.  Dissolves the food.  Begins the chemical breakdown of the food. Salivary Glands  The mucous membrane of the mouth and tongue contains many small salivary glands that open directly, or indirectly via short ducts, to the oral cavity.  The principal glands of salivation are:  Parotid,  Submandibular, all of which make a small contribution  Sublingual glands, to saliva.  Many very small buccal glands, Saliva  Daily secretion of saliva normally ranges between 800-1500 ml.  Saliva contains two major types of protein secretion: 1. A serous secretion that contains ptyalin (an α-amylase = digesting starches). 2. Mucus secretion that contains mucin for lubricating and for surface protective purposes.  The parotid glands secrete almost entirely the serous type of secretion.  The submandibular and sublingual glands secrete both serous secretion and mucus.  The buccal glands secrete only mucus.  Saliva has a pH = 6.0-7.0, a favorable range for the digestive action of ptyalin. Secretion of ions in saliva  Saliva contains especially large quantities of potassium and bicarbonate ions.  Conversely, the concentrations of both sodium and chloride ions are several times less in saliva than in plasma.  Salivary secretion is a two-stage operation: I. The first stage involves the acini. II. The second stage involves the salivary ducts.  The acini secrete a primary secretion that contains ptyalin and/or mucin in a solution of ions in concentrations not greatly different from those of typical extracellular fluid.  As the primary secretion flows through the ducts, two major active transport processes take place that markedly modify the ionic composition of the fluid in the saliva. 1. Na+ are actively reabsorbed from all the salivary ducts and K+ are actively secreted in exchange for the sodium. The Na+ concentration of the The K+ concentration of the saliva saliva becomes greatly reduced. becomes increased.  However, there is excess Na+ reabsorption over K+ secretion, and this creates electrical negativity of about -70 mV in the salivary ducts; this in turn causes Cl– to be reabsorbed passively.  Therefore, the Cl– concentration in the salivary fluid falls to a very low level, matching the ductal decrease in Na+ concentration.  Second, bicarbonate ions are active secreted by the ductal epithelium into the lumen of the duct.  The net result of these transport processes is that under resting conditions, the concentrations of sodium and chloride ions in the saliva are only about one seventh to one tenth their concentrations in plasma.  Conversely, the concentration of potassium ions is about seven times as great as in plasma.  During maximal salivation, the salivary ionic concentrations change considerably because the rate of formation of primary secretion by the acini can increase as much as 20-fold.  This acinar secretion then flows through the ducts so rapidly that the ductal reconditioning of the secretion is considerably reduced.  Therefore, when copious quantities of saliva are being secreted, the sodium chloride concentration rises only to one half or two thirds that of plasma, and the potassium concentration rises to only four times that of plasma. Function of saliva for oral hygiene  Under basal awake conditions, about 0.5 ml of saliva (almost mucous type) is secreted each minute; but during sleep, secretion becomes very little.  This secretion plays an exceedingly important role for maintaining healthy oral tissues.  The mouth is loaded with pathogenic bacteria that can easily destroy tissues and cause dental caries. Function of saliva for oral hygiene  Saliva helps prevent the deteriorative processes in several ways. 1. The flow of saliva itself helps wash away pathogenic bacteria as well as food particles that provide their metabolic support. 2. Saliva contains several factors that destroy oral bacteria, like thiocyanate ions and several proteolytic enzymes (the most important, lysozyme). 3. Saliva often contains antibodies (IgA) prevents attachment of microbes so they can’t penetrate the epithelium. In the absence of salivation, oral tissues often become ulcerated and otherwise infected, and caries of the teeth can become rampant. Nervous regulation of salivary secretion  The salivary glands are controlled mainly by parasympathetic nervous signals all the way from: I. The superior salivatory nucleus. II. The inferior salivatory nucleus.  The salivatory nuclei are located approximately at the juncture of the medulla and pons. I. The superior salivatory nucleus contains parasympathetic neurons that project, through a branch of the facial nerve (CN VII), to the pterygopalatine ganglion. The postganglionic fibers supply the lacrimal glands, which produce tears.  Another branch of the facial nerve carries preganglionic fibers to the submandibular ganglion. The postganglionic fibers supply two salivary glands, the submandibular and sublingual glands. II. The inferior salivatory nucleus contains parasympathetic neurons that project through the glossopharyngeal nerve (CN IX) to the otic ganglion. The postganglionic fibers supply a third salivary gland, the parotid gland.  The sympathetic nerves originate from the superior cervical ganglia and travel along the surfaces of the blood vessel walls to the salivary glands.  Sympathetic stimulation can also increase salivation a slight amount, much less so than does parasympathetic stimulation.  A secondary factor that also affects salivary secretion is the blood supply to the glands because secretion always requires adequate nutrients from the blood.  The parasympathetic nerve signals that induce copious salivation also moderately dilate the blood vessels.  The sympathetic nerve signals cause vasoconstriction.  In addition, salivation itself directly dilates the blood vessels, thus providing increased salivatory gland nutrition as needed by the secreting cells.  Part of this additional vasodilator effect is caused by kallikrein secreted by the activated salivary cells, which in turn acts as an enzyme to split an alpha2-globulin, to form bradykinin, a strong vasodilator. I. Taste stimuli, especially the sour taste (caused by acids), elicit copious secretion of saliva (often 8 to 20 times the basal rate of secretion). II. Certain tactile stimuli, such as the presence of smooth objects in the mouth, cause marked salivation, whereas rough objects cause less salivation and occasionally even inhibit salivation. III. Central nervous system, salivation can be stimulated or inhibited by nervous signals arriving in the salivatory nuclei from higher centers of the CNS. (When a person smells or sees favorite foods, salivation is greater than when disliked food is smelled or seen).  The appetite area of the brain, which partially regulates these effects, is located in proximity to the parasympathetic centers of the anterior hypothalamus, and it functions to a great extent in response to signals from the taste and smell areas of the cerebral cortex or amygdala. IV. In response to reflexes originating in the stomach and upper small intestines (particularly when irritating foods are swallowed or when a person is nauseated because of some gastrointestinal abnormality). The saliva, helps to remove the irritating factor in the gastrointestinal tract by diluting or neutralizing the irritant substances. Swallowing (Deglutition)  Swallowing is a complicated mechanism, principally because the pharynx subserves respiration as well as swallowing.  The pharynx is converted for only a few seconds at a time into a tract for propulsion of food.  It is especially important that respiration not be compromised because of swallowing. Swallowing (Deglutition)  In general, swallowing can be divided into: I. A voluntary stage, which initiates the swallowing process. II. A pharyngeal stage, which is involuntary and constitutes passage of food through the pharynx into the esophagus. III. An esophageal stage, another involuntary phase that transports food from the pharynx to the stomach. I. Voluntary stage of swallowing:  When the food is ready for swallowing, it is voluntarily squeezed or rolled posteriorly into the pharynx by pressure of the tongue upward and backward against the palate.  From here on, swallowing becomes entirely automatic and ordinarily can’t be stopped. II. Pharyngeal stage of swallowing  As the bolus of food enters the posterior mouth and pharynx, it stimulates epithelial swallowing receptor areas all around the opening of the pharynx, especially on the tonsillar pillars, and impulses from these pass to the brain stem to initiate a series of automatic pharyngeal muscle contractions as follows: 1. The soft palate is pulled upward to close the nasopharynx, to prevent reflux of food into the nasal cavities. 2. The palatopharyngeal folds on each side of the pharynx are pulled medially to approximate each other.  In this way, these folds form a sagittal slit through which the food must pass into the posterior pharynx.  II. Pharyngeal stage of swallowing 3. The vocal cords of the larynx are strongly approximated, and the larynx is pulled upward and anteriorly by the neck muscles.  These actions, combined with the presence of ligaments that prevent upward movement of the epiglottis, cause the epiglottis to swing backward over the opening of the larynx.  II. Pharyngeal stage of swallowing  All these effects acting together prevent passage of food into the trachea.  Most essential is the tight approximation of the vocal cords, but the epiglottis helps to prevent food from ever getting as far as the vocal cords. 4. The upward movement of the larynx also pulls up and enlarges the opening to the esophagus.   At the same time, the upper esophageal sphincter relaxes, thus allowing food to move easily and freely from the posterior pharynx into the upper esophagus. 5. Then entire muscular wall of the pharynx contracts, beginning in the superior part of the pharynx, then spreading downward over the middle and inferior pharyngeal areas, which propels the food by peristalsis into the esophagus.  Between swallows, the upper esophageal sphincter remains strongly contracted, thereby preventing air from going into the esophagus during respiration. Nervous initiation of the pharyngeal stage of swallowing  The most sensitive tactile areas of the posterior mouth and pharynx lie in a ring around the pharyngeal opening, with greatest sensitivity on the tonsillar pillars.  Impulses are transmitted from these areas through the sensory portions of the trigeminal and glossopharyngeal nerves into the medulla oblongata. Nervous initiation of the pharyngeal stage of swallowing  The successive stages of the swallowing process are then automatically initiated in orderly sequence by neuronal areas of the reticular substance of the medulla and lower portion of the pons (the deglutition or swallowing center).  The motor impulses from the swallowing center to the pharynx and upper esophagus that cause swallowing are transmitted successively by the 5th, 9th, 10th, and 12th cranial nerves and even a few of the superior cervical nerves. Effect of the pharyngeal stage of swallowing on respiration  The entire pharyngeal stage of swallowing usually occurs in less than 6 seconds, thereby interrupting respiration for only a fraction of a usual respiratory cycle.  The swallowing center specifically inhibits the respiratory center of the medulla during this time, halting respiration at any point in its cycle to allow swallowing to proceed. III. Esophageal stage of swallowing  The esophagus functions primarily to conduct food rapidly from the pharynx to the stomach, and its movements are organized specifically for this function.  The esophagus normally exhibits two types of peristaltic movements:  Primary peristalsis.  Secondary peristalsis.  III. Esophageal stage of swallowing  Primary peristalsis is simply continuation of the peristaltic wave that begins in the pharynx and spreads into the esophagus during the pharyngeal stage of swallowing.  If the primary peristaltic wave fails to move into the stomach all the food that has entered the esophagus, secondary peristaltic waves result from distention of the esophagus itself by the retained food; these waves continue until all the food has emptied into the stomach.  The secondary peristaltic waves are initiated:  partly by intrinsic neural circuits in the myenteric nervous system.  partly by reflexes that begin in the pharynx and are then transmitted upward through vagal afferent fibers to the medulla and back again to the esophagus through glossopharyngeal and vagal efferent nerve fibers. When the vagus nerves to the esophagus are cut, the myenteric nerve plexus of the esophagus becomes excitable enough after several days to cause strong secondary peristaltic waves even without support from the vagal reflexes. Therefore, even after paralysis of the brain stem swallowing reflex, food fed by tube or in some other way into the esophagus still passes readily into the stomach. Function of the lower esophageal sphincter  This sphincter normally remains tonically constricted, in contrast to the midportion of the esophagus, which normally remains relaxed.  As a peristaltic wave travels down the esophagus during swallowing, the lower esophageal sphincter relaxes ahead of the wave, facilitating the smooth propulsion of swallowed food into the stomach. Function of the lower esophageal sphincter  The stomach secretions are highly acidic and contain many proteolytic enzymes.  The esophageal mucosa is not capable of resisting for long the digestive action of gastric secretions.  Fortunately, the tonic constriction of the lower esophageal sphincter helps to prevent significant reflux of stomach contents into the esophagus except under very abnormal conditions.  Another factor that helps to prevent reflux is a valve-like mechanism of a short portion of the esophagus that extends slightly into the stomach.  Increased intra-abdominal pressure caves the esophagus inward at this point.  Thus, this valve-like closure of the lower esophagus helps to prevent high intra- abdominal pressure from forcing stomach contents backward into the esophagus.  Otherwise, every time we walked, coughed, or breathed hard, we might expel stomach acid into the esophagus. Receptive Relaxation of the Stomach  When the esophageal peristaltic wave approaches toward the stomach, a wave of relaxation, transmitted through myenteric inhibitory neurons, precedes the peristalsis.  Furthermore, the entire stomach and, to a lesser extent, even the duodenum become relaxed as this wave reaches the lower end of the esophagus and thus are prepared ahead of time to receive the food propelled into the esophagus during the swallowing act. Achalasia  It is a condition in which the lower esophageal sphincter fails to relax during swallowing.  As a result, food swallowed into the esophagus then fails to pass from the esophagus into the stomach.  It occurs as a result of damage in the neural network of the myenteric plexus in the lower two thirds of the esophagus.  When achalasia becomes severe, the esophagus often can’t empty the swallowed food into the stomach for many hours, instead of the few seconds that is the normal time. Achalasia  Over years, the esophagus becomes tremendously enlarged until it often can hold as much as 1 liter of food, which often becomes putridly infected during the long periods of esophageal stasis.  The infection may also cause ulceration of the esophageal mucosa, sometimes leading to severe substernal pain or even rupture and death. Esophageal secretion  The esophageal secretions are entirely mucous in character and principally provide lubrication for swallowing.  The main body of the esophagus is lined with many simple mucous glands (prevent mucosal excoriation by newly entering food).  At the gastric end and to a lesser extent in the initial portion of the esophagus, there are also many compound mucous glands (protect the esophageal wall from digestion by acidic gastric juices that often reflux from the stomach back into the lower esophagus).