2 Digestive -Oral Cavity

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2 Digestive -Oral Cavity
  DIGESTION IN THE ORAL CAVITY The oral cavity is part of the orofacial system, which consists of a large variety of tissues and organs that act as a whole in performing important functions such as chewing, swallowing, communication through vocalization and facial expression, characterizing of compounds by the sense of taste, smell and touch. Initial  processing of the food occurs in the mouth and involves saliva secretion, chewing and swallowing. Food is ingested through the oral cavity. In the oral cavity the food is processed into smaller pieces, is mixed with saliva and is prepared for being swallowed. Degradation of carbohydrates also begins in the mouth. As the food is moved through the mouth, taste buds are stimulated. Sensation of taste contributes to stimulation of salivary and gastric secretion. I.   The teeth In adults there are 32 permanent teeth, which replace the 20 temporary teeth. The teeth are embedded in the sockets of the mandible (lower jaw) or the maxilla (upper jaw). On one arcade (superior or inferior) there are 4 incisors and 2 canines that cut food, 4 premolars and 6 molars that grind food. Teeth are consisted of:    crown, protruding from the gum, is consisted of an thin outer layer, the hard enamel, surrounding an inner layer, the dentine    root, embedded in the bone beneath. The root is surrounded by the tooth cementum, which keeps the tooth in its socket. In the center of the tooth is the pulp cavity, which contains nerves and blood vessels. They pass to the tooth by a small opening in the root. The tooth is kept in place also by the periodontal ligamentum. II.   The tongue The tongue is a muscular organ inserted to the hyoid bone in the back and attached to the floor of the mouth by the tongue frenulum. The tongue has a very important roles    in mastication, as it helps prehension, keeping the food on the masticatory surfaces of the food, mixing the food particles with saliva    in swallowing    in speech production    in the perception of taste   III.   MASTICATION Mastication or chewing is the first stage of digestion in most mammals. It is an intermittent rhythmic act, during which food is taken into the mouth in bites ranging from a few cubic millimeters upward; there it is broken up, mixed with saliva and lubricated. Thus mastication is meant to prepare the food for swallowing and further processing in the digestive system. During chewing, the food bolus or food particles are reduced in size, saliva is produced to moisten the food and flavors are released. Taste and texture of the food are  perceived and have their influence on the chewing process. The water in the saliva moistens the food  particles, whereas the salivary mucins bind masticated food into a coherent and slippery bolus that can be easily swallowed. There are several factors determining the chewing result. The mastication is realized by the masticator system, that comprises distinct anatomic structures, such as the masticator muscles, the temporomandibular  joint, the teeth, the tongue, the soft tissues of the oral cavity, the mandible and the superior maxillar. All these structures act as a whole in performing the masticator function. The mandible is able to move in relationship to the skull and is guided by two temporomandibular joints through contractions of the masticatory muscles. The muscles are the dominant determinants of jaw movement, because they determine the jaw movements. The muscles not only move the jaw but also maintain joint stability during midline movements. The human masticator system contains more muscles than are apparently necessary to accomplish its tasks (the system is mechanically redundant), which means that there is an infinite number of muscle contraction patterns which can cause the same movement. The teeth are important in the masticatory system. They form the occlusal area where the food  particles are fragmented. This fragmentation depends on the total occlusal area and thus on the number of teeth. Another important factor in mastication is the bite force. The bite force depends on muscle volume,  jaw muscle activity, and the coordination between the various chewing muscles. Also the movement of the  jaw, and thus the neuromuscular control of chewing, plays an important role in the fragmentation of the food. Another aspect of chewing is how well the tongue and cheeks manipulate the food particles between the teeth. Finally, the production of sufficient saliva is indispensable for good chewing. Taste and texture of the food are perceived and have their influence on the chewing process. The effects of sensory factors were most evident at the beginning of meals and decreased until the end of meals The masticatory act varies with each individual; different individuals people do not perform the same mandibular movements when they chew. Some people will need more chewing cycles to treat the same bolus of food than others will and the amplitude and duration of muscular contraction will vary from individual to individual. It has been shown that the granularity of bolus of food at the end of mastication is the same in all  individuals; to arrive at such a granularity different individuals develop different masticatory strategies. In one individual, the masticatory pattern is the same over long periods of time. This pattern depends on the age, the dental condition and the food characteristics. Regulation of mastication: The act of chewing is partly voluntary, partly reflex. Intake of food into the oral cavity is a voluntary act. After that, mastication becomes a reflex act. According to the modern theory, it is generally accepted that the motor command for the basic pattern of rhythmical oral-facial movements is generated by a neuronal population in the brainstem (central pattern generator, CPG). This mastication center is represented by a group of cells in the medial bulbar reticular formation (RF) between n. V. motor and the inferior olive. The central pattern generator may be switched on by activity of higher centers or by intra-oral stimuli. The masticatory forces are controlled precisely by peripheral feedback and that these forces change from bite-to-bite depending on the consistency of the bolus. Sensory feedback from a variety of intraoral, joint, and muscle receptors interact with the central control system at several levels to adapt the program to the characteristics of the food. This is the source of the variability in the pattern of mastication.   The roles of mastication 1.  Digestive role : An immediate role of mastication concerns deglutition. To be safe, the food bolus must  be smooth, plastic and cohesive. Cohesiveness is particularly necessary because otherwise bolus scattering would favour particle aspiration into the airways, linked to high morbidity. Chewing of food is important for digestion of all foods, because the digestive enzymes (in digestive juices) act only on the surface of the food; the larger the surface, the better the digestion. 2.A calming down role  on nervous system 3. Trophic role  – mastication has an trophic role on orofacial system, as it activates the local circulation, thus increasing the salivary flow rate and increasing of the thickness and the resistance of maxillar bones. 4.  Regulation of    energy metabolism  - it seems that mastication suppresses food intake (probably by influencing the satiety centers in the hypothalamus). Mastication also accelerates   lipolysis predominantly in the visceral adipocytes. Indeed, therapeutic application of mastication   has been found effective to reduce visceral fat in obese animals. IV.   THE SECRETION OF SALIVA The daily secretion of saliva usually ranges between 1000 and 1500 ml; this gives an average flow rate of 1 ml/minute. The salivary flow rate varies a lot over a 24 hours period. The salivary flow rate in basal condition (unstimulated) is about 0.25-0.5 ml/min; higher rates are encountered during stimulation; the rate is about 5 ml/min when stimulated by parasimpathomimetics (also called cholinomimetics) (pilocarpine, metacholine). The highest rates occur just before, during and a short periods after a meal, up to 7 ml/min.   Hypersalivation, hypersialism, hypersialosis or  sialorrhea  defines an excessive basal flow rate, of over 0.5 ml/min. Higher than normal flow rates are found during teeth eruption, during periods and in the first trimester of pregnancy; smoking increases salivary flow in the short term; mechanical stimulation (mastication, during the first days of dental prosthesis wearing), olfactory stimulation (the smell of food),  psychological stimuli (thinking to food) or exposure to light cause hypersalivation. Sialorrhea causes a range of physical and psychosocial complications, including discomfort, dehydration, odor and phonation, mastication and neurovegetative disorders.  Hyposalivation, hyposialism or hyposialosis  define a basal salivary flow rate of 0.01-0.06 ml/min. Salivary secretion decreases with age, at menopause, due to acute intake of alcoholic drinks, in dehydration, as well as during stress. Hypo- and asyalism are accompanied by mouth dryness (xerostomia), thirst, vocalization, mastication and swallowing difficulties, irritation of the oral mucosa, disorders of the sense of taste. The most serious condition accompanying hyposialia are dental caries. Mixed saliva, total saliva or oral fluid  is the secretion product of major salivary glands (parotid, submaxillar, sub,mandibular), minor sakivary glands (hundreds, spread into the mucosa of the oral cavity), to which the gingival or crevicular fluid is added. The major salivary glands are 3 pairs of compound exocrine glands: - The parotid glands are large and are located near the ears; they release their secretion into  parotid ducts, that open into the oral vestibule near the upper second molars. - The submandibular glands are medium-sized, are located in the oral cavity near the angle of each mandible and release their secretion through submandibulary ducts that open into the oral cavity near the lower central incisors. - The sublingual glands are small, are located beneath the tongue and have ducts that open into the floor of the oral cavity. The functional anatomy of the salivary glands The three major salivary glands consist of a secretor part and an excretion part.    Secretory part: the parenchyma of the salivary glands is consisted of acini. A secretor unit is called salivon and is composed of an acinus and a small intercalated duct. The acinnar cells are of 2 kinds: serous cells and mucous cells. Serous cells are found in large number in parotid gland and in very small number in sublingual glands. The secretion of this type of cell is serous. Mucous cells are found in large number in sublingual glands, in small number in submaxillary glands and are very rare in parotid glands. Acini are surrounded by the basal membrane. Between the basal membrane and the cells in acini there are located myoepithelial cells, which contain actin and myosine in their cytoplasm with a disposal similar to that in the smooth muscle. Because the parotid and submaxillary glands are encapsulated, the contraction of myoepithelial cells exerts a pressure upon the parenchyma and thus voids the secretion into the excretory
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