Digestion, its types and functions. Oral cavity role in digestion.

Digestion – is an integrity of food products physical and chemical processing, their transformation into components without species specificity and suitable for absorbtion and participation in substances exchange.

Digestion types have been formed in course of alive organisms development and nowadays we differentiate:

· Intracellular – food products hydrolysis realized inside cells (it is very limited in human being, the example of which is phagocytosis).

· Extracellular – is performed in special cavities (oral cavity, stomach, intestine); enzymes synthesized by secretory cells are released in extracellular environment (cavity).

· Membrane – has intermediate state between extra- and intracellular digestion and performed by enzymes localized on enterocytes membrane structures (in zone of enterocytes mucosa striggillate margin).

Alimentary tract main functions:

1) Secretory – alimentary juices (saliva, stomach, intestinal, bile) secretion and releasing by glandulocytes.

2) Motor-evacuational – food growing shallow, its mixturing with juices, passage through alimentary tract.

3) Absorbtional – transport of ending digestion products, water, salts, vitamins through alimentary tract epithelium in blood and lymph.

4) Excretory – excretion of non-assimilated food components, some metabolism products, hard metals salts, medicines (drugs) out off organism.

5) Incretory – releasing of hormones regulated digestion organs functions.

6) Protective – bacteriocide, bacteriostatic, detoxicative action.

7) Receptor – many receptive zones existance in alimentary tract for excretory, circulatory system reflexes and so on.

8) Erythropoietic – there exists iron depot in stomach, small intestine mucosa, liver participating in haemoglobine synthesis; there is so-called internal Kastl’s factor necessary for vitamine B₁₂ absorbtion responsible for erythropoiesis regulation.

Digestion process is originated from oral cavity. This part of alimentary tract performs 2 functional groups:

1) Specific functions – food suitability assessment performs by means of chemo-, mechano, thermo-, nociceptors, gustatory receptors in oral cavty. Information comes in central nervous system from these receptors and then – to oral cavity organs (masticatory muscles, salivatory glands, tongue). Food gustatory features determining, food mechanic processing and swallowing are performed due to their action. Food chemical processing is also originated from oral cavity (mainly of carbohydrates). Absorbtion can also perform in oral cavity.

2) Non-specific -

· participation in behavioural reactions forming (hunger, thirst);

· thermoregulatory;

· protective;

· excretory;

· incretory;

· participation in articulation and speech forming.

Digestion in oral cavity is mainly realized due to salivary galnds secretory function. Salivary glands secretory function is provided by functionning of 3 pairs of large (parotid, sublingual and submandibular) and great amount of small glands disseminated in oral mucosa. Saliva is a mixture of secretes. With the addition of epitheliocytes, food particles, mucus, lymphocytes, neutrophils and microorganisms (they are in oral cavity in large amounts) it formes oral liquid. Daily saliva secretion is 0,5-2,0 litres. Its pH fluctuates from 5,25 to 8,0.

Saliva contains up to 99,5% of water. There are many organic and inorganic substances in solid residue. One can say that almost all Mendeleev’s table is in saliva (even gold!). There are many organic substances in saliva. They are protheins – albumins, globulins, aminoacids. Nitrogen-containing substances – urea, ammonia, creatine. Bacteriocydic substances – lyzozyme; enzymes – alpha-amylase or ptyalin, maltase, proteases, peptidases, lipase, alkaline and acid phosphatase.

Saliva role in digestion: it gives the beginning to food chemical processing. It occurs due to amylase acting on polysacharides (starch) while their destruction to maltose. Under other enzyme maltase influence maltose destruction to glucose can occur. But enzymes action is very limited because food is in oral cavity very little time. One of the most important digestion rules: careful (durable) food mastication due to which saliva can influence on food (in oral cavity) more effectively.

But saliva is not only restricted by food possible chemical processing. Saliva takes part in preparation of food portion to swallowing and further digestion. Food is mixed with saliva in course of mastication and is swallowed better. Saliva equally covers teeth in neutral environment forming special tunic on them. In acid environment releasing mucin covers teeth surface an encourages teeth coating and stones forming. That’s why after food taking it’s necessary either to brush teeth or to wash oral cavity. Teeth and mucosa state depends on saliva content and features. Saliva volume, chemical content and features change can underline many diseases of oral cavity. For example, saliva, while contact with dental enamel is the calcium, phosphorus, zinc and other microelements sourse for it. If saliva pH is 7,0-8,0 it oversaturated by calcium that creates ideal conditions for ions passage into enamel. At environment acidification (pH 6,5 and lower) oral liquid becomes deficient on calcium ions content that encourages its releasing form enamel and caries development.

According to saliva chemical analysis and even smell, colour one can tell about inner organs diseases. For instance, at nephritis, stomach and duodenum ulcer disease residual nitrogen amount is increased in saliva. At stroke on the injury (haemorrhagia) side salivary glands excrete great number of protein.

You know about oral mucosa increased regenerative ability. Quickly mucosa restoration after its wounding (it occurs practically every day) is connected not only with tissular immunity but also with saliva antibacterial features. Besides, there are substances in saliva influencing on blood coagulation and fibrinolysis. That’s why oral cavity protective function is also delt with this saliva ability to influence on local haemostasis and fibrinolysis.

Saliva formation mechanism. Saliva is formed both in acinuses and in salivary glands ducts. Secretory granules are in glandulocytes cytoplasm. Granules size, amount and localization are changed in course of secretion. They are moved to cellular apex from Golgi complex. Organic substances synthesis passed with water through cell on endoplasmic net is performed in granules. Saliva formation first stage is realized in acinuses – primary secrete forming containing amylase and mucin. Ions content in it insignificantly differs from their concentration in extracellular space. Secrete content changes significantly in salivary ducts: sodium ions are actively reabsorbed and potassium ions are actively secreted. As a result, sodium amount in saliva becomes less and potassium – bigger. And after reabsorbtion and seretion secondary saliva is formed.

Salivary glands in new borns secrete little saliva – 0,4 ml per minute in course of sucking, less – out off sucking. It is in average in 8 times less than in adulthood. Salivation volume is increased from 4 months and reaches up to 150 ml per day to 1 year (it is 1/10 of adult secretion). Amylase activity in new-borned saliva is low and it is increased in second half-year, reaching adult level in course of 1-2 years after birth.

Salivation regulation is performed by complicated-reflectory and humoral ways. Special place in regulation has complicated-reflectory mechanism. It consists of conditioned-reflectory and unconditioned-reflectory. Conditioned-reflectory salivation regulation way is connected with food appearence, its smell (in humans and animals), communication about it and other conditioned stimuli (pictures, writings, symbols) delt with alimentary motivation. Unconditioned-reflectory appears as an answer to oral cavity mechano-, chemo-, thermo- and gustatory receptors irritation. Nervous impulses flow comes from these receptors through V, VII, IX and X pairs of cranio-cerebral nerves to medulla oblongata in salivation center. Efferent fibres of given reflectory acts go from this center to salivatory glands. They can carry information to salivary glands through sympathetic and parasympathetic fibres that innervate salivary glands. Sublingual and submandibular salivary glands are innervated by preganglionar parasympathetic nervous fibres coming in composition of chorda tympani (facial nerve branch) to corresponding ganglions located in glands body. Postganglionar nervous fibres innervate glands secretory cells and vessels.

Parotid glands are innervated by preganglionar parasympathetic fibres of inferior salivatory nucleus of medulla oblongata coming in the composition of IX pair in auricular node. Postganglionar nervous fibres are directed to secretory cells and vessels. Sympathetic innervation is represented by preganglionar nervous fibres from lateral corns of spine II-IV thoracic segments and is finished in superior cervical node, then postganglionar fibres to salivary glands come.

At sympathetic nerve excitement small saliva amount containing mucin doing it viscous and dense is released. At parasympathetic nerve – on the contrary, saliva becomes fluid and its amount is big.

Hypothalamic anterior and posterior nuclear groups participate in salivation regulation. Salivation reflectory regulation is not unique though it is main.

Humoral mechanism is delt with hypophyseal, pancreatic, thyroid, sexual hormones action. Excessive salivation occurs due to salivatory center irritation by carbonic acid. Saliva releasing may be stimulated by vegetothropic pharmacologic substances – pilocarpine, proserine, atropine. Saliva production can decrease too. It may be connected with noceoceptive and emotional reactions, with fever states, at systematic sleeping pills usage, diabetes mellitus, anaemia, uraemia, salivary glands diseases.

Oral cavity motor activity. Essence:

· food biting;

· getting small;

· grinding;

· mixturing with saliva;

· alimentary piece forming;

· swallowing.

Oral cavity motor function main part is realized in course of mastication.

Mastication – is a complicated act. Its essence is in consequent contractions of masticatory muscles, mandibule, tongue and soft palate movements. Masticatory muscles are fixated to moveless skull part by their one end, by other end – to unique movable skull bone – mandibule. They provide mandibule status change as for maxilla while their contraction. Mimic muscles are close to masticatory muscles on their functions. They participate in food catching, its supporing in oral cavity vestibule, oral cavity closure at mastication. They are essential at sucking in new-borns and at liquid food taking. Tongue has definite role in mastication. It takes active part in food mixture, definition of its place for getting smaller on teeth.

Mastication act by its mechanism is partially arbitrary, partially – reflectory. Human being can free inhibit or enforce masticative movements, change their character. Food biting and mastication is performed at superior jaw teeth occlusion (contact) with inferior jaw teeth. Mandibule performes rhythmic movements in 3 main directions: vertical, sagittal, transversal. Mastication is originated from assessment of received food after which food piece irritates located in oral cavity touch, temperature, gustatory, nociceptive receptors. Besides, due to sense of smell impulses occuring in these receptors come into mentioned above nervous stems in medulla oblongata in mastication center. Then they on trygeminal nerve second and third rami, facial, glosso-pharyngeal and hypoglossal nerve come to masticatory muscles. In parallel with food getting smaller its washing with saliva occurs for better swallowing. Food getting smaller degree is under oral mucosa receptors control. Non-food elements are pushed at this by tongue (bones, stones, paper et al.). One should remember about necessity of careful food processing in oral cavity. It’s an essential preventive measure for many diseases not only of alimentary tract. In babies sucking corresponds to mastication which is provided by mouth and tongue muscles reflectory contractions.

Swallowing – is a complicated reflectory act due to which food is transported from oral cavity into stomach. Phases:

· Oral arbitrary – from food common mass in oral cavity small piece is separated which by tongue movements is pressed to hard palate. Jaws are closed, soft palate is rised closing entrance into choanes. Simultaneousely with this palato-pharyngeal muscles are contracted. Septum is formed which closes passage between oral and nasal cavity in the result of these processes. Tongue moving ahead pressses onto palate and pushes food piece into pharynx. Because of this food piece is pushed down into pharynx. Entrance into larynx is closed by epiglottis, vocal cord is closed to prevent food coming into trachea. As food piece comes into pharynx, soft palate anterior arch are contracted and together with tongue root prevent food returning into oral cavity.

· Pharyngeal-inarbitrary – is originated when food piece is pushed ahead and pharyngeal-oesophageal sphincter, closing under rest state the entrance to oesophageus, is opened. Sphincter’s muscles are relaxed and pressure is decreased in it, food piece passes into oesophageus and sphincter is closed again because of pressure increasing in it. Such reaction prevents food piece passage from oesophageus into pharynx.

· Oesophageal inarbitrary: food piece transmits from oesophageus oral part to cardial.

Swallowing process as reflectory act is performed due to irritation receptor endings of trygeminal, superior and inferior laryngeal, glosso-pharyngeal nerves located in soft palate and pharynx mucosa. Swallowing center is located in medulla oblongata near respiratory center and is in reciproqual (antagonist) interrelations. At swallowing center excitement respiratory center activity is inhibited; respiration is stopped in this moment that prevents food particles passage into respiratory ways. Swallowing act afferent ways – superior and inferior pharyngeal, recurrens nerve and vagus fibres. They direct nervous impulses to muscles participating in swallowing.

Oral cavity is an initial link of reflectory reactions influencing on digestion in stomach and intestine. Oral cavity receptors irritation stimulates stomach juice forming, stomach motor function. Stomach and pancreas secretion depends on mastication act duration. The mastication is less the stomach juice is less. Oral mucosa and tongue is alimentary tract mirror. One can see problems which may occur in stomach and other alimentary tract parts in it. Pathological processes in oral cavity organs can encourage some inner organs diseases occurence, cause or support different complications. In particularly, teeth pathological agility and loss leads to incomplete food processing in oral cavity that in first turn influences on stomach and intestine motor and secretory activity. But digestion disorders in oral cavity caused by mastication change at teeth loss don’t always lead to one or another pathology in alimentary tract other parts. Alimentary tract initial link (oral tract) periodically undergo to action of removable substances (solid subjects, acids, alkalins, excessively warm or cold bodies, strong mechanical actions) that causes hypersalivation occurence as mean as oral cavity and alimentary channel tissues integrity providing. Rich microbe flora containing pathogenic microorganisms comes into oral cavity with alimentary substances. It was the reason of forming tissular and cellular barriers as well as oral cavity specific and non-specific resistance in course of evolution.

Oral cavity protective function mechanisms (see also lecture on blood physiology).

Oral cavity protective function systemic mechanisms are functional integrity of behavioural, conditioned- and unconditioned-reflectory, barrier and immuno-chemical reactions. Information about threat to tissues integrity occurs at super intensive influencings onto mechano-, thermo- and chemoreceptors of tongue, lips, cheeks mucosa, palate, periodont and others. Besides, at oral cavity tissues injury special chemoreceptors (chemonociceptors) percept substances forming in course of cells destruction and direct the information to CNS. On the base of this information compensatory mechanisms are formed the ending aim of which to provide tissues integrity, to protect organism from injury.

One of protective mechanisms is behaviour directed to injured factors avoiding (head turn, jaws closure, running from irritator, avoiding dangerous places et al.). Defencive behaviour may be passive and active. Pricking up, covering, harboring, avoiding something (remind children’s behaviour in dental clinics) belong to behaviour passive forms; aggression, resistance belong to active ones. But the most important urgent oral cavity protective mechanism is salivation – saliva releasing as answer reaction to removable substances coming. Mechano-, thermo- and chemoreceptors strong irritation and action to nociceptors lead to releasing of great number of saliva poor on enzymes and performing next tasks: ability to the fastest removal of harmful substances out off oral cavity, normalization of coming products temperature. Buffer saliva features are essential and allow to neutralize acids and alkalins of removable substances. Buffer features are connected with alkaline salts existance in a secretion (sodium and potassium chloridum et al.). Besides, saliva possesses other protective qualities. Alongside with alimentary or removable substances toxines and microbe flora (particularly pathogenic one) comes to oral cavity. There are more than 400 types of bacterias in human mouth, some of them may be the reason of infectionning of gums and osseal tissue below them. There exist rather favourable conditions in oral cavity for microflora development – food residues existance, weakly alkaline saliva environment (pH), humidity, optimal temperature. Microorganisms are up to 70 per cent of dental covering. It was estimated that approximately 250 microbe cells are in 1 mg of dental covering dry mass, 1 ml of saliva contains more than 10⁸ of microbes. Microbes and viruses distribution in oral cavity is unequal - their main part is located in dental-gingival pockets, mucosa plicas and interdental spaces. Pathogenic microflora is of essential importance at gums injuries. Special attention should be payed to parodontosis development. Gums inflammation, in course of which they become sensitive to different irritators action and are bleeded – it is the first stage of this disease, affecting millions of people. But not only gums are affected at this disease. But while sick gims are exfoliated out off teeth deeper and deeper pockets are formed where infection penetrates destructing osseal tissue. Teeth are sitting in their nests undensely and that’s why finally human being losses them. But simultaneousely parodonthosis may accelerate other diseases development in organism or make their course complicated. How defence from pathogenic microflora is performed in oral cavity? In course of oral cavity microflora it was established that it possesses relative stability preventing pathogenic microorganisms spreading. Such stability is defined by saliva content, bacteriocydic and bacteryostatic substances containing in it. Enzyme lyzozyme (muromidase) plays important role in oral cavity homeostasis supporting. This enzyme bacteriolytic action is delt with muramic acid destruction in some bacterias wall changing its permeability that causes their content diffusion in surrounding environment. Salivary lactoperoxidase makes bacteriocyde action (participates in gram-negative bacterias lysis). Myeloperoxidase - enzyme encouraging lipid peroxidative oxidation that results in bacterias death. Lactoferrine competes with bacterias for iron ion. If bacterias have highly-developed cytochromic system lactoferrine leads to their death. Mucin encourages bacterias big amount fixation to desquamating epitheliocytes. There are betta-lysines in oral liquid which penetrate here from blood and cause bacterial cellular membrane lysis. Saliva contains i nterpherones having the ability to suppress viral replication, possess antitumorogenic features. Salivary protheolytic enzymes of wide activity spectrum can injury some bacterias membranes. Lithium ions, zyanides presence and other components also leads to microorganisms death. As for complement components, immunoglobulins, phagocytosis and haemostatic reactions – see their description in lection on blood physiology.

Finally, it should be mentioned in conclusion that alimentary organs chronic diseases sometimes are accompanied by appearance of antibodies in circulating blood that react to food proteins antigens and glycoproteids of milk, eggs, fish, citruses, chocholate and other foods. These antibodies to foods participate in alimentary allergy pathogenesis (toxico-allergic stomatitis). But we would like to pay your attention to the fact that antibodies against alimentary antigens are in blood of healthy people too. That’s why, feeding according to blood group, to our point of view, is one of the most important ways of oral cavity diseases prevention as well as other factors of a healthy life style (physical activity, harmful habits et al.).

Thus, oral cavity protective mechanisms providing the integrity of tissues of alimentary channel initial part and organism in a whole are a very complicated system.

 

Lecture 8