Stomach secretion regulating 


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Stomach secretion regulating



is realized in 3 phases:

1. Cerebral - is determined by complex of conditioned and unconditioned reflexes. It is originated from conditioned-reflectory phase, because food appearence, smell and everything delt with its preparation (sounds, for instance) cause gastric juice releasing. Unconditioned-reflectory phase begins at the moment when food comes into oral cavity. Here excitement of receptive zones known for you from last lecture is accompanied by informational flow in alimentary center bulbar part (medulla oblongata) through nervi vagi, from where through secretory fibres of the same nerves, to secretory cells. This gastric juice prepares stomach to food taking before it. It possesses high acidity and large proteolytic activity. This juice is called “appetite” one.

2. Gastric phase. When food comes into stomach than gastric juice releasing continues mainly due to reflectory-humoral mechanisms linked with stomach activity. That’s why this phase was named gastric. Stomach juice releasing on this stage is connected with vagus participation and local (intramural) reflexes as well as it is performed due to stomach local hormones secretion. Vagus sensory fibres excitement occurs while mechanical and chemical irritators (food, hydrochloric acid, salts, digestion products) action onto stomach mucosa. They transmit information into bulbar center and return to stomach glands through its secretory fibres. Acethylcholine released on vagi endings excites gastric glands main and parietal cells and helps progastrine releasing (the latest transforms into gastrine under hydrochloric acid influence and acts to these cells). Also acethylcholine enforces hystamine formation in stomach mucosa. This phase of gastric secretion is main.

3. Intenstinal phase. When food begins its gradual transition into duodenum, gastric secretion is still continue. It is possible due to next phase performance – intestinal. Stomach juice amount released during this phase is only 10 per cent of general juice volume. This phase is humorally-chemical. Gastric glands hypersecretion at this moment is linked with fresh food portion coming. Such portion didn’t manage to get saturated with hydrochloric acid. Enterogastrine formed in duodenum mucosa excites stomach secretion too. One of powerful intestinal factors stimulating gastric secretion are food digestion products (especially proteins) which increase gastrine and hystamine secretion.

But gastric secretion is inhibited at some stage. First of all, it is linked with food passage from stomach. But further stomach secretion inhibiting is delt with gastrine antagonist secretine occurence in duodenum mucosa. Secretine is formed from its predecessor prosecretine under hydrochloric acid action. Especially sharp stomach secretion inhibition appears at fats coming into duodenum as well as peptide substances secreted in gastro-intestinal tract (somatostatine, vaso-active peptide or VIP, cholecystokinine, glucagon and others). Hormone enterogastrone secreted in duodenum mucosa as well as adrenaline (noradrenaline) inhibit gastric secretion too. Emotional reactions connected with hypersympathicotony inhibit gastric secretion. But not all emotional reactions and emotional excitement influence similarly on gastric secretion. Such reactions as stress, fury may cause in separate people both gastric juice activation and inhibition. Fear and anguish – inhibit gastric secretion.

Stomach juice character and amount depend on food type. Regulational mechanisms are important for this. In course of meat eating (protein food) stomach secretion increases during the 1-st hour and reaches its maximum to the 2-nd hour. It occurs due to reflectory reactions connected with oral cavity activity (tasty, organoleptic saliva features) and proteins – stocks, received at their digestion in stomach, possess the same features. Then stomach juice secretion begins its gradual decreasing and finishes approximately in 8 hours after beginning. Reaction is expressed on carbohydrate food (bread, for example) during 1-st hour. It is connected with the same reasons like on meat: stomach juice reflectory releasing on food components which are in oral cavity and stomach. Then secretion is sharply reduced and lasts about 10 hours at low level. One can observe 2 phases at milk (fat) action: inhibitory and exciting. Secretion maximum is developed only on the 3-rd hour and may last up to 6 hours. Gastric glands secretion has not only digestive tasks, but it provides several other organism reactions delt with neutral mucopolysacharides, sialomucines and glycoproteins (that is the mucus base) as it was mentionned above.

Stomach motor activity. Stomach preserves, heats up, mixes, diminishes, transforms into semiliquid state, sorts and moves content to duodenum with different velocity and force. All this is performed due to motor function, caused by its smooth-muscular wall contractions. Stomach is in recessive state, without wide cavity between its walls out of digestion phase. After 45-90 minutes of rest state periodic stomach contractions appear lasting 20-50 minutes (hungry periodic activity). At filling with food stomach aquires sac-like shape, one part of which comes into cone.

When stomach is filled, its motor activity consists of some movements types. Peristaltic waves appear in initial contractive period. They are distributed from oesophageus to pylorus with velocity of 1 cm/sec, have the duration about 1,5 sec and involve 1-2 cm of stomach wall. Waves duration in pylorus is 4-6 per minute and its velocity increases up to 3-4 cm/sec. These low-haltitude peristaltic movements encourages food mixture with stomach juice and transmission of its small portions into stomach body. Carbohydrates decomposition with salivatory amylase continues inside food piece. These movements last approximately 1 hour. Strong and freaquent contractions occur periodically which mix food more active with gastric juice enzymes and pass stomach content. Peristaltic waves in pylorus were named as propulsive contractions. They provide content evacuation to duodenum. These waves appear with the freaquency 6-7 per minutes.

Stomach musculature state and activity is changed with reflectory mechanism at oral cavity irritation by food and removed substances. Liquid and semiliquid substances usage and psychical excitement inhibit stomach movements by reflectory way and close pyloric sphincter. Solid food substances cause by the same reflectory way stomach movements decreasing with oral cavity receptors.

Mastication is accompanied by stomach musculature tonic contractions; swallowing – stomach smooth muscle inhibiting and weakening. Stomach contractive force and its musculature hypertony degree depend on mastication intensivity and its musculature initial state. The more swallowed piece volume is, the more stomach contraction inhibition is expressed.

Under usual digestion coditions stomach contractions occur as a result of mechanical irritation and stretching of its walls with food. This is percepted by nervous plexi neurons processes situated in intermuscular and submucosal layer. Vagus enforces and sympathic nerve weakens stomach motor activity.

Stomach motor activity humoral causative agents are gastrointestinal hormones – gastrine, motiline. Motor activity is increased under serotonin, insuline. Glucagon, secretine and cholecystokinine under stomach acid content action inhibit stomach motor activity and food evacuation from it. The same action have adrenaline, noradrenaline, enterogastrone.

Food transition from stomach in duodenum is performed portially in course of antrum strong contractions. Pyloric sphincter prevents chimus regurgitation to stomach. Pyloric sphincter is opened at empty stomach. It is periodically opened and closed in course of digestion. Sphincter opening reason is pylorus mucosa irritation with hydrochloric acid. At this time food part transits in duodenum and reaction in it becomes acid despite alkaline, that causes pylorus musculature reflectory contraction and sphincter is closed. It is observed at fat introduction in duodenum that allows its lack in stomach.

Other essential factors for food transition from stomach into duodenum are the following:

· stomach content consitention (liquid and semiliquid food leaves stomach);

· chimus osmotic pressure (hypertonic solutions retard evacuation and leave stomach only after their dissolving with stomach juice up to isotonic concentration);

· duodenum filling degree: at its stretching evacuation from the stomach is retarded and even can finish;

· badly swallowed and fat food are retarded in stomach for long;

· vagus and enterogastrine enforces chimus transition, sympathic nerve and enterogastrone – inhibit it.

Stomach content may leave it in opposite direction. It is delt with cardial sphincter working peculiarity. Food piece passing into oesophageus low end, irritates its mucosa that causes cardial sphincter reflectory opening which in the adult always closes entrance into stomach. That’s why stomach content can’t fall down even if you are standing your head down. Cardial sphincter contraction is supported from the stomach side with reflectory way usage. Cardial sphincter tone is absent in small children and that’s why stomach content is pushed ahead into oral cavity at child’s turning over his head down. There can be other variant of such reaction. In course of gastro-intestinal tract receptors irritation with toxines and metabolites nausea occurs – sensation connected with central nervous system activity at reticular formation excitability significant increasing. Vomiting comes after nausea; nausea is accompanied by vegetative disorders (salivation, perspiration increasing). Vomiting – protective reaction, occuring at vomiting center, medulla oblongata reticular formation structures excitement as well as impulsation from alimentary tract and vestibular receptors. It may be caused by olfactory, visual, gustatory irritations which excites vomiting center at intracranial pressure increasing. Efferent influencings through vagus and partially splanchnic nerve are transmitted to intestine, stomach, eosophageus as well as motor nerves to abdominal wall muscles and diaphragm. At vomiting bone and larynx are rised up, oesophageal superior sphincter is opened, pharynx is closed, soft palate is opened with choans closure. Then diaphragm and abdominal wall strong contraction begin, finally, lower eosophageal sphincter is relaxed and stomach content is pushed through eosophageus. Antiperistalsis and nausea come before vomiting. Antiperistaltic waves occur in alimentary tract dystal parts and are distributed through small intenstine with velocity of 2-3 cm/sec while intestinal content return in duodenum and stomach for 3-5 min. Vomiting occurs by reflectory principle at gastro-intenstinal tract receptors irritation and automatically – at some substances (toxines) action through blood to nervous center. Sometimes vomiting is caused consciousely, specially with the aim of stomach releasing (for example, at intoxications).

In some occasions, stomach motor activity is disturbed and is realized slowly. One should take into account that bad stomach releasing – is ulcerogenesis risk factor.

Motor periodics on an empty stomach in new-borns is absent. It is delt with nervous regulatory mechanisms immaturing. Stomach content evacuation after baby feeding with breast milk occurs for 2-3 hours. It determines feedings freaquency. Feeding mixture with cow milk of the same volume at artificial feeding is retarded in stomach more – for 3-4 hours. Proteins and fats amount increasing in food retards evacuation from the stomach up to 4,5-6,5 hours. In babies evacuation inhibiting with proteins is more expressed, in teenagers and adults – with fats.

 

Lecture 24.



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