The Digestive System For Dentest

The Digestive System • All living organisms require energy to carry out the daily activities. • The required energy is provided by food. • The food is consists of complex organic substances. • The digestive system is responsible for 1- Ingestion of food 2- Digestion of food. 3- Absorption 4- Elimination of indigestible foodstuffs as Dr. Yaser Ashour feces.

The Digestive System gland Esophagus

Longitudinal organization

Stomach

Small Intestine

Large Intestine

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• The adult gastrointestinal tract is about 5 meter long. • It starts by mouth and ended by anus. • It consists of: - Mouth - Pharynx - Oesophagus - Stomach - Small intestine - Large intestine (rectum and anal canal) Dr. Yaser Ashour

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Digestive Tract Anatomy • • • •

1- Oral cavity 2- Salivary glands 3- Esophagus 4- Stomach – – – – –

Fundus Body (rugae) Antrum pyloris

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More Digestive Tract Anatomy • 5- Small intestine – Duodenum – Jejunum – Ileum

• 6- Pancreas • 7- Liver • 8- Large intestine – Colon – Rectum

• 9- Anus Dr. Yaser Ashour

Gastrointestinal System

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Digestive system functions • • • • • • •

1- Ingestion 2- Digestion 3- Motility 4- Secretion 5- Absorption 5- Elimination 6- Self protection Dr. Yaser Ashour

Functions representation of main regions of the gastrointestinal tract

REGION: Oesophagus

Stomach

FUNCTION: - Transit - storage, H+/peptic digestion & intrinsic factor

duodenum Small intestine

jejunum

- fat, protein, carbohydrate digestion & absorption - water and electrolyte transport

ileum

- bile salt transport

Colon

- Storage water and electrolyte transport

Large intestine

rectum & anus Dr. Yaser Ashour

- defecation

Layers of GI Tract

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The gut wall Lumen epithelium Lamina propria

MUCOSA

1

Muscularis mucosa SUBMUCOSA ( blood vessels, lymph 2 vessels and Meissner`s plexus) Circular muscle Myenteric plexus

MUSCULARIS PROPRIA

3

Longitudinal muscle Mesothelium (SEROSA) Dr. Yaser Ashour

4

Layers of GI Tract

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GIT WALL The wall of the gut is formed by four layers:1- The serous layer: all abdominal organs are covered by a connective tissue called peritoneum. 2- Muscle layer: it is consists of two layers outer longitudinal and inner circular in between these there are blood vessels and Myenteric plexus (Auerbach`s). 3- Submucous layer: it consists of loose connective tissue and it contains blood vessels and nerve plexus (Meissner`s). 4- Mucous layer: it consists of surface epithelium Dr. Yaser Ashour

Innervation of the GI tract Smooth muscle has inherent rhythmicity • Intensity, duration and frequency of contraction modulated by parasympathetic nervous system • Ganglia present in wall – Meissner’s plexus in submucosa – Auerbach’s plexus in between layers of the muscularis (myenteric plexus). Dr. Yaser Ashour

GIT Innervations

• The GIT is innervated by an intrinsic nervous and extrinsic nerves (sympathetic and parasympathetic) The intrinsic nervous system consists of: 1- The Meissner’s (submucosa). 2- The Auerbach’s plexus (myenteric). The Meissner’s (submucosa), control the secretory activity and the blood flow of the gut. It also serves a sensory function. The Auerbach’s plexus (myenteric), controls the motility of the gut. Dr. Yaser Ashour

GIT Innervations (cont.

The intrinsic nervous system has many neurotransmitters (acetylcholine, norepinephrine, vasoactive intestinal peptide, substance P and somatostatin. The extrinsic nerve supply: The parasympathetic fibers to the gut, extending from esophagus to the large intestine, pass via the vagus nerve, which also supply pancreas and gall bladder, the distal half of the large intestine receives its parasympathetic through pelvic nerve. Dr. Yaser Ashour

GIT innervations (cont. The parasympathetic stimulation increase the motility of GIT wall, relaxation of the sphincters and increase of secretory activity of the GIT. The sympathetic fibers to the GIT: Are supplied through the splanchnic nerves. Stimulation of the sympathetic nerves causes inhibition of the motility, contraction of the sphincters and vasoconstriction in the GIT. Dr. Yaser Ashour

Intrinsic and extrinsic nerves of the digestive tract e.g. secretory cell

e.g. Stretch & chemosensitive neurons Submucosal plexus

INTRINSIC NERVES

e.g. post-ganglionic nonadrenergic or cholinergic fibres

Myenteric plexus

Visceral afferents

EXTRINSIC NERVES

Parasympathetic efferents

Dr. Yaser Ashour

Regulation of GIT motility and secretion The motility and secretion depend upon Reflex arc: • The Receptors: present in the mucosa. • The stimulus: is the distention of gut by the luminal contents, acidity of the chyme, osmolality of the chyme, peptides and fats. • The effector organs: the smooth muscle wall and exocrine glands. • The aim of the reflex activities: is to maintain conditions that are optimal for digestion and the absorption of foodstuff. Dr. Yaser Ashour

Regulation of GIT motility and secretion Neural regulation • The intrinsic nervous system regulate the motility and secretory activity of GIT. • Extrinsic innervations is involved in the regulation of GIT motility and secretion in response to changes in the environment (emotions, taste, smell). Hormonal regulation • local hormones control and regulate GIT motility and secretion. Dr. Yaser Ashour

GIT Hormones

• Released from small intestine epithelium in response to acid and some of them secreted from stomach wall. • Synthesized in entero chromaffin like cells, which has diffuse distribution. Gut Hormone Families • Gastrin family: Gastrin, Cholecystokinin pancreozymin. • Secretin family (Secretin, Glucagon, Vasoactive intestinal polypeptide (VIP) and Gastric inhibitory peptide (GIP). • Other Candidate Hormones (Motilin, Somatostatin, Substance P, Neurotensin, Gastrinreleasingpeptide, Bulbogastrone, Urogastrone, Villikinin, Calcitonin generelated peptides, Endorphins) Dr. Yaser Ashour

• • • • • • •

Physiological Roles of GIT Hormones Stimulate secretion of enzymes for digestion. Required to move material across cell membranes. Stimulate secretion of acids and bases. Responsible for optimal pH for enzymes action. Stimulate smooth muscle contraction. Stimulate release of hormones from pancreatic islets Provide satiety signals to the brain Dr. Yaser Ashour

Gastrin

Source: Antral mucosa of the stomach and G cells of duodenal mucosa. Released into: both lumen and into blood at basal lamina. Released in response to: food, Peptides, amino acids, to a lesser extent, FFA’s distension of stomach after food intake • Release under autonomic nervous system control.

Physiological Function of Gastrin • Stimulate HCl and pepsinogen secretion in the funds of the stomach Dr. Yaser Ashour

Site targeted for stimulating release • Histamine-releasing cells • Acid-secreting parietal cells • Indirect release of pepsinogen through HCl release Effect of Gastrin hormone release: • Stimulates lower esophageal sphincter. • Relaxation of pyloric sphincter. • Stimulates release of pancreatic enzymes. • Enhance motor activity of intestine. • Stimulate pancreatic bicarbonate and water secretion. Dr. Yaser Ashour

Cholecystokinin (CCK)

Release in response to the presence of food particularly lipids. Site of release: T cells of duodenum and jejunum. Response to stimulation: • Gallbladder contractions. • Pancreatic enzyme secretion. • Emptying of gallbladder. • Inhibits gastric emptying. • Potentates Secretin-induced pancreatic bicarbonate secretion. • Trophic actions on acinar cells of the pancreas. • Relaxation of sphincter of Oddi by VIP release. • Regulates bile flow. • Inhibition of gastric emptying by inhibiting vago-vagal reflex by vasoactive intestinal peptide. • Acts as satiety hormone. • Anorexia of aging may be related to increased levels of CCK. Dr. Yaser Ashour

Secretin • Concentration greatest in duodenum • Granular S cells between crypts and villi of mucosa. • HCl reaching mucosa stimulus for secretion. • Stimulates pancreatic bicarbonate secretion. • Potentiate CCK-stimulated pancreatic enzyme secretion. • Interact with CCK to stimulate bicarbonate secretion Dr. Yaser Ashour

Vasoactive Intestinal Peptide (VIP) Physiological effects: • Relaxes smooth muscles • Antagonizes smooth muscle constrictor agents • Inhibits histamine and pentagastrin-stimulated gastric acid release • Stimulates electrolyte and water secretion by pancreas • Stimulates bile secretion • Stimulates – Lipolysis – Glycogenolysis – Insulin secretion Dr. Yaser Ashour

Gastric Inhibitory Peptide (GIP) • Localized to K cells of duodenum and jejunum • Released by fat present in ingested meal. Physiological effects: • Inhibits gastric-acid secretion. • Stimulates release of pancreatic water and electrolyte secretion. • Stimulates water and electrolyte release from Brunner’s glands on intestinal mucosa. • Adipose tissue. • Activates lipoprotein lipase. • Inhibits glucagon-induced lipolysis • Potentiates effect of insulin on fatty acid incorporation into triglycerides • Increases glucose use in peripheral tissue (muscle) • Increase volume or electrolyte composition of saliva and intestinal secretions Dr. Yaser Ashour

Motilin • peptide hormone • Highest concentrations localized to duodenum • Stimulates gastric motor activity • Regulates emptying of stomach

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Movements of the GIT

• Digestion is a process by which complex food material is broken down into simple particles, and then absorbed. • Digestion is of two types:– Mechanical (physical). – Chemical.

• Mechanical digestion: is broken of food into small particles. • Chemical digestion: is the act of various digestive enzymes on the complex food to convert it into a simple form. Dr. Yaser Ashour

Movements of the GIT There are three types of GIT movements: • Neurogenic, Myogenic and passive movements

1- The neurogenic: are brought by nervous control. 2- The Myogenic: are brought about by smooth muscles which are independent of nervous control. 3- The passive movements: are neither neurogenic nor Myogenic in nature. Dr. Yaser Ashour

Movements of the GIT (con. Purpose of gastrointestinal movements:

1- Converting the food into fine particles. 2- Mixing the food with juices. 3- Moving the food onward through the alimentary canal. 4- Help in absorption of food. 5- Help in maintenance of alimentary tract in an active and normal condition. 6- Help in excrete waste products from the food and body itself, and eliminate the undigested and Yaser Ashourof feces. unabsorbed food inDr.the form

Movements of the GIT (con. Types of Gastrointestinal Movement: • 1- Mastication. • 2- Deglutition (swallowing). • 3- Stomach movements. • 4- Small intestine movements. • 5- Large intestine movements.

Dr. Yaser Ashour

Movements of the GIT (con. • Mastication – It is the preliminary stage of motor function. – It is the first mechanical process to which food is subjected in the GI tract. – It is a process of crushing food under the grinding action of the teeth. Dr. Yaser Ashour

Mastication is a reflex actìon: Stimulus: The pressure of food in the mouth. Receptors: The mechanical receptors in the oral mucosa. Afferent: 5th, 7th and 9th cranial nerves. Center: Mastication center in the reticular formation of the medulla. Effector organs: The digastric and mylohyoid muscles. Response: Inhibition of the muscles of mastication, which allows the lower jaw to drop, and Dr. Yaser Ashour opening of mouth.

• The drop initiates a stretch reflex of the jaw muscles (the masseters, temporalis and pterygoids, which are supplied by the fifth cranial nerve). • This leads to rebound contraction, which automatically raises the jaw, causing the teeth to close and exert their crushing effect. • The rotatory movements of the molars are carried out by the pterygoid muscles, which make the jaws move sideways. Dr. Yaser Ashour

• The tongue muscles (supplied by the 12th cranial nerve) move inside the mouth. • The muscles of the oropharynx (supplied by the 9th cranial nerve) and the cheek muscles (supplied by the 7th cranial nerve) also move.

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Purpose of mastication – Help the swallowing processes. – Help in food broken into small particles, – With saliva it helps in formation of bolus. – Helps in protection of mucous membrane against damage. – Facilitate digestion. – Helps in the secretion of digestive juice. – Stimulation of the taste buds. Dr. Yaser Ashour

Deglutition (swallowing) •

It is the process by which food is passed from the oral cavity into the stomach. • It starts after mastication is complete, that is, when the food has been converted into a bolus. Deglutition may be divided into:• Oral stage: A voluntary stage, which initiates the swallowing process. • Pharyngeal stage: A pharyngeal stage, which is involuntary and constitutes the passage of food through the pharynx into the esophagus. • Esophageal stage: An esophageal stage, which is an involuntary phase that promotes the passage of food from the pharynx to the stomach. Dr. Yaser Ashour

Deglutition (swallowing)

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Oral stage (voluntary Stage) • It started when the food in the mouth is ready to be swallowed. • This is done by the tongue pushing upwards and backwards against the palate to move the food. • This is a voluntary movement which humans can control in normal swallowing. • When the bolus is allowed to pass into the pharynx, the nasopharynx is shut off to prevent the regurgitation of food through the nose. At this point, respiration reflexly stops (deglutition apnea). • The nasopharynx is shut off by the upward movement of the soft palate and the forward movement of the Dr. Yaser Ashour posterior pharyngeal wall.

Oral Phase

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Pharyngeal stage • The pharyngeal stage begins once the swallow reflex has been activated at the end of the oral stage. • The events of this stage cannot be voluntarily interrupted and will not take place if the reflex is not activated by the swallowing receptor areas around the opening of the pharynx, particularly the tonsillar or faucial pillars, which are stimulated as the bolus of food enters the posterior mouth and pharynx. Dr. Yaser Ashour

Pharyngeal stage (cont. • A person does not breathe during the pharyngeal stage of swallowing. • During the pharyngeal stage of swallowing it is important to: • Protect the airway during deglutition so that none of the swallowed matter is ingested into the lungs. • The whole sequence of events in this stage is completed quickly so that breathing can be rapidly resumed. • The whole stage occurs in less than 2 seconds. Dr. Yaser Ashour

Pharyngeal stage (cont. • The swallowing receptor areas pass impulses to the brain stem to start a series of automatic pharyngeal muscle contractions. • Reflux of food into the nasal cavities is prevented by the soft palate pulling upwards, by contracting the tensor veli palatini, levator veli palatini and musculus uvulae. • On either sides of the pharynx are pillars of fauces which are pulled together, to form a saggital slit. This slit acts as a gateway to the posterior pharynx, letting through food which has been masticated enough to pass easily. Larger objects usually can't pass. Dr. Yaser Ashour

Pharyngeal Stage

Dr. Yaser Ashour

Pharyngeal stage (cont. • The vocal folds of the larynx are closed and the larynx is pulled upward and anteriorly by the neck muscles. • The epiglottis is attached to ligaments that prevent it from moving upwards, thus causing it to swing backward to cover the entrance to the larynx. • These two actions prevent food passing into the trachea, particularly the action of the vocal folds. • If the vocal folds were removed or unable to close (i.e. muscle defects) then strangulation could occur.

• However, if the epiglottis was to be removed then swallowing could still occur. Dr. Yaser Ashour

Pharyngeal stage (cont. •

•

•

As the larynx moves upwards, it effectively pulls up and enlarges the opening to the esophagus. The upper oesophageal sphincter relaxes to allow food to move easily down from the posterior pharynx to the upper esophagus. This sphincter remains strongly contracted in between swallows so as to prevent air from going into the esophagus during respiration.

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Protection of air ways

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•

Protection of air ways

The glottis is lifted out of the main flow of food when the larynx moves upwards, so that food passes on either side of the epiglottis rather than over its surface. This action can count as another protective precaution to prevent the entry of food into the trachea. • At this stage the whole muscular wall of the pharynx contracts as a rapid peristaltic wave, beginning in the superior part of the pharynx and spreading downwards. This wave then spreads down into the esophagus, propelling food down into this part of the digestive tract. Dr. Yaser Ashour

Esophageal Stage This stage involves the esophagus which functions to transport food from the pharynx to the stomach via the relaxed gastro-oesophageal (or cardiac) sphincter at the entrance to the stomach. In this stage the esophagus moves the food by:1- primary peristalsis. 2- secondary peristalsis. Dr. Yaser Ashour

Primary peristalsis • Continues the peristaltic wave that begins in the pharynx and spreads down into the esophagus during the pharyngeal stage mentioned earlier. • The primary peristaltic wave passes from the pharynx to the stomach in approximately 8-10 seconds. • However, when a person is sitting upright, food swallowed is transmitted to the lower end of the esophagus in about 5-8 seconds.

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Secondary peristalsis

• occurs to move any left food down from the esophagus into the stomach, which the primary peristaltic wave may have left behind. • This second wave results from the distension (stretching) of the esophagus by the food within it. • The esophagus is always closed flat except during the passage of food. • The secondary peristaltic waves continue until all the food has moved down into the stomach. • During this stage, the larynx descends and the airway is reopened. The tongue moves forwards and respiration resumes Dr. Yaser Ashour

Swallowing of fluids The descriptions above apply to human adults swallowing solid or semisolid food. • There is a slight difference with the swallowing of liquids. • First suction is created as a seal is created between the tongue and palate and simultaneously the tongue is retracting. • The posterior part of the tongue then depresses from the palate and allows the liquid to run into the pharynx. Dr. Yaser Ashour

Salivary glands • Three major glands (paired) – Parotid – Submandibular – Sublingual

• Secretion – – – –

Parasympathetic stimuli Chemical stimuli Physical stimuli Psychological stimuli

• Numerous accessory glands in submucosa – continuous secretion Dr. Yaser Ashour

Salivary glands

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Salivary glands One and half liter of saliva is produced daily in normal adult persons to start chemical digestion in the mouth, when the food comes in contact with saliva. • Saliva is secreted by:– Three pairs of salivary glands (parotid, Submaxillary and sublingual.) – Many buccal glands, which secrete mucous.

• The salivary gland is an exocrine gland; its secretion is as follow: – Parotid glands: secrete about 25% of saliva and it is serous (thin and watery) – Submaxillary glands: Secrete about 70 % of saliva and it is mixed (serous and mucous). – Sublingual glands: Secrete about 5% of saliva and it is mucous (thick and viscid). Dr. Yaser Ashour

Salivary glands • Saliva – Mucus -Enzymes (amylase, lysozyme) – Antibodies (IgA) - Inorganic ions

• Cells – Serous and mucus secreting • Serous cells – Round central nuclei – Zymogen granules – Basophilic cytoplasm

• Mucus cells – Clear staining mucigen granules – Flattened eccentric nuclei – Lightly stained Dr. Yaser Ashour cytoplasm

Salivary Gland Functions 1- Digestion of starch: - salivary amylase/ptyalin acts on polysaccharides and converts boiled starch into maltose. - It acts best at a pH of 6.8. - The digestion of carbohydrates begins in the oral cavity, but the food remains in the oral cavity for only a short time. - Therefore, the action of amylase is short-lived, continuing only until the food enters the stomach and till the ph of gastric contents becomes too acidic for amylase activity. Dr. Yaser Ashour

Salivary Gland Functions 2- Facilitation of swallowing: By moistening and lubricating food bolus. 3- Facilitation of speech: by moistening the mouth cavity, tongue and lips. 4- Regulation of water balance: in cases of dehydration when secretion is decreased giving the sensation of thirst.

Dr. Yaser Ashour

Salivary Gland Functions 5- Immunity function: The immunoglobulin present in saliva acts against bacteria in the mouth. 6- Cleaning: saliva washes ood remnants, which acts as a good medium for bacteria. 7- Excretion: The salivary glands excrete heavy metals, such as lead and mercury, and viruses such as rabies and poliomyelitis. 8- Solvent: Saliva dissolves many food materials so that taste buds in the tongue can be stimulated. 9- Dilution: of irritating substances and excessively hot or cold food. Dr. Yaser Ashour

Regulation of salivary secretion - The secretion of saliva is regulated basically by parasympathetic innervations of the salivary glands. -This brings about reflex secretion of saliva in response to tactile stimuli, or stimuli which arise in the CNS itself. - Parasympathetic stimulation causes vasodilatation, while stimulation of glandular tissue gives rise to profuse, watery secretion. - Vasodilatation by the parasympathetic nerves is brought about by the formation of bradykinin. - Parasympathetic innervations of the salivary glands arises from the superior and inferior salivary nuclei in the reticular formation of the medulla.

Dr. Yaser Ashour

Mechanism of salivary secretion

• Salivary secretion is a rapid process; therefore it occurs through nervous mechanism only, through two pathways:• Conditioned reflex • Unconditioned reflex

• Conditioned reflex: • It is an acquired reflex which depends on presence of cerebral cortex and pervious training. It produces salivary secretion in empty mouth • Stimulus: Seeing, hearing and smelling of food • Receptors: In the organs of special senses. • Afferent: Sensory nerves from organs of special senses (optic, olfactory and auditory). • Center: Cerebral cortex. • Efferent: Chorda tympani, glossopharyngeal and sympathetic nerve. • Response: Salivary secretion. Dr. Yaser Ashour

Mechanism of salivary secretion

Unconditioned reflex: • Is present since birth, do not need learning. • It started on the presence of food in the mouth, which stimulate taste receptors in the mouth. • Stimulus: food, (chemical) or mechanical (e.g. chewing gum) stimuli. • Receptors: taste buds receptors. • Afferent: Chorda tympani (anterior 2/3 of tongue) and glossopharyngeal nerve (posterior 1/3 of tongue). • Center: Superior and inferior salivary nuclei. • Efferent: Chorda, glossopharyngeal and sympathetic nerves. • Effectors organs: Salivary gland. • Response: Salivary secretion. Dr. Yaser Ashour

Salivary secretion • Clinical Importance • - It is important to maintain oral asepsis in high fever and coma, since the secretion of saliva decreases. • - Decreased salivary secretion can cause dental caries, so the mouth has to be rinsed after taking food. • - Atropine is administered before general anesthesia to decrease salivary secretion. • - The rabies virus, caused by dog bite, and the virus of poliomyelitis are transmitted through Dr. Yaser Ashour saliva.

Vomiting Vomiting: It is the forceful expulsion of contents of the stomach and often, the proximal small intestine. • It is a manifestation of a large number of conditions, many of which are not primary disorders of the gastrointestinal tract. Regardless of cause, vomiting can have serious consequences, including acid-base derangements, volume and electrolyte depletion, malnutrition and aspiration pneumonia. The Act of Vomiting • Vomiting is usually experienced as the finale in a series of three events, • Nausea is an unpleasant and difficult to describe psychic experience in humans and probably animals. Physiologically, nausea is typically associated with decreased gastric motility and increased tone in the small intestine. Additionally, there is often reverse peristalsis in the proximal small intestine. Dr. Yaser Ashour

• Retching ("dry heaves") refers to spasmodic respiratory movements conducted with a closed glottis. While this is occurring, the antrum of the stomach contracts and the funds and cardia relax. • Emesis: is when gastric and often small intestinal contents are propelled up to and out of the mouth. It results from a highly coordinated series of events that could be described as the following series of steps Mechanism of vomiting • 1-A deep breath is taken, the glottis is closed and the larynx is raised to open the upper esophageal sphincter. Also, the soft palate is elevated to close off the posterior nares. • 2- The diaphragm is contracted sharply downward to create negative pressure in the thorax, which facilitates opening of the esophagus and distal esophageal sphincter. • 3- Simultaneously with downward movement of the diaphragm, the muscles of the abdominal walls are vigorously contracted, squeezing the stomach and thus elevating intragastric pressure. With the pylorus closed and the esophagus relatively open, the route of exit is clear. Dr. Yaser Ashour