Gastrointestinal Physiology Introduction & Activities in the mouth PDF

Gastrointestinal Physiology General Objectives of the Course 1. Understand the functions of the gastrointestinal System. 2. Describe the organization of the gastrointestinal system. 3. Explain the regulation of the functions of the gastrointestinal system. 4. Demonstrate the most important activiti...

Gastrointestinal Physiology General Objectives of the Course 1. Understand the functions of the gastrointestinal System. 2. Describe the organization of the gastrointestinal system. 3. Explain the regulation of the functions of the gastrointestinal system. 4. Demonstrate the most important activities that occur in each segment of the gastrointestinal tract during passage of food. 5. Integrate the facts and concepts of physiology to solve clinical problems. 6. Use the principles of gastrointestinal physiology in the laboratory and bedside technique commonly encountered in clinical practice. 7. Acquire skills of self learning. Functions of the Gastrointestinal System 1. Nutrition function: Ingestion, digestion & absorption of food (assimilation of food), considered to be the primary function. 2. Excretion function: Undigested materials as stool & excretion of certain wastes which can not be excreted in urine. 3. Defense function: Bayer's batches, chemical & physical mechanisms. Organization of the Gastrointestinal System 1. Structural Organization. 2. Functional Organization. 1. Structural organization Digestive tract: - Tube extends from mouth to anus. - Divided into segments & there are sphincters which control passage of food from one segment to another. - Different activities & functions occur in each segment. - Basic structure of the layers of wall of the gastrointestinal tube with slight variation from one segment to another. Accessory glands: - Three accessory glands: salivary glands, pancreas & liver. - Elaborate their secretion into the lumen of GIT. - Contribute by their secretions in digestion & absorption. Anatomical Structure of the Gastrointestinal System Basic Structure of the Wall of the Gastrointestinal Tract 2. Functional organization Food is sequentially digested as it passes through the gastrointestinal tract from one segment to another. Digestion & absorption is complete by the food leave the small intestine & the remaining undigested material are excreted in the form of stool. No further digestion & absorption of food occur in the large intestine, only absorption of water electrolytes & processing of undigested material takes place in the large intestine. The plan of study of physiology of the gastrointestinal is to follow a bolus from the mouth until it excreted as stool in the anus see (study) the activities (most important) in each segment There are two activities by which the gastrointestinal system carry its function in digestion & absorption of food: 1. Secretory activities. 2. Motility activities. Regulation of the Functions of the Gastrointestinal System Regulation of the gastrointestinal function generally depends on: 1. Neural regulation. 2. Humoral regulation. 1. Neural regulation Can be divided into two: A. Intrinsic innervation (enteric nervous system), act independent of the extrinsic innervation, mediate their effect through a very large number of neurotransmitters, depends on two nerve pluxes: 1. Submucous plexus (Meissner's pluxes), in the submucosa. 2. Myenteric plexus (Auerbach's pluxes), in between the circular & longitudinal muscle layers. B. Extrinsic innervation, act directly or indirectly through the enteric nervous system, depends on: 1. Parasympathetic NS. 2. Sympathetic NS. Regulation of the Gastrointestinal Functions 2. Humoral Regulation Depends on a large number of peptides secreted from endocrine cells in the wall of the gastrointestinal tract. Can be divided into: A. Hormone: Gastrin, CCK, Secretin, GIP. B. Paracrine: Somatostatin, Histamine. C. Neurocrine: Ach, Ad, Enkephalin, Substance P, VIP, GRP, Neuropeptide Y. Activities in the Mouth Secretory activity: - Secretion of saliva. Motility activities: - Mastication. - Swallowing or deglutition. Secretion of saliva Secreted by three pairs of salivary glands: 1. Submandibular, 60%, Mixed (serous & mucus), Facial nerve. 2. Parotid, Largest, 25%, Mainly Serous, Glossopharyngeal nerve. 3. Sublingual, 5%, Mixed (mainly mucus), Facial nerve. Small glands scattered throughout the mouth, 10-15%. Volume 1 – 1.2 L/d. Most of the volume come from parotid upon stimulation of secretion. The Salivary Glands Functions of Saliva: 1. Keep the oral tissues moist & protect against irritants and desiccation. 2. Bacteriostatic functions. 3. Protect teeth: (wash & form pellicles) & prevent dental caries. 4. Important in speech. 5. Important in mastication. 6. Important in swallowing. 7. Important in forensic medicine. 8. Important in taste. 9. Digestive function. 10. Buffering function. Microscopic Structure Grape-like structure consists of: - Acini: made up of acinar cells. - Duct system: intercalated ducts, intralobular (striated ducts), interlobular. - myoepithelial cells. Microscopic Structure of Salivary Glands Mechanism of salivary secretion. Primary secretion: - Secreted by acinar cells. - Secreted as isotonic secretion. Secondary secretion: - Modified primary secretion by ductal cells. - Becomes hypotonic because of absorption of sodium & chloride. Mechanism of salivary secretion Composition of saliva: Volume of saliva secrete per day is 05 to 1.5 liter/day. Hypotonic, pH 6 to 7 & becomes alkaline with high rates of secretion. Composition is flow dependent. Consists of: 1. Mainly water. 2. Organic constituents: enzymes, blood group antigens, lysozymes, lactoferrin, mucus, Ig A, growth factors, mucin, amylase, glucose, urea, uric acid, creatinine, cholesterol, cAMP. 3. Inorganic constituents: electrolytes, Na, K, Cl, Fe, HCO3, Ca, HPO4. 4. Hypotonic, alkaline, flow dependent composition. Relation Between Composition of Saliva & the Salivary Flow Rate Control of Salivary Secretion Only neural by Sympathetic & parasympathetic divisions of the ANS. - Parasympathetic centers are the superior & inferior salivary nuclei in the medulla oblongata. - Impulses from superior salivary nucleus are carried by the facial nerve (7th cranial nerve) to reach submandibular ganglion. - Postganglionic fibers then supply both submandibular & sublingual glands. - From the inferior salivary nucleus, the glossopharyngeal nerve (9th cranial nerve) preganglionic fibers synapse in otic ganglion that lies very close to parotid gland with postganglionic fibers supply the parotid gland. - Sympathetic centers are present in thoracic spinal cord (TI -T4). - Short preganglionic fibers synapse in superior cervical ganglion & postganglionic fibers supply each of the three glands. Cont. Autonomic nervous system produces its effects by acting on four different sites: 1. Acini. 2. Ducts. 3. Myoepithelial cells. 4. Blood vessels.  Effect of the parasympathetic & sympathetic on the salivary glands Site Parasympathetic Sympathetic Acini Increases activity of acini and Ducts stimulating Helps to produce organic components of water, electrolytes And salivary amylase saliva poor in water and electrolytes secretion Myoepithelial Contraction Contraction cells Blood vessels Vasodilatation Vasoconstriction, But poor as it is Counteracted by vasodilatation produced by parasympathetic Cont. Control of salivary secretion Controlled through neural reflexes, divided into: conditioned reflexes & non conditioned reflexes Salivary secretion is increased by: 1. Taste, especially of sour substances 2. Smell. 3. Mastication, even of tasteless substances. 4. Conditional reflexes, which operates mainly in animals, but not very prominent in humans. 5. Presence of acid in lower esophagus, which causes an increase in both flow rate and bicarbonate content of saliva. Cont. Control of salivary secretion Salivary secretion is inhibited by: 1. Dehydration. 2. Fear. 3. Sleep. 4. Severe mental effort. Swallowing or Deglutition The process of passage of food from the mouth to the stomach. Depend on coordinated muscle activity of the mouth, pharynx & esophagus. It is a reflex that start voluntarily & end involuntarily. Coordinated by swallowing center in the medulla. Divided into three phases: 1. Buccal phase (voluntary). 2. Pharyngeal phase (involuntary or reflex). 3. Esophageal phase (involuntary or reflex). 1. Buccal phase - This is a voluntary phase. - Mouth is first closed (swallowing is extremely difficult with an open mouth). - Tongue collect bolus, the tip of the tongue is pressed against the hard palate thus creating a chamber between the tongue & the palate. - Then contraction of mylohyoid & styloglossus muscles causes the tongue to moves upward & backward, pushes the bolus into pharynx. - The soft palate rises & the posterior pharyngeal wall moves forward as a result of the of contraction of the superior constrictor muscle of the pharynx as the result they meet & close of the nasopharynx. - Respiration is inhibited & the larynx starts to rise signaling the end of the oral phase & this help to push the larynx out the pass of the food & prevent entry of food into the respiratory passages. - From this phase onward swallowing becomes involuntary. 2. Pharyngeal phase - This phase is involuntary & controlled by the swallowing center in the medulla. - This requires highly coordinated muscle contractions to propel the bolus to the upper entrance of the esophagus while preventing the entrance to the air passages at the larynx. - During this phase the tongue continue to move backwards pushing the bolus against the epiglottis which bends over the larynx & divide the bolus into two part, the epiglottis is however not important for swallowing & can be removed without significant effect on swallowing. - On the posterior pharyngeal wall there are lots of receptors which when stimulated by food send impulses through IX and X cranial nerves to swallowing center in the medulla & efferent impulses come back through the same nerves to cause all events in this phase. - The opening of the larynx is closed by contraction of muscles guarding its opening & vocal cords come close together such that food does not pass into lungs. Cont. Pharyngeal phase - The bolus is propelled downwards by the contractions of the middle & inferior constrictors of the pharynx & reach the upper end of the esophagus. - The cricopharyngeal sphincter relaxes for about one second & closes again as soon the bolus enters the esophagus & this is the end of the pharyngeal phase of swallowing. - Then the focal folds relax, the larynx drops & the epiglottitis returns to its previous position - Palatopharyngeal folds are pulled towards the center to prevent passage of large objects so as not to damage the delicate esophageal epithelium. - Upper & lower sphincter relax therefore food can pass into & out of the esophagus. - Superior constrictor muscle produces a wave of contraction (peristaltic wave) that pushes food into the esophagus, this is known as primary peristaltic wave. 3. Esophageal phase - This phase starts with opening of the upper sphincter. - The bolus of food goes into the esophagus. - The sphincter immediately contracts to prevent entry of air into the esophagus. - The peristaltic wave, which started in the pharynx continues to push bolus to the end of esophagus, normally travel 4 cm/s & takes 5-10 seconds, known as primary peristaltic wave, occur by efferent impulses in the vagi from the swallowing center. - If the bolus sticks into the esophagus, it stimulates mechanical receptor in esophageal wall leading to impulses travelling in sensory vagal fibers to swallowing center in medulla & efferent impulses are then sent through motor vagal fibers to cause peristaltic contractions. Cont. Esophageal phase - This is referred to as secondary peristaltic waves, which is a vagovagal reflex as both afferents & efferent are present in the same nerve. - (A peristaltic wave is defined as a wave of contraction proceeded by a wave of relaxation; the contraction wave occurs at the level of the bolus & results from local distension caused by the bolus). - By the time the bolus reaches the lower part of the esophagus the lower esophageal sphincter is relaxed allowing the bolus to enter the stomach. Esophageal Motility The esophagus is a tube of about 25 cm long with an upper & a lower sphincter. The wall of the upper third of the esophagus is a skeletal muscle, the middle third is a mixture of smooth and skeletal muscles & the lower third is exclusively smooth muscle. Esophageal motility overlap with esophageal phase of swallowing & with receptive relaxation component of gastric motility. This is because primary peristalsis open the upper & lower esophageal sphincter & cause receptive relaxation of the stomach. Secondary peristalsis occur as the result of distension of esophagus. Esophageal sphincters: Upper esophageal sphincter: - Also known as cricophayngeal sphincter. - It is an anatomical sphincter consisting of a thick muscle band. - Normally it is closed & opens by primary peristalsis only when the posterior pharyngeal wall is stimulated by food & closes immediately when food enters the esophagus. - This is important for guarding against entry of air, which may cause distension of stomach. Cont. Lower esophageal sphincter - Also known as gastroesophageal sphincter. - It is a functional sphincter i.e. no obvious muscle thickening. - Relaxation of this sphincter may be caused by: 1. Distension of the pharynx by food. 2. Vagal stimulation releasing VIP. 3. Carminative drugs e.g. mint directly acts on sphincter and also causes relaxation of stomach so causes belching. 4. Progesterone hormone, the level of which increases during pregnancy, a fact which makes pregnant ladies to complain of hyperacidity (heart burn). Cont. The sphincter is contracted (closed) by: 1. Rise of intragastric pressure such as by bending so guarding against acid reflux. 2. Gastrin hormone secreted by G cells of the stomach. This hormone stimulates acid secretion. 3. Products of protein digestion, which stimulates Gastrin secretion. 4. Sympathetic stimulation 5. Vagus nerve releasing acetylcholine.