Digestion.docx

Saliva production is continuous secretion under autonomic nervous system Not essential for digestion, needed for speech, chewing + swallowing. Can be controlled by simple and conditioned reflexes. Simple reflex when chemoreceptors and pressure receptors when oral cavity respond to presence of food i...

Saliva production is continuous secretion under autonomic nervous system Not essential for digestion, needed for speech, chewing + swallowing. Can be controlled by simple and conditioned reflexes. Simple reflex when chemoreceptors and pressure receptors when oral cavity respond to presence of food initiates impulses via afferent nerve to salivary centre in medulla Conditioned reflex occurs unhat ora simulation, just thinking/ seeing/ smelling food causes salvation -This activates cerebral cortex which then activates salivary centre in medulla. Both reflexes cause autonomic nerves to be innervates which stimulates the salivary glands and therefore production of saliva The sublingual, the submandibular and the parotid glands produce saliva. Both parasympathetic and sympathetic control saliva production. Not antagonistic like elsewhere in body so they were together Autonomic control of saliva production Parasympathetic (chlorogenic) Innervated by medulla. Increases production of watery saliva- mouth watering Main control of saliva- dominant Sympathetic (noradrenergic) Innervation from superior cervical ganglion Increase production of thick mucus saliva Dry mouth Eg in stressful situations. Swallowing Voluntarily initiated followed by reflex. Reflex triggered by pressure receptors in pharynx. Controlled by swallowing centre in medulla. Highly complex, involved 31 pair of muscles. 3 stages of swallowing Oral/Oropharyngeal stage Once the food is chewed into a bolus Voluntary swallowing is initiated by pushing the tongue against the hard palate. The bolus is pushed to the back to pharynx. The ovula is elevated and lodges at the back of the throat to seal off nasal passage. Food is prevented from entering trachea by elevation of larynx and closure of vocal folds- glottis and epiglottis. Epiglottis prevents food from entering the respiratory tract. Swallowing centre inhibits respiratory centre. Once the bolus is pushed into the oesophagus, the pharyngoesophageal sphincter closes. Oesophageal stage Peristaltic movements push the bolus down Gastroesophageal sphincter relaxed to push the bolus into the stomach and then closes to prevent the food from entering back into the oesophagus again. Exocrine secretions of pancreatic juice Watery alkaline solution (sodium bicarbonate)- duct cells Proteolytic enzymes- acinar cells Pancreatic amylase and lipase- acinar cells Proteolytic enzymes are releases in inactive form and later activated by enzymes in duodenal membrane in small intestine. Trypsinogen Chymotryspinogen Procarboxypeptidase Activates to Trypsin Chymotrypsin Carboxypeptidase Each different type of Proteolytic enzymes attacks different peptide linkages. Once chyme enters the duodenum, it causes one of two things to be released based on the composition of chyme. Secretin Triggered by acid. Stimulates duct cells to produce more sodium bicarbonate to neutralise chyme. Cholecystokinin (CCK) Triggered by fat or protein. Stimulates acinar cells to produce panarctic digestive enzymes. Absorption in small intestines Doesn’t secrete any digestive enzymes. Special hairlike projections on liminal surface of small intestine = microvilli form the bush border → contain 3 types of membrane spanning proteins that function as membrane bound enzymes. Enteropeptidase → activates pancreatic Proteolytic enzyme called trypsinogen as its inactive when released by pancreas. Disaccharidases → breaks down disaccharides such as maltose into monosaccharides. Aminopeptidases→ hydrolyse small peptide fragments into amino acids. These hairlike projections are constantly being replaced due to harsh environment and sensitive to cancer treatment. Carbohydrate reabsorption Carbohydrates are normally found in form of disaccharides such as maltose sucrose and lactose. These disaccharides are broken down into absorbable monosaccharides units of glucose galactose and fructose. Glucose and lactose are absorbed into epithelial cells by symport with Na+ (SGLT= sodium glucose transporters). SGLT is energy dependence secondary active transport located on luminal membrane. Fructose enters the cell passively by facilitated diffusion by a Glut-5 Glucose, galactose, and fructose exit the cell at basal membrane by passive facilitated diffusion via Glut-2 and enter the blood via simple diffusion. Protein reabsorption Amino acids absorbed into epithelial cells via Na+ symporters. Amino acid symporters are selective for different amino acids. Some small peptides are also absorbed by H+ symporter- these small peptides are broken down into amino acids by intracellular peptidases. Amino acids exit the cell at basal membrane passively via carriers (facilitated diffusion) Amino acids enter the blood via simple diffusion. Fat absorption The pancreatic lipase and bile salts breakdown fats into micelles Micelles are able to passively diffuse through membrane the monoglycerides and free fatty acids leave the micelles and passively diffuse through the luminal membrane. Inside epithelial cells the monoglycerides and fatty acids re synthesised into triglycerides These triglycerides aggregate coated with lipoprotein from the ER to form water soluble chylomicrons. Chylomicrons leave the basal membrane via exocytosis. Enters the lymphatic system are there unable to cross the basement membrane of the capillaries.