GIT Physiology PDF

Faculty of Physical Therapy GIT Physiology Regulatory mechanisms in the GIT Neural Regulation The gut is innervated by two sets of nerves: I- The intrinsic nervous system, (enteric nervous system, ENS), has cell bodies that are located within the wall of the gut (submucosal and my...

Faculty of Physical Therapy GIT Physiology Regulatory mechanisms in the GIT Neural Regulation The gut is innervated by two sets of nerves: I- The intrinsic nervous system, (enteric nervous system, ENS), has cell bodies that are located within the wall of the gut (submucosal and myenteric plexuses). II- The extrinsic nervous system is defined as nerves that innervate the gut, with cell bodies located outside the gut wall; these extrinsic nerves are part of the autonomic nervous system (ANS). Regulatory mechanisms in the GIT I- The intrinsic nervous system The gastrointestinal tract has a nervous system all its own called the enteric nervous system (ENS). It lies entirely in the wall of the gut, beginning in the esophagus and extending to the anus. The enteric nervous system is composed mainly of two plexuses: (1) an outer plexus called the myenteric or Auerbach's plexus, and (2) an inner plexus, called the submucosal or Meissner's plexus. Regulatory mechanisms in the GIT I- The intrinsic nervous system (1) an outer plexus lying between the longitudinal and circular muscle layers, called the myenteric or Auerbach's plexus, and control the motility function. (2) an inner plexus, called the submucosal or Meissner's plexus, that lies in the submucosa and control the secretory function, absorbtion, and contraction of submucosal muscles that affect the degree of infolding of GIT epithelium. Regulatory mechanisms in the GIT I- The intrinsic nervous system Stimuli in the wall of the gut are detected by afferent neurons, which activate interneurons and then efferent neurons to alter function. In this way the ENS can act autonomously from extrinsic innervation. However, neurons in the ENS, are innervated by extrinsic neurons. Regulatory mechanisms in the GIT Regulatory mechanisms in the GIT II- The extrinsic nervous system via the two major subdivisions of the ANS, 1- Parasympathetic innervation The vagus nerve, innervates the esophagus, stomach, gallbladder, pancreas, small intestine, cecum, and the proximal part of the colon. The pelvic nerves (S2,3,4) innervate the distal part of the colon and the anorectal region. These nerves results mainly in stimulation of GIT motility and secretion. Regulatory mechanisms in the GIT 2- Sympathetic innervation Sympathetic innervation to GIT arise from T6- L2 segments. These nerves results mainly in inhibition of GIT motility and secretion. Also, it innervates the smooth muscle cells of blood vessels, leading to vasoconstriction Sensory nerve endings that arise from GIT epithelium and relay in cell bodies of enteric nervous system, in prevertebral ganglia of sympathetic, in vagus nerve or the spinal cord causes local reflexes. Regulatory mechanisms in the GIT  Stomach The stomach is the most distensible part of the GI tract. Its functions are: to store food, to initiate the digestion of proteins, to kill bacteria with the strong acidity of gastric juice, to move the food into the small intestine as a pasty material called chyme. Stomach Gastric glands Gastric glands contain several types of cells that secrete different products: 1. goblet cells, secrete mucus; 2. parietal (or oxyntic) cells, secrete HCl; and intrinsic factor 3. chief (or zymogenic) cells, secrete pepsinogen, 4. enterochromaffin-like (ECL) cells, secrete histamine Pyloric glands contain several types of cells: ❑ goblet cells, secrete mucus; ❑ G cells, secrete the hormone gastrin into the blood; ❑ D cells, secrete the hormone somatostatin. Hydrochloric Acid The high concentration of HCl makes gastric juice very acidic, This strong acidity serves some functions: 1. Activate pepsinogen into pepsin and provide optimum pH for its action. 2. Killing most ingested bacteria. 3. Help absorption of iron and calcium. 4. Stimulate bile flow and pancreatic secretion. Hydrochloric Acid Secretion Regulation of Hcl Acid Secretion Histamine, gastrin and ACh are important in stimulating acid secretion. They facilitate the second messengers which help the expression of H,K- ATPase on the cell surface. While somatostatin inhibit acid secretion. Regulation of Gastric Function The extrinsic control of gastric function is divided into three phases: (1) Cephalic phase; (2) Gastric phase; (3) Intestinal phase (1) The cephalic phase: (30%) Stimuli: Sight, Smell, Taste, Chewing. Pathways: Parasympathetic nerves to ENS results in the release of: ✓ ACh from the plexus neurons, ✓ gastrin from the gastrin- releasing cells, and ✓ histamine from ECL cells Response: stimulation of gastric acid secretion 2) The gastric phase (60%) Stimuli: gastric distension by the volume of ingested material Pathways: Vagovagal reflex Response: stimulation of gastric acid secretion (3) The intestinal phase Stimuli: Acid, distension, hypertonic solutions, and solutions containing amino acids, and fatty acids in the small intestine ( food in small intestine) Pathways: Long and short neural reflexes; Hormones released by the intestinal tract: secretin, cholecystokinine CCK, gastric inhibitory peptide, vasoactive intestinal polypeptide, and somatostatin Response: inhibition of gastric acid secretion and motility. Digestion and Absorption in the GIT  Digestion and Absorption in the GIT The major foods can be classified as carbohydrates, fats, and proteins. They generally cannot be absorbed in their natural forms. Therefore, it needs to be digested into small enough compounds for absorption: carbohydrates monosacharrides fats FA + monoglycerides Proteins A.A.  Digestion of Carbohydrates In the mouth: the digestion of starch begins with the action of salivary amylase. In the duodenum: by the action of pancreatic amylase to produce disaccharide maltose (2 molecules of glucose) and trisaccharide maltriose (3 molecules of glucose) and oligosaccharides (2 to 10 molecules of glucose). In the small intestine: the brush border enzymes hydrolyze maltose, maltriose and oligosaccharides to their monosaccharides  Absorption of Carbohydrates In the small intestine At intestinal brush border: At low concentrations is transported through the mucosal lining into the epithelial cells of the intestine by secondary active transport, via a sodium dependant transporter. At higher concentrations, is transported facilitated diffusion. At basolateral border: is transported facilitated diffusion. Absorption of Carbohydrates  Digestion of Protein In the stomach: protein digestion begins with the action of pepsin to produce short-chain polypeptides. In the duodenum and jejunum: most protein digestion occur by the action of trypsin, chymotrypsin, elastase, carboxypeptidase (in pancreatic juice) and aminopeptidase (in intestinal juice). As a result of the action of these enzymes, polypeptide chains are digested into free amino acids.  Absorption of Protein At intestinal brush border: Amino acids is absorbed by secondary active transport. At basolateral border: is transported facilitated diffusion.  Digestion of Lipids In a process called emulsification, bile salt micelles (spherical shape formed of 20 to 40 bile salts) are secreted into the duodenum and act to break up the fat droplets into tiny fat droplets of triglycerides. Fat digestion occurs at the surface of the droplets through the enzymatic action of pancreatic lipase, Free fatty acids and monoglycerides quickly become associated with micelles of bile salts to form “mixed micelles” to be absorbed from brush border membrane. Digestion of Lipids  Absorption of Lipids At intestinal brush border: The micelles is absorbed by simple diffusion. At basolateral border: Chylomicrons is transported exocytosis to enter the lymph vessels not blood capillaries. THANK YOU