UWCM/SONMS/nutrition/MJohn The Large Bowel Defaecation UWCM/SONMS/nutrition/MJohn

UWCM/SONMS/nutrition/MJohn Digestive System GI tract divided into: Alimentary canal. Accessory digestive organs. GI tract is 30 ft long and extends from mouth to anus. UWCM/SONMS/nutrition/MJohn

UWCM/SONMS/nutrition/MJohn Layers of GI Tract Mucosa Submucosa Muscularis Serosa UWCM/SONMS/nutrition/MJohn

UWCM/SONMS/nutrition/MJohn Mucosa Lines the lumen of GI tract. Consists of simple columnar epithelium. Lamina propria: Thin layer of connective tissue containing lymph nodes. Muscularis mucosae: Thin layer of smooth muscle responsible for the folds. Increase surface area. Goblet cells: Secrete mucus. UWCM/SONMS/nutrition/MJohn

UWCM/SONMS/nutrition/MJohn Submucosa Thick, highly vascular layer of connective tissue. Absorbed molecules enter the blood and lymph vessels in this layer. Submucosal plexuses: Provide autonomic nerve supply to the muscularis mucosae. UWCM/SONMS/nutrition/MJohn

UWCM/SONMS/nutrition/MJohn Muscularis Responsible for segmental contractions and peristaltic movement through the GI tract. Inner circular smooth muscle. Outer longitudinal smooth muscle. Contractions of these layers move food through the tract and pulverize the food. Myenteric plexus located between 2 muscle layers. Major nerve supply to GI tract. UWCM/SONMS/nutrition/MJohn

Regulation of the GI Tract Extrinsic innervation: Parasympathetic Nervous System: Stimulate motility. Sympathetic Nervous System: Reduce peristalsis and secretory activity. Enteric Nervous System: Submucosal plexus and myenteric plexus: Local regulation of the GI tract. Paracrine secretion: Molecules acting locally. Hormonal secretion: Secreted by the mucosa. UWCM/SONMS/nutrition/MJohn

UWCM/SONMS/nutrition/MJohn

Intestinal Contractions and Motility 2 major types of contractions occur in the small intestine: Peristalsis: Slow movement. Pressure at the pyloric end of small intestine is greater than at the distal end. Segmentation: Major contractile activity of the small intestine. Contraction of circular smooth muscle. Mix chyme. UWCM/SONMS/nutrition/MJohn

Contractions of Intestinal Smooth Muscles Occur automatically in response to endogenous pacemaker activity. Rhythm of contractions is paced by graded depolarizations. Slow waves produced by interstitial cells of Cajal. Slow waves spread from 1 smooth muscle cell to another through nexuses. UWCM/SONMS/nutrition/MJohn

Contractions of Intestinal Smooth Muscles Slow waves decrease in amplitude as they are conducted. May stimulate contraction in proportion to the magnitude of depolarization. When slow waves above threshold, it triggers APs by opening of VG Ca++ channels. UWCM/SONMS/nutrition/MJohn

Smooth Muscle Action Potentials At threshold, VG Ca++ channels open. Repolarization: VG K+ channels open. Stimulates contraction. Parasympathetic NS, stretch and gastrin increase the amplitude of slow waves. Stimulate APs. SNS decrease APs. UWCM/SONMS/nutrition/MJohn

UWCM/SONMS/nutrition/MJohn

Absorption within the large Intestine Aid absorption of H20, electrolytes, vitamin B complex vitamins, vitamin K, and folic acid. Secrete H20, via active transport of NaCl. Guanylin stimulates secretion of Cl- and H20 and inhibits absorption of Na+. Contains lymphocytes, lymph nodes, goblet cells. UWCM/SONMS/nutrition/MJohn

Defence systems within the large Intestine immune cells distributed in follicles & Peyer's patches dispersed in the mucosal lamina propria & epithelium T helper cells + CD4 (in the lamina propria) T cells suppressor/Cytotoxic + CD8 (in the epithelium) B lymphocytes, produces mostly IgA & some IgG producing cells Enterocytes, produce cytokines (Interluekin with pro-inflammatory anti-inflammatory properties Goblet cells populations also rise in response to infections UWCM/SONMS/nutrition/MJohn

UWCM/SONMS/nutrition/MJohn Fluid & Electrolyte Absorption in the Intestine:COMPARING LARGE SMALL BOWEL Small intestine: Most of the fluid & electrolytes are absorbed by small intestine (90% of the volume) Absorption of H20 occurs passively as a result of the osmotic gradient created by active transport. Aldosterone stimulates NaCl and H20 in the ileum. Large intestine: Absorbs about 90% of the remaining volume. Absorption of H20 occurs passively as a result of the osmotic gradient created by active transport of Na+ and Cl-. UWCM/SONMS/nutrition/MJohn

Enteric Nervous System Submucosal and myenteric plexuses contain 100 million neurons. Include preganglionic parasympathetic axons, ganglion cell bodies, postganglionic sympathetic axons, and sensory neurons. UWCM/SONMS/nutrition/MJohn

Enteric Nervous System Peristalsis: Acetylcholine and substance P stimulate smooth muscle contraction. Nitrous Oxide, Vasoactive Intestinal Peptide, and ATP stimulate smooth muscle relaxation. UWCM/SONMS/nutrition/MJohn

UWCM/SONMS/nutrition/MJohn Intestinal Reflexes Ileogastric reflex: Distension of ileum causes decreased gastric motility & this may bring about nausea. Intestino-intestinal reflex: Overdistension in 1 segment, causes relaxation of rest of intestine. UWCM/SONMS/nutrition/MJohn

UWCM/SONMS/nutrition/MJohn Defaecation Waste material passes to the rectum. Defaecation reflex: Longitudinal rectal muscle contracts to increase rectal pressure(pressure receptors stimulated) Relaxation of of internal anal sphincter. Excretion is aided by contractions of abdominal & pelvic skeletal muscles. Pushes faeces from the rectum. UWCM/SONMS/nutrition/MJohn