Large Intestine Lecture Notes Fall 2024 PDF

1502 Large Intestine Fall 2024 Lecture 17 Tuesday Oct 1: 9am Dr. L. Al-Nakkash [email protected] 1  Learning Objectives 1. Ident...

1502 Large Intestine Fall 2024 Lecture 17 Tuesday Oct 1: 9am Dr. L. Al-Nakkash [email protected] 1  Learning Objectives 1. Identify the structure and role of the LI. 2. Examine what motility patterns exist in the LI. 3. Describe the gastrocolic reflex. 4. Describe how the defecation reflex is mediated. 5. Pathophysiology for the LI: Hirschprung’s disease. 6. Identify causes and symptoms of constipation. 7. Examine LI secretion. 8. Identify where intestinal gas comes from. 2 © L. Al-Nakkash 2024  Large Intestine Consists of: Transverse colon – colon, cecum, appendix & rectum. Cecum – blind ended pouch below junction of SI & LI at the Haustra Taeniae coli ileocecal valve. Descending Appendix – fingerlike projection colon Ascending at bottom of the cecum. colon Lymphoid tissue. Ileocecal Colon – comprises most of LI - valve non coiled –3 straight portions; Appendix ascending, transverse & descending colon. Cecum Rectum Terminal part becomes S Sigmoid shaped = sigmoid colon. colon Internal anal sphincter External anal (smooth muscle) sphincter It straightens out forming the (skeletal muscle) rectum. © L. Al-Nakkash 2024 1. Identify the structure and role of the LI. 3  Large Intestine Colon - receives ~ 500 ml chyme/day from SI. What is the major role of the LI? Drying of contents & storage Contents delivered to LI: – Indigestible food residues (i.e., cellulose), – unabsorbed biliary compounds, some fluid. Luminal surface of LI: – → smooth, ↓ absorptive capacity. Colon absorbs: – H20 & salt Na+ actively absorbed, Cl- follows passively. H20 follows osmotically Forming firm fecal matter: 500ml enters colon/day and 350ml absorbed → ______ 150 ml eliminated/day. 4 © L. Al-Nakkash 2024 1. Identify the structure and role of the LI.  Structure-histology – No villi on the mucosa: Only absorbs water – Mucosa is thicker: crypts are deeper with more goblet cells (see image). function? __________→provides Secrete mucus lubrication & protection from colon contents. 5 © L. Al-Nakkash 2024 1. Identify the structure and role of the LI.  Haustral Contractions Outer longitudinal SM Transverse colon does NOT completely surround LI. It consists of 3 separate Taeniae coli Haustra bands of muscle. Descending These are known as? taeniae coli colon Ascending colon Taeniae coli are shorter Ileocecal than underlying circular valve Appendix SM layer → this causes it to Cecum Sigmoid bunch up and form Rectum colon pouch like sacs. External anal Internal anal These are known as? Haustra. sphincter sphincter (smooth muscle) (skeletal muscle) 6 © L. Al-Nakkash 2024 2. Examine what motility patterns exist in the LI.  Haustral Contractions Movement of LI: Like segmentation contractions – slow and not propulsive – fits the absorptive/storage functions Major motility method = haustral contractions – Initiated by autonomous rhythmicity controlled locally by the intrinsic plexuses. – They are similar to SI segmentation BUT much less frequent. SI segmentation contractions =