uwcm/sonms/nutrition/mjohn the large bowel defaecation

UWCM/SONMS/nutrition/MJohn

The Large Bowel

Defaecation


Digestive System

GI tract divided into: Alimentary canal. Accessory digestive

organs.

GI tract is 30 ft long and extends from mouth to anus.


Layers of GI Tract

MucosaSubmucosaMuscularisSerosa


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.

Mucosa


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.

Submucosa


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.

Muscularis


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.

Regulation of the GI Tract


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.


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.

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.

Contractions of Intestinal Smooth Muscles


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.

Smooth Muscle Action Potentials


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.

Absorption within the large Intestine


Defence systems within the large Intestineimmune 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


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-.

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.


Submucosal and myenteric plexuses contain 100 million neurons.

Include preganglionic parasympathetic axons, ganglion cell bodies, postganglionic sympathetic axons, and sensory neurons.

Enteric Nervous System


Enteric Nervous System

Peristalsis: Acetylcholine and

substance P stimulate smooth muscle contraction.

Nitrous Oxide, Vasoactive Intestinal Peptide, and ATP stimulate smooth muscle relaxation.


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.


DefaecationWaste 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 the large bowel defaecation

Documents

smooth muscle cell

lumen of gi tract

muscle layers

inner circular smooth

amplitude of slow waves

slow movement

goblet cells

rhythm of contractions