Movements of large intestine & defecation reflex Dr. Wasif Haq

Anatomy of Large Intestine

Functions of Large Intestine Absorption of water & electrolytes from Chyme to form solid feces (Proximal half of colon) Storage of fecal matter till a convenient time (Distal half of colon) Harbors bacteria that synthesize Vitamin K

Movements of Large Intestine Same as that of small intestine Sluggish in nature Two types Mixing movements/ Haustrations Propulsive/Mass movements

Haustrations Haustration=Sac like. After contraction of large intestine, portions of large intestine bulge outward in baglike sacs called Haustrations. Peak intensity in 30 seconds, fades in 1 minute. Slow, forward propulsion towards anus. New contractions dig & roll over fecal matter to enhance absorption of useful substances.

Series of pouches along the length of colon. Haustrations Series of pouches along the length of colon.

Mass movements Modified type of Peristaltic movement. Transverse colon-Sigmoid colon. Slow, persistent haustral contraction propelling Chyme through Transverse colon. Chyme becomes fecal in nature e.g. Semi solid form from semi liquid form.

Sequence Peristaltic constrictive ring develops over distended portion of Colon, causing the distal colon to constricted ring contract as a unit pushing fecal matter downwards. When feces reaches the rectum, the desire for defecation is felt.

Defecation The act of eliminating feces from digestive tract. Average feces quantity 80-200 ml/day Rectum remains empty most of the time; sphincter between the rectum and sigmoid colon & sharp curvature at this junction offers resistance to filling of rectum.

Defecation Reflex Involves voluntary & involuntary control. Two anal sphincters; internal and external. Intrinsic defecation reflex; a weaker reflex, needs fortification by Parasympathetic defecation reflex. Intrinsic defecation reflex: Upon entry in rectum, rectal wall distention causes peristaltic movements in descending colon, sigmoid colon & rectum pushing fecal analwards.

Parasympathetic Augmentation Nerve fibers in rectum send signals to spinal cord through Parasympathetic fibers through “Sacral nerves” of spinal cord. Descending colon, sigmoid colon, rectum & anus receive amplification signals from spinal cord causing powerful peristaltic movements with the simultaneous relaxation of internal anal sphincter.

Voluntary Component External anal sphincter: Striated, voluntary muscle. Remains subconsciously constricted all the time until decision to defecate. “Pudendal nerve” supplies fibers to this sphincter. Upon suitable time, defecation reflex can be generated by taking deep breath to move diaphragm downward, contracting the abdominal muscles to increase pressure in abdomen to cause movement of feces in rectum initiating defecation.

Why Children? Continous dribbling of feces prevented by tonic constriction of anal sphincters (internal & external) Why do children defecate without any control?

Thank You http://coolbluez.t15.org

Do not create disturbance and unpleasantness for others. Take Home Message Do not create disturbance and unpleasantness for others.