Lower GIT: Large intestine
Lec.10 21 Apr. 2016Objectives
Physiological anatomy
innervation
Functions
Secretion
Motility:Physiology of Defecation
Physiological Anatomy
Its length is about 110 cm. Large intestine is the terminal or end portion of GIT ,composed of the same four bases of tract .Its divided into :cecum , ascending colon, transverse colon, descending colon, sigmoid colon , rectum, , and anus.1. Cecum: first part of large intestine, small and rounded in structure, which lies below ileocecal valve.
2. Appendix: is a blind, wormlike structure attached to the posteromedial surface of the cecum. Appendix contains masses of lymphoid tissue. The appendix has an important structural shortcoming –its twisted provides an ideal location for enteric bacteria to accumulate and multiply. Acute inflammation of appendix called appendicitis.
3. Colon: a portion of large intestine that extended from the cecum to the sigmoid colon, its further divided into ascending colon, transverse colon, descending colon and S-shaped sigmoid colon.
Sigmoid colon: a curved portion of large intestine that terminates to the rectum.
Colon is divided into two main parts: proximal colon and distal
colon. Diameter is greater than that of the small intestine
4. Rectum: a portion of large intestine that has three valves and
Serves to store feces until elimination.
5. Anal canal: terminal portion that has internal (composed of smooth muscles i.e. it's under involuntary control) and external anal sphincters (composed of skeletal muscles i.e. it's under voluntary control) that control defecation.
The fibers of the external muscular larger of large intestine, are collected into three longitudinal bands, the teniae coli. The wall of the colon forms outpunching (haustra) between the teniae. Colonic glands are short inward projections of the mucosa that secret mucus.
No digestive enzymes so no digestion of nutrients in large intestine also no villi on mucosa so no absorption of nutrients.
Innervations: vagus (parasympathetic cranial outflow) innervate proximal half of colon. Pelvic nerve (parasympathetic sacral outflow) innervate distal half of colon. Parasympathetic stimulation increases motility and secretion while sympathetic stimulation decreases motility and secretion.
Functions
1. Absorption: Proximal half of colon is concerned with the of absorption of water, electrolytes, minerals, and some vitamins such vitamins such as K, B-complex, B12, and folic acid (that are synthesized by intestinal flora).Bacterial flora of large intestine is made up of hundreds types of bacteria that either enter from the anus or small intestine or colonize. Bacteria serve the purpose of breaking down proteins and indigestible starches resulting in formation of gases that approximate around 0.5 liter of flatus per day.
2. Storage: the distal half of colon concerned principally with storage, and elimination of wastes in form of feces through defecation (process of elimination of solid wastes from bowel).
Secretion
About 1000 ml – 2000 ml of small intestinal secretion is entering colon from ileum daily, about 800 – 1750 ml is absorbed and 200 – 250 ml.excreted in feces.
Mucus secretion. The great secretion in the large intestine is mucus which is secreted by mucous cells. This mucus contains moderate amounts of bicarbonate ions. The rate of secretion of mucus is increased by principally by stimulation of the pelvic nerves from the spinal cord .Functions of mucus: 1. protects the intestinal wall against excoriation, and the great amount of bacterial activity that takes place inside the feces.
2. It provides an adherent medium for holding fecal matter together.
3. Protects the intestinal wall from 4. The mucus plus the alkalinity of the secretion provides a barrier to keep acids formed in the feces from attacking the intestinal wall.
Motility
1. Mixing contractions. 2. Propulsive contractionsa. Mixing movement – haustrations: Are large circular constrictions of circular and longitudinal smooth muscle of colon, as a result colon bulge outward into baglike sacs called haustrations. Mix the contents of colon and by exposing more of the contents to the mucosa and help absorption in the proximal portion of colon. They are very slow contractions and perform two functions: a. mixing mainly and , b. Propelling chyme from cecum to ascending colon.
Propulsive movements: Mass movements: Propulsive occurs by:
1. Haustral contractions – are weak and propel chyme from cecum and ascending colon to transverse colon.
2. Mass contraction: these contractions move fecal materials from transverse colon to descending colon, sigmoid and finally to rectum and distension rectal initiate the defection reflex. These mass movements occur only a few minutes each day, most abundantly for about 15 minutes during the first hour after eating breakfast. Mass Movements is stimulated by 1.gastrocolic and duodenocolic. These reflexes result from a distension of the stomach and duodenum. They are transmitted through the myenteric plexus. 2. Irritation in colon e.g. ulcerative colitis.
3. Intense stimulation of the parasympathetic nerves system or by over- distension of a segment of colon.
The movements of colon are coordinate by the BER, 9 contractions
/ min. in cecum and 16 Contractions / min .in sigmoid.
Physiological of defecation
Defecation is the process of elimination of solid wastes from bowel.
Distension of the rectum with feces initiates reflex contraction of its musculature and the desire to defecate. In humans, internal anal sphincter is under involuntary control. Its innervates by sympathetic which is excitatory (↑ tone) whereas the parasympathetic supply is inhibitory (i.e. ↓ tone) this sphincter reflexes when the rectum is distended. External anal sphincter is under voluntary control, the nerve supply to external anal sphincter comes from somatic motor pudendal nerve. This sphincter is maintained in the state of tonic contraction, and moderate distension of the rectum increases the force of its contraction.
Defecation reflexes: Ordinarily, defecation is initiated by defecation reflexes. One of these reflexes is1. Involuntary or natural defecation reflex which is subdivided into:
a. intrinsic defecation reflex of colon itself. Distension of rectum initiates afferent signal that spread through myenteric plexus to initiate peristaltic waves in the descending colon, sigmoid colon and rectum forcing the feces towards the intrinsic anal sphincter which is relaxed by inhibitory nerves and if the external anal sphincter is voluntary relaxed at the same time, defecation will occur. The intrinsic defecation reflex itself is weak and to be effective in causing defecation, it must be fortified by: parasympathetic defecation reflex: that involves the sacral segments of the spinal cord. Distension of rectum, impulses are first carried to the spinal cord by afferent fibers and then reflexly back to the descending colon, sigmoid colon and rectum and internal anal sphincter by parasympathetic fibers in the pelvic nerves. These parasympathetic stimuli greatly intensify the peristaltic waves and relaxing internal anal sphincter and thus convert the weak intrinsic defecation into powerful defecation that is effective in emptying the colon in one movement all the way from the descending colon to anus.
2. Voluntary defecation reflex: From higher centers, impulses are carried to spinal cord nerve initiate other effects such as taking a deep breath, closure of the glottis and contraction of abdominal muscles, and voluntary relaxation of the external anal sphincter, thus aiding the reflex emptying of the distended rectum.
When it becomes convenient for the person to defecate, the defecation reflexes can purposely be activated voluntary by taking a deep breath to move the diaphragm downward and then contracting the abdominal muscles to increase the pressure in the abdomen, thus forcing fecal contents into the rectum to cause new reflexes. Reflexes initiated in this way are almost never as effective as those that arise naturally, for which reason people who too often inhibit their natural reflexes are likely to become severely constipated.
Defecation is a spinal reflex that can be voluntarily inhibited by keeping the external sphincter contracted or facilitated by relaxing the sphincter and contracting the abdominal muscles.
Distension of the stomach by food initiates contractions of the rectum and a desire to defecate. This response is called the gastrocolic reflex, because of the response, defecation after meal is the rule in children. In the newborn baby and in some persons with transected spinal cords, the defecation reflexes cause automatic emptying of colon without the normal controls through contraction of the external anal sphincter because of lack of conscious control exercised through voluntary contraction or relaxation of the external anal sphincter.