Large Intestine Motility Flashcards
Functions of large intestine
- To store fecal material, which consists of indigestible food products such as cellulose.
- To extract water from the luminal contents.
- To move fecal material towards the rectum.
Secretions and absorptions of large intestines
There are no digestive enzymes secreted by the colon and nutrient absorption does not take place in this organ.
The colonic epithelium secretes mucus and absorbs ions and water.
Innervation of colon
As in the small intestine the intrinsic nerves in the colon exert a net inhibitory influence. The “right side” of the colon (proximal colon; ascending colon) receives parasympathetic input from the vagus as well as from the pelvic nerves. Vagal stimulation produces segmental contractions in the proximal colon. Pelvic nerve stimulation induces tonic propulsive contractions in the distal colon. Splanchnic nerves provide sympathetic innervation to the proximal colon while the lumbar colonic nerves innervate the entire colon. Parasympathetic stimulation generally leads to contraction while sympathetic stimulation
causes relaxation.
Motility types of colon
Mixing movements are the most common motility pattern and occur primarily in the proximal colon. During this activity there is no net movement but the luminal contents are shuttled back and forth between the haustra. This contractile behavior occurs in association with short bursts of spiking activity on slow waves.
Haustral migration results in the net movement of chyme in an aboral direction. In this case slow waves are associated with long bursts of spike activity.
Mass movement or mass peristalsis is the least frequent motility pattern. This moves the luminal contents over long distances in an aboral direction. When this occurs slow waves are associated with long bursts of spike potentials originating in the proximal colon and migrating aborally.
Gastrocolic reflex
distension of the stomach by food increases the motility of the colon and increases the frequency of mass movements in the colon.
The internal anal sphincter (IAS)
is composed of circular smooth muscle which is tonically contracted. Activation of the parasympathetic nerves relaxes the IAS via the release of VIP while activation of sympathetic nerves causes it to contract.
The external anal sphincter (EAS)
is composed of skeletal muscle that surrounds the internal sphincter extending distally. It receives excitatory innervation from the pudendal nerve and is under voluntary
control. This is one of only a few regions in the body where skeletal muscle forms a functional cylinder.
Defecation
involves the distal colon, rectum, anal sphincters and the skeletal muscle of the pelvic floor, abdominal wall and diaphragm.
It is under both voluntary and involuntary control.
Defecation is initiated by mild distension which activates the rectosphincteric reflex which relaxes the internal anal sphincter and elicits the urge to defecate. Mild distension of the rectum also induces a reflexive contraction of the external anal sphincter. The rectum then relaxes to accommodate the increased volume, the IAS regains its tone and the external anal sphincter relaxes - this is continence.
The urge to defecate is then suppressed until the next entry of feces into the rectum.
In response to mild distension, the urge to defecate can be continued by voluntarily relaxing the external anal sphincter. Defecation is facilitated by voluntary contraction of the abdominal muscles (Valsalva maneuver).
In response to increased distension of the rectum defecation can be prevented by voluntarily contracting the external anal sphincter.
CONSTIPATION
is the term used to describe the slow transit of chyme through the colon. This results in excess water removal. This condition may be exacerbated by a life-long voluntary
suppression of the urge to defecate, beginning with toilet training in infancy.
MEGACOLON or HIRSCHPRUNG’S DISEASE
is the result of an absence of intrinsic innervation of the colon (aganglionic). Usually this occurs over a short distance but occasionally the whole colon may be involved. As there are no intrinsic inhibitory neurons the affected segment is tonically (continuously) contracted.
DIVERTICULAR DISEASE
is a term used to describe those diseases associated with herniation of the mucosa and submucosa through the muscularis propria. It is the most common disorder of the large intestine (frequently in the sigmoid colon) and usually occurs in the elderly.
Diverticulosis is associated with a thickening of the rings of circular smooth muscle (hypertrophy) which increases luminal pressure and causes the diverticula.
Diverticulitis is an inflammatory condition that is a consequence of the perforation of the mucosa in a diverticulum.
IRRITABLE BOWEL SYNDROME (IBS)
involves altered bowel function and abdominal pain in the absence of any detectable structural abnormality. This may be related to alterations in the threshold of afferent fibers resulting in visceral hypersensitivity, or there may be some abnormality in the normal brain-gut interactions. In the small intestine a corresponding phenomenon known as NON-ULCER DYSPEPSIA is seen as epigastric pain, bloating nausea and vomiting. As with IBS there is no apparent pathophysiological basis for these events.
INFLAMMATORY BOWEL DISEASE (IBD)
includes ulcerative colitis and Crohn’s disease. These are diseases of the mucosa (ulceration leading to loss of absorptive surface area) but patients often present with symptoms consistent with altered motility e.g. abdominal pain, diarrhea or constipation. Crohn’s disease is distinguished from ulcerative colitis by being associated with transmural inflammation (as opposed to only mucosal inflammation) and the more frequent presence of anal lesions.
PSEUDOMEMBRANOUS COLITIS (antibiotic-associated colitis or C. difficile colitis)
can occur when there is a disruption in the balance of ‘good’ versus ‘bad’ bacteria. Prolonged antibiotic treatment
kill off bacteria allowing proliferation of certain bacteria such as Clostridium difficile. Production of toxins leads to inflammation and diarrhea.
microflora of the large intestine
contains both aerobic and anaerobic bacteria. A major factor in the number of bacteria in the colon is its slow motility.
