The large intestine is a reservoir for remnants of undigested and unabsorbed meal. Epithelial cells of the colon absorb sodium ions, water, and electrolytes from chyme that enters it converting the chyme to semi-solid feces.
Therefore, the principal function of the large intestine is twofold:
First, the colon absorbs water and minerals from semi-liquid chyme to form a semi-solid slush; second, the colon stores the fecal matter until the volume is enough to trigger a defecation reflex.
Motility in the large intestine is extremely slow so that materials entering the large intestine from the small intestine appear in stool in about 18 to 24 hours.
The ileocecal juncture
The ileocecal valve links the ileum to the colon and prevents reflux/backflow of colon contents into the ileum. Most importantly, this valve prevents the large numbers of bacteria that colonize the large intestine from flowing backwards into the relatively sterile ileum. The ileocecal valve is essentially a projection of the end of the ileum into the cecum. The leaflets of the valve arrange in such a way that increased pressure in the colon closes the valve, whereas, increased pressure in the cecum opens the valve briefly. As the valve opens for a brief moment,
The ileocecal valve usually closes; however, each peristaltic wave of contraction that reaches the valve from the ileum creates a pressure that opens the valve only for a brief period allowing a portion of chyme to squeeze through the valve into the colon.
Patterns of movements in the colon
- Segmentation contraction
- Mass action contractions also called mass movement
Segmentation contractions occur in the colon and mix chyme. Forward movement of chyme during the mixing contractions is rather minimal. The segmentations in the colon occur due to the contraction of circular muscles in the large intestine to produce segmentation waves/contractions. The rhythm of these contractions is much slower when compared to that of the small intestine.
Mass movements are the propulsive movements of the colon
Mass movements are waves of intense contractions that generally occur in the large intestine following a meal. Waves of mass contraction spread rapidly along the distal portion of ascending colon over the transverse colon towards the rectum.
A mass movement is arguably a specialized form of peristalsis. The stimulus for the initiation of mass movement is distension or irritation of the colon. The colon distension elicits a ring of contraction at the point of distension. Suddenly, about 18 cm of colon contract as a single unit propelling fecal matter in the affected area of the colon down the colon.
In contrast to peristaltic waves, in which smooth muscles at one point relaxes after the wave of contraction passes, smooth muscles of colon remains in a state of contraction for about 1 to 2 minute and relaxation takes place 2 to 3 minutes after the contraction begun.
Both parasympathetic and sympathetic nerve fibers innervate the large intestine. Parasympathetic stimulation increases contractions and movements of colon whereas sympathetic stimulation decreases contractions and movements.
A mass movement gradually forces a mass of semi-solid feces into the rectum, and triggers the defecation reflex.
Defecation, a spinal reflex
Mass movement off fecal matter into the rectum produces sudden distension of the wall of the rectum. This sudden distension of the rectum initiates the neural-mediated reflex called defecation reflex. Mechanoreceptors (i.e. stretch receptors) along the walls of the rectum stretch and relay the impulses to the spinal cord for integration.
Efferent signals of the reflex causes the following effect
- Contraction of rectum
- Relaxes the internal anal sphincter
- Contracts the external anal sphincter but just for a short while
- Increase peristaltic activity in the sigmoid colon.
As fecal materials collect in the rectum, rectal pressure rises gradually; the conscious urge to defecate first appears when pressure in the rectum rises to about 20mmHg. At this point, we still can voluntarily contract the external anal sphincter to delay defecation until a time when it socially convenient to do so.
However, at rectal pressure of 55mmHg, the external anal sphincter relaxes by reflex and reflex expulsion of rectal contents occurs. Moreover, if conditions are socially convenient, we can strain a bit to initiate defecation. Straining causes contraction of the abdominal muscles contract and forces down rectal contents, which easily passes through the external anal sphincter, which relaxes at this point.
Autonomic nerves supply the internal anal sphincter with parasympathetic nerve inhibiting/relaxing the sphincter whereas the sympathetic nerve is excitatory, and contracts the sphincter.
However, we can voluntarily inhibit defecation by maintaining the external anal sphincter contracted, or voluntarily initiate it by contracting the abdominal muscles and at the same time relaxing the external anal sphincter.
In children, defecation after meals is rather common due to the gastro-colic reflex. In the gastro-colic reflex, stretch of stomach wall by food initiates contractions of the distal colon and rectum and usually triggers a desire to defecate. Thus, the gastro-colic reflex causes the contraction of the colon immediately following a meal.
The external anal sphincter is a skeletal muscle supplied by fibers from the pudendal nerve. This sphincter stays in a state of tonic contraction. Nevertheless, brain centers can send signals through descending pathways to the pudendal nerve and the external anal sphincter to override reflex signals, which would otherwise relax the external sphincter, thereby closing the sphincter and allowing humans the ability to delay defecation.
Usually, the sustained distension of the rectum initiates a reverse peristalsis, which forces the rectal contents back into the sigmoid colon. The desire to defecate gradually disappears only to return stronger when the next mass movement propels more fecal matter into the rectum increasing its volume and pressure, which act to initiate yet another defecation reflex.