In this article we will discuss about the small and large intestine of digestive system.
Small Intestine of Digestive System:
Motility of Small Intestine:
The movements of small intestine are brought about by the smooth muscle present in its wall.
a. Wall of small intestine has an outer longitudinal and an inner circular layer.
b. The movements are subjected to neural control through the plexus of nerves, namely myenteric and Meissner’s plexuses, through the influence of extrinsic autonomic nerves. Movements are subjected to hormonal control and to local control as well.
Aims of Small Intestinal Motility:
1. Agitation of food or chyme:
This helps in three ways:
a. Mixing the food with the enzymes of pancreatic and intestinal juice thus facilitating enzyme action.
b. Breaks the food into very small particles which again helps in digestion and absorption.
c. Renewal of the layers in contact with the villi which helps in absorption.
2. Propulsion:
The chyme is moved over the large area of small intestine to facilitate digestion and absorption and the residues are propelled downwards to the ileocecal junction to reach the large intestine, mostly for excretion.
In man, the time taken for the food to travel in the small intestine as well as in stomach can be easily estimated radiologically with the help of barium meal. It takes about 2-12 hours for the food to travel from pylorus to ileocecal junction.
Types of Intestinal Movements:
Rhythmic Segmentation Contractions (Figs 5.27 and 5.28):
Ability of the muscle to contract, stretch when the walls are distended and generation of impulse almost at regular intervals (rhythmicity) are the fundamental properties of visceral smooth muscle all over GIT. Rhythmic segmentation contractions are ring like contractions a few cm long as small segments in small intestine.
These contractions are caused by distension of walls due to chyme. The contractions are so organized that in a given segment, contractions and relaxations alternate and at the next instant, contracted segment relaxes and relaxed segment goes into contraction. This type of movement chops the food and helps in mixing food with intestinal secretions.
These contractions depend on the presence of myenteric plexus. They can occur even when the extrinsic vagus and sympathetic are cut. These contractions occur at a faster rate in the duodenum (12/min) and slow down towards the ileum (8/min). Parasympathetic stimulation increases these contractions and sympathetic stimulation inhibits them.
Peristalsis:
It is defined as a wave of contraction preceded by a wave of relaxation which travels down a hollow organ. In the intestine, the contractions proceed towards aboral region.
Peristaltic wave always travels in the aboral direction is called law of the intestine which is dependent on myenteric reflex (Fig. 5.29).
Stimulus:
Usually distension of intestine caused by food. Sometimes chemical irritation also can act as stimulus.
i. Receptor:
Stretch receptors in the mucous membrane of small intestine.
ii. Afferent:
Short vagal fibers in the intestinal wall itself
iii. Center:
Ganglion cells of myenteric plexus
iv. Efferent:
Short parasympathetic fibers in the intestinal wall itself.
v. Effector:
Circular muscle of the wall.
Response:
Contraction of circular muscle resulting in peristalsis.
In the genesis of peristalsis, the wave of contraction occurs a few centimeters prior to the part of intestine where food (chyme) is present and the receptive relaxation occurs beyond the area of chyme. This sort of movement helps to propel the food, so peristalsis is more of propulsive movement. It can also occur in absence of extrinsic nerve supply.
Sympathetic stimulation inhibits it and parasympathetic stimulation promotes it. Peristaltic movements are more frequent in the upper small intestine (12/min) and less so in the lower intestine (8/min).
This leads to a postulation that there is a pacemaker in the duodenum (similar to the one in the heart), which directs the peristalsis down towards the ileocecal junction. Theoretically, distention by food can result in a peristaltic wave traveling in both directions but the one towards the mouth dies out.
Thus it is believed that it is due to pacemaker function located in the duodenum.
Peristalsis in the reverse direction is called antiperistalsis.
There is an interesting experiment where a small loop of small intestine is cut and anastomosed in the reverse direction. The food does not cross the segment the area of anastomoses and stop. This experiment proves the directional integrity of myenteric plexus is essential for peristalsis and the propulsion of food.
Pendular Movements:
Here, long segments of about 20 cm move forwards and backwards, up and down helping mainly agitation of food. These and peristalsis are superimposed on rhythmic segmentation contractions. These can also occur in absence of extrinsic nerve.
Movements of Villi:
These are due to contractions of muscularis mucosa which extend into each villus.
There are two types of movements:
a. Lashing movement
b. Rhythmic shortening and elongation of the villus.
These movements help in the absorption of nutrients into the blood vessels as well as lacteals. In the case of villi, parasympathetics inhibit the movement, sympathetics promote it. It is believed that there is hormone villikinin (yet to be isolated) secreted from the duodenum and promotes contractions of villi.
Regulation of Intestinal Motility:
1. Neural:
Except in the case of villi, vagal stimulation promotes intestinal movements whereas sympathetic stimulation inhibits them. Yet intestinal movements can go on without the influence of extrinsic nerves.
Certain emotional factors influence intestinal motility, e.g. grief inhibits the movements and causes constipation. Similarly fear causes stimulation of movements and causes diarrhea. These emotional factors act through cerebral cortex and autonomic nervous system.
Gastroileal reflex:
Intake of food into the stomach stimulates intestinal movement and empties the contents from ileum and other parts of large intestine. This may be acting through vagal connections between stomach and small intestine.
Peristaltic rush:
Here, extreme distension or irritation of small intestine by food leads to exaggerated peristalsis in which, in one sweep of a movement the food from duodenum reaches the ileocecal junction.
2. Hormonal:
Acetylcholine, 5-HT cause contractions of small intestine. These are local hormones. Among general hormones, thyroid hormone stimulates intestinal motility. So in hyperthyroidism, there is diarrhea. In hypothyroidism, there is constipation.
3. Local regulation:
In the absence of extrinsic nerves the presence of food alone by way of distension or chemical stimulation can cause intestinal movements. This is more important mechanism of regulation.
When the person is fasting, the GIT is completely empty; the small intestine exhibits peristalsis-like movements called migrating myenteric complexes. This shows that smooth muscle of small intestine is always active.
Large Intestine of Digestive System:
Functional Anatomy:
The mucosa has no villi. It has short glands containing numerous goblet cells. The smooth muscle is made up of two layers: the inner circular similar to what is present in rest of the gastrointestinal tract, whereas the outer longitudinal is arranged in three bands called tenia coli. Since these bands are shorter than the entire length of the colon, the colon is compressed to form puckering or haustrations.
Functions of Large Intestine:
1. Secretion:
The glands secrete a watery fluid containing H2O and HCO3– to which mucin is added from globlet cells. Mucin is meant for lubrication of chyme.
2. Absorption:
About 1-2 liters of chyme enter the colon. This volume is reduced to less than 200 ml by the time is reaches rectum mainly by active reabsorption of Na+, water and Cl– following passively. The ability of colonic mucosa is made use of in administration of drugs especially in children by way of suppositories and also enemas usually for constipation.
3. Synthesis:
Colon has got a variety of bacteria. These bacteria are in a way beneficial because they can synthesize vitamins of B complex group, vitamin K. This fact is taken into consideration when broad- spectrum antibiotics are administered. B-complex vitamins are always given with broad-spectrum antibiotics because the latter also kills the beneficial bacterial flora of intestine.
4. Movements:
The presence of smooth muscle in the colon is responsible for colonic motility.
There are two major types of movements:
a. Mixing movement:
Rhythmic segmentation contractions occur as a result of circular muscle activity while tenia coli cause haustral contractions. These contractions roll the colonic wall back and forth against the contents helping in the absorption of Na+ and water and reducing the bulk of chyme. These are also called kneading movements.
b. Mass peristalsis (movements):
Peristalsis does not occur in large intestine the way it occurs in small intestine. Instead, mass peristalsis is found. It occurs a few times in a day usually in the hour after breakfast lasting for duration of 10 minutes. It can start in any part of the colon, it occurs usually in the transverse or descending colon.
When it occurs, it propels the colonic contents right up to the rectum. Once the rectum is filled up the desire for defecation is felt.
The mass peristalsis is initiated by following factors:
i. Gastrocolic reflex: Food in the stomach through neural pathways (may be vagus) stimulates mass peristalsis in the colon.
ii. Duodenocolic reflex: Distension of duodenum with food can stimulate mass peristalsis.
iii. Irritation of colon due to infection
iv. Over distension of colon, e.g. enema
v. Parasympathetic stimulation
5. Storage and expulsion of feces:
Composition of feces:
i. Weight/day = 75-170 g/day
ii. pH: 7-7.5
iii. Color is due to stercobilinogen
iv. Odor is due to indol and skatol and also by bacterial action in the intestine
Actual composition:
75% water and 25% solids.
Other important constituents are:
1. Fiber
2. Bacteria
3. Fats and fatty acids
4. Desquamated mucous cells
5. Unabsorbed digestive juices containing small quantities of enzymes
6. Mucin
7. Inorganic substances particularly Ca++ and phosphates.