MOVEMENTS OF SMALL INTESTINE

MOVEMENTS OF SMALL INTESTINE

Movements of small intestine are essential for mixing the chyme with digestive juices, propulsion of food and absorption.

Types of Movements of Small Intestine

Movements of small intestine are of four types:

1. Mixing movements:

i. Segmentation movements

ii. Pendular movements.

2. Propulsive movements:

i. Peristaltic movements

ii. Peristaltic rush.

3. Peristalsis in fasting – migrating motor complex

4. Movements of villi.

1. MIXING MOVEMENTS

Mixing movements of small intestine are responsible for proper mixing of chyme with digestive juices such

as pancreatic juice, bile and intestinal juice. The mixing movements of small intestine are segmentation

contractions and pendular movements.

i. Segmentation Contractions

Segmentation contractions are the common type of movements of small intestine, which occur regularly or irregularly, but in a rhythmic fashion. So, these move ments are also called rhythmic segmentation contractions. The contractions occur at regularly spaced intervals along a section of intestine. The segment of the intestine involved in each contraction is about 1 to 5 cm long. The segments of intestine in between the contracted segments are relaxed. The length of the relaxed segments is same as that of the contracted segments. These alternate segments of contraction and relaxation give appearance of rings, resembling the chain of sausages. After sometime, the contracted segments are relaxed and the relaxed segments are contracted. Therefore, the segmentation contractions chop the chyme many times. This helps in mixing of chime with digestive juices.

ii. Pendular Movement

Pendular movement is the sweeping movement of small intestine, resembling the movements of pendulum ofclock. Small portions of intestine (loops) sweep forward and backward or upward and downward. It is a type of mixing movement, noticed only by close observation. It helps in mixing of chyme with digestive juices.

2. PROPULSIVE MOVEMENTS

Propulsive movements are the movements of small intestine which push the chyme in the aboral direction

through intestine. The propulsive movements are peristaltic movements and peristaltic rush.

i. Peristaltic Movements

Peristalsis is defined as the wave of contraction followed by wave of relaxation of muscle fibers. In GI

tract, it always travels in aboral direction. Stimulation of smooth muscles of intestine initiates the peristalsis. It travels from point of stimulation in both directions. But under normal conditions, the progress of contraction in an oral direction is inhibited quickly and the contractions disappear. Only the contraction that travels in an aboral direction persists.

Starling’s law of intestine

Depending upon the direction of the peristalsis, ‘Law of intestine’ was put forth by Starling.

According to the law of intestine, the response of the intestine for a local stimulus consists of a contraction of smooth muscle above and relaxation below the stimulated area.

Peristaltic contractions start at any part of the intestine and travel towards anal end, at a velocity of

1 to 2 cm/sec. The contractions are always weak and usually disappear after traveling for few centimeter.

Because of this, the average movement of chime through small intestine is very slow and the average

velocity of movement of the chyme is less than 1 cm/ sec. So, the chyme requires several hours to travel from duodenum to the end of small intestine. Peristaltic waves in small intestine increase to a great extent immediately after a meal. This is because of gastroenteric reflex, which is initiated by the distention

of stomach. Impulses for this reflex are transmitted from stomach along the wall of the intestine via myenteric plexus.

ii. Peristaltic Rush

Sometimes, the small intestine shows a powerful peristaltic contraction. It is caused by excessive irritation of intestinal mucosa or extreme distention of the intestine. This type of powerful contraction begins in duodenum and passes through entire length of small intestine and reaches the ileocecal valve within few minutes. This is called peristaltic rush or rush waves. Peristaltic rush sweeps the contents of intestine into

the colon. Thus, it relieves the small intestine off either irritants or excessive distention.

3. PERISTALSIS IN FASTING – MIGRATING MOTOR COMPLEX

Migrating motor complex is a type of peristaltic contraction, which occurs in stomach and small intestine

during the periods of fasting for several hours. It is also called migrating myoelectric complex. It is different from the regular peristalsis because, a large portion of stomach or intestine is involved in the contraction. The contraction extends to about 20 to 30 cm of stomach or intestine. This type of movement occurs once in every 1½ to 2 hours. It starts as a moderately active peristalsis in the body of stomach and runs through the entire length of small intestine. It travels at a velocity of 6 to 12 cm/min. Thus, it takes about 10 minutes to reach the colon after taking origin from the stomach.

Significance of Peristalsis in Fasting

Migrating motor complex sweeps the excess digestive secretions into the colon and prevents the accumulation of the secretions in stomach and intestine. It also sweeps the residual indigested materials into colon.

Post a Comment

0 Comments