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Villi possess independent movements of their own. These movements are due to the contraction of strips of muscle fibres which proceed from muscularis mucosae and get attached to the sides of the villus.
Generally, two kinds of movements are seen:
i. Side-to-Side Movement:
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In this movement the villi bend to one side which may be in any direction. This is caused by the contraction of muscle strip on that side. The function of this movement is to help admixture and also to help absorption by bringing the villi in contact with different portions of food mass. During active digestion these movements are very rapid.
ii. Pumping Movements:
In this movement all the muscular strips around the villus contract simultaneously and the villus shorten in length as a whole. At the height of digestion and absorption this movement takes place at the rate of about 1-6 per minute. The function of this movement is the increase of flow of blood and lymph.
Beside these movements, the villus performs protecting action on the inflammation due to the contraction of muscular is mucosae as a result of contact of sharp foreign body in the intestinal contents.
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Factors Controlling Movements of Villi:
i. Mechanical Irritation:
Mechanical irritation of food acting through the local nerve plexus.
ii. Neural Stimuli:
Stimulation of the splanchnic nerve augments the movement of villi. Stimulation of the sympathetic nerves causes contraction of the muscularis mucosae. The nerves are adrenergic and respond to sympathomimetic drugs. The muscularis mucosae also respond to parasympathomimetic, agents, e.g., acetylcholine with contraction. They are depressed by atropine and nicotine.
iii. Chemical Stimuli:
Some products of food digestion, specially some amino acids, as well as yeast extracts (vitamin B) act as strong stimulants.
iv. Villikinin:
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This is a hormone that can be extracted from the intestinal mucosa. The structure of it is not definitely known but possibly not protein in nature. It. is believed that during active digestion villikinin is absorbed through blood and stimulates the movements of villi.
It will be seen from the above that all these factors operate during digestion of foodstuffs. Hence, during digestion the movements of villi are very brisk. During fasting they are very slow. Movements of villi cease during asphyxia, loss of blood, circulatory failure, etc.
