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Introduction to smooth muscle

© Narayan Dattatray Wadadekar

Basic types of animal movements are amoeboid, ciliary / flagellar and muscular. Different animals show a given single type of movement or more than one type of movement. All three types of movements can also be seen in a single animal.

Human body has the white blood corpuscles which move in an amoeboid fashion, cells lining our trachea show ciliary movement whereas, human sperms exhibit flagellar movement. Muscle movement enables us to move our limbs, lips etc. We can go from place to place because with our muscles.

After getting to know about amoeboid and ciliary / flagellar type in earlier articles, let us now have a look at the muscular movements.

Muscles are of three types.

The bulky muscles that the body builders so proudly exhibit, are the skeletal muscles.

The muscle in the middle layer of heart is the type, called cardiac muscle. The third type is the smooth muscle, also called - the visceral, non-striated or involuntary muscle.

Of the three types smooth muscle is more primitive and hence, let us consider it first.

In our bodies the smooth muscle is found in many hollow bodyparts, such as, the ureters, hepatic and pancreatic duct, uterus, arteries, veins, esophagus (food pipe), trachea (wind pipe), bronchi and the bronchioles - which are the branches of trachea; stomach and intestines (small and large), fallopian tubes, epididymis, vas deferens of the male reproductive system and iris of the eye. It is also associated with the hair follicles.

Certain cells called myoepithelial cells are found in breasts, salivary glands and some sweat glands. They have like smooth muscle, contractile proteins but other wise look and act like epithelial lining cells.

Hydra, a coelenterate, also shows such a variety of cells in their two layered body wall. These cells have contractile proteins as in muscles. Such cells in hydra are called epitheliomuscular and nutritio - epitheliomuscular cells.

The epitheliomuscular cells of the ectoderm perform the function of giving mechanical protection and can contract on account of myofibrils in their body.

The nutitio - epitheliomuscular cells of endoderm give mechanical protection, can contract on account of myofibrils in their body and can also carry out digestive functions.

A hydra can bend and stretch, swim and somersalt, coil around a stem and climb up an aquatic plant. It can walk on its tentacles. All this becomes possible due to the epitheliomuscular and nutritio - epitheliomuscular cells. These cells in bodywall of hydra may be regarded as progenitors of smooth muscle cells in higher animals.