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Smooth muscle structure

© Narayan Dattatray Wadadekar

As we have already seen, smooth muscle is seen in hollow muscular tubes. It originated perhaps in epithelial cells like the epithelio-muscular cells of animals such as hydra. Hydra belongs to the phylum coelenterata. The platyhelminthes e.g. liverfluke like animals have smooth muscle cells and free living flatworms such as planaria, crawl by using such cells.

Round worms are the nematodes and have feeble smooth muscle with which they crawl. Annelids like earthworm have smooth muscles in their gut wall and so also we have. In fact, even the arrangement of muscle layers - the outer longitudinal and inner circular - in the gut, is the same in earthworm like animals and mammals like us.

Smooth muscle cells are long and spindle shaped i.e. broad in the middle and tapering at both ends. Such a shape is also known as fusiform. Each cell has its nucleus in its broadest central part. Cytoplasm of the cells shows mitochondria, rough endoplasmic reticulum, strings of ribosomes (polyribosomes), and Golgi complex. Peripherally located pinocytic vesicles are also seen.

Like in striated muscles thin filaments of actin and tropomyosin, and thick filaments of myosin are seen in the smooth muscles. Smooth muscle cells also show intermediate filaments of desmin or skeletin. In addition to these in smooth muscle cells of blood vessels, another protein called vimentin, has been shown to be present. But in the smooth muscles these contractile filaments are not arranged in a microscopically visible parallel regular pattern. Moreover, the myosin content of smooth muscle is only about one-fourth that of striated muscle, while the actin and tropomyosin content is about twice that of striated muscle. There are bundles of myofilaments running obliquely through out the cytoplasm of smooth muscle cells. This absence of a precise transverse sarcomere alignment accounts for the absence of "striations" (bands) in smooth muscles. The unstriated smooth appearance of cytoplasm has given the name smooth muscle. Smooth muscle cells also do not show any T-tubules. T-tubules are invaginations of cell membrane of muscle cells. They are seen in striated muscles.

Narrowest portion of one cell of smooth muscle is in touch with broadest part of neighboring cells. Long axes of all cells are parallel to each other. Such an arrangement ensures tight packing.

Smooth muscle cells can be as small as 20 micrometers in length to as big as 500 micrometers. Their width varies from 2-5 micrometers. The smallest of the smooth muscle cells are seen in the blood vessel walls and the biggest in the advanced stage of pregnant uterus.

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