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contractile tissue which, unlike striated muscle, consists of cells (and not symplasm) and is without transverse striation.
In invertebrates (except all arthropods and certain representatives of other groups) the smooth muscles form the entire body musculature; in vertebrates they are part of the sheaths of internal organs—the intestine, blood vessels, respiratory passages, excretory and sex organs, and many glands. Smooth muscle cells in invertebrates are varied in shape and structure. In vertebrates, they are in most cases fusiform and much elongated, with a rodlike nucleus 50-250 microns in length (up to 500 microns in the uterus of a pregnant animal). They are surrounded by fibers of connective tissue, which form a dense casing. The contractile material (protofibril) is usually distributed through the cytoplasm in isolated fashion; only in a few animals is it gathered into fascicles, or myofibrils. All three types of contractile protein—actin, myosin, and tropomyosin—are found in smooth muscle. Protofibrils with an approximate diameter of 100 angstroms are the predominant type. There are fewer cell organoids (mitochondria, Golgi complex, and elements of the endoplasmic reticulum) in smooth muscle than in striated muscle. They are distributed predominantly at the poles of the nucleus in the cytoplasm, which is devoid of contractile elements. The cell membrane often develops pockets in the form of pinocytotic vacuoles, which indicates resorption and absorption of substances by the cell surface. Soviet scientists, including A. A. Zavarzin and N. G. Khlopin, have established that the smooth muscles are a group of tissues of various origin, united by a single functional characteristic— the ability to contract. Thus, in invertebrates the smooth muscles develop from mesodermal layers and coelomic epithelium; in vertebrates the smooth muscles of the salivary, sweat, and mammary glands originate from ectoderm, and the smooth muscles of the internal organs are descended from mesenchyme. Neighboring cells of smooth muscles are in contact with one another by means of projections such that the membranes of two cells touch one another; in the muscles of the mouse intestine the zones of contact occupy 5 percent of the surface of the cell membrane. It is probably here (the synapse) that the transmission of excitation from one cell to another occurs.
In contrast to the striated muscles, smooth muscles are characterized by slow contraction and the ability to sustain prolonged contraction, expending comparatively little energy and without experiencing fatigue. The motor innervation of smooth muscle is accomplished by projections of the cells of the autonomic nervous system. Sensory innervation is supplied by cell processes of the dorsal root ganglia. There is not a specialized nerve ending for every smooth muscle cell.
REFERENCESZavarzin, A. A.Izbr. trudy, vols. 1-4. Moscow-Leningrad, 1950-53.
Policard, A., and C. A. Baud. Submikroskopicheskie struktury kletok i tkanei v norme i paloiogii. Leningrad, 1962. (Translated from French.)
Elektronno-mikro skopie he skaia anatomiia. Moscow, 1967. (Translated from English.)
E. S. KIRPICHNIKOVA