Smooth muscle tissue

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Smooth muscle is one of three types muscle tissue occurring in the body of mammals, including the human body. It is found mainly in the muscular layers in walls of organs of the digestive, respiratory, genitourinary systems as well as in blood vessels. Smooth muscle is also present in the skin (arrector pili muscles), and in the iris or ciliary body in eye. It is an involuntary, non-striated type of muscle whose contractions are slow and sustained and It is controlled by the autonomic nervous system.

Smooth muscle development[edit | edit source]

Smooth muscle develops mainly from the splanchnopleuric mesodern surrounding the alimentary canal and its derivatives. It gives rise to the tunica muscularis of the alimentary canal, trachea and bronchi. The smooth muscle of the vascular wall originates from local mesenchymal cells which serve as a potential source of smooth muscle throughout the body.

Structure of smooth muscle cells[edit | edit source]

Smooth muscle cells; stained with hematoxylin-eosin.

Mechanism of contraction

smooth muscle

Smooth muscle is composed of individual spindle-shaped (fusiform) cells, each cell contains a signal elongated nucleus located centrally in the wider part of the cell. The size of smooth muscle cells varies from about 20 μm to 500 μm in the uterine wall during pregnancy. Each cell is surrounded by basal lamina and a network of reticular fibers which help distribute the contractile forces evenly and ensure a coordinated contraction.

The plasma membrane (sarcolemma) contains specialized areas known as dense bodies, located on its inner surface. These dense bodies serve as attachment sites for thin and intermediate filaments, and are composed of the protein α-actinin, so they are similar to the Z-discs of striated muscle. The sarcolemma forms numerous pinocytic invaginations (caveolae), and adjacent cells are connecter by gap junctions (nexuses), allowing ionic and electrical coupling. Intermediate filaments, primarly desmin and vimentin, contribute to the cytoskeletal framework.

Smooth muscle cells contains numerous mitochondria (their energy supply is provided mainly by glycolysis). The Golgi apparatus is located near the nucleus, the granular endoplasmic reticulum is abundantly represented, and many free ribosomes are also present. Sarcoplasmic reticulum is reduced, T-tubules are absent. We can also find glycogen inclusions. The cell produces collagen type III, proteoglycans or elastin (similar to a fibroblast).

Contraction of smooth muscle[edit | edit source]

The contraction of smooth muscle, like in skeletal muscle, is based on the interaction of actin and myosin, but the mechanism and organization of the contractile apparatus is different. The myofilaments are arranged in a lattic-like network, rather than in the regular transverse striations characteristic of skeletal and cardiac muscle. The thin filaments are composed of actin and tropomyosin, (unlike in striated muscles troponin is missing). The thick filaments consist of myosin.

Contraction is initiated by an influx of Ca²⁺ ions, which bind tocalmodulin to form a Ca²⁺ -Calmodullin complex. This complex subsequently activates myosin light chain kinase (MLCK), which phosphorylates the regulatory light chains of myosin, enabling the formation of the actin-myosin cross bridges and the generation of tension.

The myosin filaments in smooth muscle differ from those in striated muscle: their central region contains myosin heads, while the ends are bare, allowing sliding in opposite directions and contributing to the smooth, sustained nature of contraction.