How can smooth muscle contract if it has no striation or sarcomeres?

Answer 1

Actin and myosin are present in all three muscle types. In skeletal and cardiac

muscle cells, these proteins are organized in sarcomeres, with thin and thick

filaments. The internal organization of a smooth muscle cell is very different:

• A smooth muscle fibre has no T tubules, and the sarcoplasmic reticulum

(SR) forms a loose network throughout the sarcoplasm. Smooth muscle

tissue has no myofibrils or sarcomeres. As a result, this tissue also has no

striations and is called nonstriated muscle.

• Thick filaments are scattered throughout the sarcoplasm of a smooth

muscle cell. The myosin proteins are organized differently than in skeletal

or cardiac muscle cells, and smooth muscle cells have more cross-bridges

per thick filament.

• The thin filaments in a smooth muscle cell are attached to dense bodies,

structures distributed throughout the sarcoplasm in a network of

intermediate filaments composed of the protein desmin. Some of the dense

bodies are firmly attached to the sarcolemma. The dense bodies and

intermediate filaments anchor the thin filaments such that, when sliding

occurs between thin and thick filaments, the cell shortens. Dense bodies

are not arranged in straight lines, so when a contraction occurs, the muscle

cell twists like a corkscrew.

• Adjacent smooth muscle cells are bound together at dense bodies,

transmitting the contractile forces from cell to cell throughout the tissue.

• Although smooth muscle cells are surrounded by connective tissue, the

collagen fibres never unite to form tendons or aponeuroses as they do in

skeletal muscles.