Structure and Organization of Smooth Muscles

In the human body, smooth muscle tissue is found lining the visceral and tubular organs such as the liver, lungs, blood vessels, and respiratory tract.

They are organized into two types— clusters of cells that are controlled as a single unit, called visceral muscle, and individually controlled cells that form the multi-unit muscle.

Both these tissue types contain spindle-shaped cells with tapering ends and a single nucleus in the center.

The cell length varies between 30 and 200 µm with a diameter of about 5 to 10 µm.

Smooth muscles lack organized sarcomeres and as a result, have a non–striated appearance.

This is because the thick and thin filaments in these cells are irregularly organized along the long axis.

The thin filaments are anchored to the sarcolemma by dense bodies, which are interconnected via the intermediate filament network.

Additionally, instead of T-tubules, these muscle fibers have caveolae — small pockets of invaginated sarcolemma that help the influx of Ca²⁺ ions during contraction cycles.