Smooth muscle, also known as involuntary muscle, is a type of muscle that has no cross stripes when examined under a microscope. It is made up of slender spindle-shaped cells with a single nucleus in the center.

Unlike striated muscle, smooth muscular tissue contracts gently and automatically. Much of the musculature of internal organs and the digestive system is made up of it.

Smooth muscle cells measure 3-10 m in thickness and 20-200 m in length. The cytoplasm is predominantly made up of myofilaments and is uniformly eosinophilic. During contraction, the nucleus, which is positioned in the core, takes on a cigar-like shape.

Caveolae are small pouch-like invaginations in the cytoplasm formed by the cell membrane, which act similarly to T-tubules in the skeletal muscle. A basal lamina anchors smooth muscle cells to the surrounding connective tissue.

Smooth muscle fibers are grouped together in branching bundles. These bundles, unlike skeletal muscle fibers, are not precisely parallel and organized, but rather form a complicated structure. As a result, the cells can contract significantly more forcefully than striated muscle.