Smooth muscle

Smooth muscle
Smooth muscle shown in the tunica media in the walls of arteries and veins
Details
Identifiers
Latin	muscularis levis; muscularis nonstriatus
MeSH	D009130
TH	H2.00.05.1.00001
FMA	14070
Anatomical terminology [edit on Wikidata]

Smooth (soft) muscle is an involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations (bands or stripes).^[1][2] It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit muscle, the whole bundle or sheet of smooth muscle cells contracts as a syncytium.

Smooth muscle is found in the walls of hollow organs, including the stomach, intestines, bladder and uterus. In the walls of blood vessels, and lymph vessels, (excluding blood and lymph capillaries) it is known as vascular smooth muscle. There is smooth muscle in the tracts of the respiratory, urinary, and reproductive systems. In the eyes, the ciliary muscles, iris dilator muscle, and iris sphincter muscle are types of smooth muscles. The iris dilator and sphincter muscles are contained in the iris and contract in order to dilate or constrict the pupils. The ciliary muscles change the shape of the lens to focus on objects in accommodation. In the skin, smooth muscle cells such as those of the arrector pili cause hair to stand erect in response to cold temperature or fear.^[1]