Smooth muscle

Smooth muscle
Smooth muscle shown in the tunica media in the walls of arteries and veins
Latinmuscularis levis; muscularis nonstriatus
Anatomical terminology
smooth muscle tissue
Smooth muscle tissue, highlighting the inner circular layer (nuclei then rest of cells in pink), outer longitudinal layer (nuclei then rest of cells), then the serous membrane facing the lumen of the peritoneal cavity

Smooth 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 alter 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]

  1. ^ a b Betts, J. Gordon; Young, Kelly A.; Wise, James A.; Johnson, Eddie; Poe, Brandon; Kruse, Dean H.; Korol, Oksana; Johnson, Jody E.; Womble, Mark; Desaix, Peter (6 March 2013). "Smooth muscle". Retrieved 10 June 2021.
  2. ^ "Thesaurus results for Striated". Retrieved 22 April 2022.

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