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Physical
Smooth Muscle
Smooth muscle is the automatic, unstriped muscle in the walls of hollow organs such as the stomach, bladder and blood vessels.
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The idea
Smooth muscle is one of the three major types of muscle tissue in vertebrates, along with skeletal and cardiac muscle. It is controlled by the autonomic nervous system, so it works without you having to think about it. It is called smooth because it has no sarcomeres, so it shows no stripes, or striations, under a microscope. Smooth muscle is found in invertebrates as well.
Smooth muscle lines the walls of hollow organs such as the stomach, intestines, bladder and uterus. In the walls of blood vessels and lymph vessels, it is called vascular smooth muscle. It is also found in the airways and in the urinary and reproductive tracts. By squeezing slowly and steadily, it moves food through the gut and helps control blood flow.
In the eyes, smooth muscles in the iris widen or narrow the pupils, and the ciliary muscles change the shape of the lens to focus. In the skin, tiny smooth muscles called arrector pili make hairs stand up when you are cold or afraid. Scientists divide smooth muscle into single-unit and multi-unit types. In single-unit smooth muscle, a whole sheet of cells contracts together as one.
Check yourself
Question 1 of 3
Why does smooth muscle show no stripes under a microscope?
Composition
what it's made of
- Non-striated cells — Muscle cells without sarcomeres, so they show no stripes.
- Single-unit smooth muscle — Sheets of cells that contract together as one unit.
- Multi-unit smooth muscle — The second subgroup, in which the cells act more independently.
- Vascular smooth muscle — Smooth muscle in the walls of blood and lymph vessels.
- Autonomic nerve control — Signals from the autonomic nervous system that run smooth muscle automatically.
Connection
how it relates
Under Muscular System
Change
how and why it evolves
- Past
- In a developing embryo, most smooth muscle forms from a tissue layer called the mesoderm, after muscle cells are first created in a process called myogenesis. The smooth muscle of the aorta and the pulmonary arteries, however, grows from cells of a different origin, the neural crest.
- Now
- Today smooth muscle is known to work throughout the body, from the eyes and skin to the gut and blood vessels.
- Next
- Research on how smooth muscle behaves in conditions such as asthma and high blood pressure may lead to better treatments.