Smooth muscle
| Smooth muscle tissue | |
|---|---|
Layers of Esophageal Wall:
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| Details | |
| Identifiers | |
| Latin | textus muscularis levis; textus muscularis nonstriatus |
| TH | H2.00.05.1.00001 |
| FMA | 14070 |
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Anatomical terminology
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Smooth muscle is an involuntary non-striated muscle. It is divided into two subgroups; the single-unit (unitary) and multiunit smooth muscle. Within single-unit cells, the whole bundle or sheet contracts as a syncytium.
Most smooth muscle is of the single-unit variety, that is, either the whole muscle contracts or the whole muscle relaxes, but there is multiunit smooth muscle in the trachea, the large elastic arteries, and the iris of the eye. Single unit smooth muscle, however, is most common and lines blood vessels (except large elastic arteries), the urinary tract, and the digestive tract.
Smooth muscle is fundamentally different from skeletal muscle and cardiac muscle in terms of structure, function, regulation of contraction, and excitation-contraction coupling.
Smooth muscle cells known as myocytes, have a shape and, like striated muscle, can tense and relax. However, smooth muscle tissue tends to demonstrate greater elasticity and function within a larger length-tension curve than striated muscle. This ability to stretch and still maintain contractility is important in organs like the intestines and urinary bladder. In the relaxed state, each cell is spindle-shaped, 20-500 micrometers in length.
A substantial portion of the volume of the cytoplasm of smooth muscle cells are taken up by the molecules myosin and actin, which together have the capability to contract, and, through a chain of tensile structures, make the entire smooth muscle tissue contract with them. The smooth muscle is the only type of muscle without the ability to be voluntarily controlled in stressful situations.
Myosin is primarily class II in smooth muscle.
Different combinations of heavy and light chains allow for up to hundreds of different types of myosin structures, but it is unlikely that more than a few such combinations are actually used or permitted within a specific smooth muscle bed. In the uterus, a shift in myosin expression has been hypothesized to avail for changes in the directions of uterine contractions that are seen during the menstrual cycle.
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