Smooth muscle

Structure

The smooth muscle cell is 3-10 µm thick and 20-200 µm long. The cytoplasm is homogeneously eosinophilic and consists mainly of myofilaments. The nucleus is located in the center and takes a cigar-like shape during contraction. The cell membrane forms small pouch-like invaginations into the cytoplasm (caveolae) which are functionally equivalent to the T-tubules of the skeletal musculature. The smooth muscle cells are anchored to the surrounding connective tissue by a basal lamina.

The smooth muscle fibers group in branching bundles. As opposed to skeletal muscle fibers these bundles do not run strictly parallel and ordered but consist in a complex system. Thus the cells can contract much stronger than striated musculature. The actin filaments are stretched between dense bodies in the cytoplasm and attachment plaques at the cell membrane. The myosin filaments lie between the actin filaments. Furthermore intermediate filaments such as desmin and vimentin support the cell structure.

Function

Smooth musculature is found in (almost) all organ system such as hollow organs (e.g. stomach, bladder), in tubular structures (e.g. vessels, bile ducts), in sphincters, in the uterus, in the eye etc. In addition it plays an important role in the ducts of exocrine glands. It fulfills various tasks such as sealing orifices (e.g. pylorus, uterine os) or the transport of the chyme through wavelike contractions of the intestinal tube. On the one hand smooth muscle cells contract slower than skeletal muscle cells, on the other hand they are stronger, more sustained and require less energy.

Myofibroblasts represent a special type of smooth muscle cell which additionally have qualities of fibrocytes. They produce connective tissue proteins such as collagen and elastin for which reason they are also referred to as fixed (or stationary) connective tissue cells. Myofibroblasts are found, among others, in alveolar septa of the lung and scar tissue.

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Innervation

The innervation of the smooth musculature is utmost complex. It lies under the influence of the visceral nervous system and works autonomously at the same time.

Furthermore, it is regulated by:

• neurotransmitters: e.g. norepinephrine, acetylcholine;
• hormones: e.g. estrogen, oxytocin; 
• tissue hormones: e.g. prostaglandins, histamine.

Local changes (e.g. stretching) may have a stimulating or relaxing effect. In contrast to the skeletal musculature, the smooth musculature is contracted involuntarily.

Functionally, one differentiates between the single-unit and multi-unit type. The smooth muscle cells of the single-unit type are electrically connected by gap junctions and contract uniformly. This type of cells is found in the wall of internal organs and blood vessels (visceral smooth musculature). The multi-unit smooth cells are independent from each other and therefore need to be innervated individually allowing a more precise muscle control. They are found, among others, in the iris and hair erector muscles.

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