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Smooth Musculature

Smooth muscle is a type of tissue found in the walls of hollow organs, such as the intestines, uterus and stomach. You can also find smooth muscle in the walls of passageways, including arteries and veins of de cardiovascular system. This type of involuntary non-striated muscle is also found in the tracts of the urinary, respiratory and reproductive systems. In addition to that, you can find smooth muscle in the eyes, where it acts to change the size of the iris and the shape of the lens. The skin is also contains smooth muscle which allows hair to raise in response to cold temporatures or fear.

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.

Smooth muscle (green)

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.

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The smooth muscle tissue forms part of the wall of blood vessels and hollow internal organs, such as the stomach, intestine etc.

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.

Smooth muscle (green) - Trachea histology slide

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.

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.

Summary

Smooth muscle is found in the wall of hollow organs, passageways, tracts, eye and skin. 

Structure

Fibers of smooth muscle group in branching bundles, which allows for cells to contract much stronger than those of striated musculature.

Functions

Smooth muscle has different functions in the Human body, including:

  • Sealing orifices;
  • Transport chyme through wavelike contractions of the intestinal tube; 
  • Myofibroblasts produce connective tissue proteins such as collagen and elastin.

Innervation

Smooth muscle is regulated by the following:

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

Smooth Musculature - want to learn more about it?

Our engaging videos, interactive quizzes, in-depth articles and HD atlas are here to get you top results faster.

Sign up for your free Kenhub account today and join over 852,397 successful anatomy students.

“I would honestly say that Kenhub cut my study time in half.” – Read more. Kim Bengochea Kim Bengochea, Regis University, Denver

Show references

References:

  • U. Welsch: Lehrbuch Histologie, 2.Auflage, Urban & Fischer Verlag/Elsevier (2006), S.152-157
  • D. U. Silverthorn: Physiologie, 4.Auflage, Pearson Studium (2009), S.595-606

Author & Layout:

  • Achudhan Karunaharamoorthy
  • Christopher A. Becker
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