Smooth muscle

* Single central nucleus
* Arranged in sheets
* Not striated
* Involuntary contractions
* Gap junctions
* Muscular layers of the vessels and within internal organs - assist in moving materials.
* Found in the skin, where it is associated with hair follicles.
* Smooth muscle cells are important in the expansion of organs (e.g. kidneys, lungs, and vagina) as they are elastic.

Smooth muscle is responsible for involuntary movement, like that of the intestines during peristalsis (contraction to propel food through the digestive system). Smooth muscle is not striated as the contractile proteins, myosin and actin, are more randomly arranged and are not arranged into sarcomeres. Smooth muscle cells contain ONE central nucleus and are connected to each other by **gap junctions**. Smooth muscle cells produce their own connective tissue (**endomysium**) and can contract simultaneously as the myocytes are arranged in sheets.

Smooth muscle can be organised in two different ways, with different characteristics and locations within the body:

1. **Single-unit smooth muscle** (more common)**:** Muscle fibres joined by gap junctions so the muscle contracts as a single unit. Found in walls of visceral organs (expect the heart), therefore commonly called visceral muscle. Due to the lack of sarcomeres, single-unit smooth muscle has a stress-relaxation response - therefore as an organ enlarges, the mechanical stress on the myocytes will trigger their contraction immediately followed by relaxation (e.g. stomach, urinary bladder)
2. **Multiunit smooth muscle:** This smooth muscle rarely has gap junctions between the myocytes, therefore the myocytes are not electrically coupled and contraction does not spread and is just confined to each stimulated cell. Stimulus for this muscle does not come from stretching but from the autonomic nerves or hormones and is found in the eyes, around large blood vessels, and in the respiratory system.

#### Formation

In smooth muscle, the myoblasts that form smooth muscle cells do not fuse, therefore smooth muscle cells are NOT multinucleate. The gap junctions develop in the smooth muscle in the early stages of development.

#### Regeneration

Smooth muscle cells have the ability to hypertrophy as well as divide themselves (**hyperplasia**), therefore can increase their numbers in this way are have the greatest ability to be able to regenerate compared to skeletal and cardiac muscle. As well as division themselves, **pericytes** allow new smooth muscle cells to be produced as well.

> ***Pericytes*** *are a type of stem cell that lie close to the endothelial cells of blood capillaries and small venules into which the capillaries drain. They are a potential source of new fibroblasts and smooth muscle cells, and help to repair damage.*
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#### Cell junctions

Smooth muscle contains **gap junctions.**

* Responsible for electrical coupling - provide direct contact between the myocytes to facilitate electrical communication, allowing for waves of depolarisation to pass from cell-cell and spread causing the myocytes to contract together in unison.
* Responsible for the transfer of small molecules between cells.

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Reference list and extra reading:

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1. Baxter, R. 2022. *Types of muscle cells.* Available at: https://www.kenhub.com/en/library/anatomy/types-of-muscle-cells#section15 [Accessed: 19-03-22]
2. Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J. E., Womble, M., DeSaix, P. 2013. Development and Regeneration of Muscle Tissue. *Anatomy and Physiology.* Available at:https://opentextbc.ca/anatomyandphysiologyopenstax/chapter/development-and-regeneration-of-muscle-tissue/ [Accessed: 19-03-22]
3. Faculty of Biological Sciences at Leeds. 2003. *Muscle: Smooth Muscle.* Available at: https://www.histology.leeds.ac.uk/tissue\_types/muscle/muscle\_smooth.php [Accessed: 19-03-22]
4. Faculty of Biological Sciences at Leeds. 2003. *Connective Tissue: Endothelial cells and pericytes.* Available at: https://www.histology.leeds.ac.uk/tissue\_types/connective/connective\_endo\_peri.php [Accessed: 19-03-22]
5. Faculty of Biological Sciences at Leeds. 2003. *Muscle:* *Muscle regeneration.* Available at: https://www.histology.leeds.ac.uk/tissue\_types/muscle/muscle\_regeneration.php [Accessed: 19-03-22]
6. Faculty of Biological Sciences at Leeds. 2003. *Muscle:* *Cell Junctions.* Available at: https://www.histology.leeds.ac.uk/tissue\_types/muscle/Muscle\_cell\_junctions.php [Accessed: 19-03-22]