Smooth muscle


The human body is also made up of a large number of muscles. These are a type of tissue that is composed of muscle fiber which contracts at the moment in which the different movements of the body are carried out. In total, a total of 656 muscles can be found and there are several types of them. One of them is the smooth muscle which works without any kind of stimulus that is voluntary.


What is a smooth muscle?

A smooth muscle is one that is made up of cells that only have a nucleus and that also can contract without there being some type of external stimulus caused by humans.

  • Smooth muscle characteristics
  • Types
  • Smooth muscle function
  • Structure
  • Contraction
  • Smooth muscle location
  • How it differs from striated muscle
  • Examples

Smooth muscle characteristics

Among its main characteristics we mention the following:

  • It is also known by the name of involuntary or visceral muscle tissue .
  • It does not have any type of stretch marks .
  • It is found covering most of the organs located within the cardiovascular , respiratory , digestive and reproductive systems .
  • They are usually shaped like a bag or tube .
  • Its dilation or contraction occurs depending on the amount of fluids that are inside the muscle.
  • You can metabolize the energy more easily which gives the advantage of working efficiently and constant.
  • They are controlled by the autonomic nervous system .
  • It is the muscle tissue that has the greatest flexibility in the human body.


There are two different types of smooth muscles, one of them is unitary smooth muscle tissue . In this type of muscle, the fibers fail to shrink voluntarily and it is made up of a series of fibers that can be retracted thanks to the presence of the sarcolemma . It also has several membranes that can be connected to each other and can be found in most of the body’s viscera such as the uterus, blood vessels and gastrointestinal tract.

The other type of smooth muscle is multi-unit , which is composed of a series of muscle fibers which are separated from themselves thanks to glycoproteins and collagen . We observe this type of muscle in the hair follicles and in the iris muscle for example.

Smooth muscle function

There are many functions that smooth muscle performs within the human body and these will always depend on the activity that it must perform. One of its functions is to prevent blood from clotting and it is also one of the means by which blood can be transported . It also intervenes in the production of nitric oxide, which is essential for the proper growth of cells.

Another of its important functions is in the intestines as it is responsible for making all the digestive work and movements are carried out, such as peristalsis, agitation and relaxation. It is also lining the bronchi and bronchioles and thanks to this, air can be taken to the lungs. It also participates in the contractions that it undergoes in the uterus at the time of delivery and thanks to this muscle, the uterus has the possibility of being able to contract to house the human embryo during the gestation process, returning to its normal size after the process.


Smooth muscle has a structure formed by uninucleated cells , which are characterized by being thin and also spindle-shaped . Its lengths can vary and in addition, it is innervated by the autonomic nervous system which makes it contract involuntarily. Within its structure there are also a series of thin and thick filaments that are attached to the plasma membrane . Its shape is irregular and has filaments of actin which are located diagonally.


The contraction of smooth muscles occurs thanks to the increase in the concentration of intracellular calcium levels . This happens when, after the cell receives a signal type nervous , levels of calcium in the inside of the cytoplasm rise inducing production of the phosphorylation of myosin , when this happens, the cells move horizontally upon others, approaching and producing at the same time a reduction in size and diameter, enlarging or reducing the size of the blood vessel or the digestive tract.

Smooth muscle location

Smooth muscles can be found in the walls of the veins and arteries. In addition, they are located in all organs that have a hollow shape, by this we mean organs such as the uterus, bladder, stomach, intestines and bladder. In other words, it can be located in the vascular , gastrointestinal , respiratory , urinary , reproductive, and finally in the ocular system .

How it differs from striated muscle

The striated muscle is a type of muscle that has the ability to act completely voluntary so the human being has the ability to control their intensity and duration. The cells found in skeletal muscle are long , cylindrical, and in its cytoplasm peripheral nuclei with transverse striae can be observed.

The smooth muscle does not have the aforementioned type of morphology, cytoplasm containing is very similar to the rest of the cells and their movements they are also completely involuntary. Its main structural difference also has a lot to do with its operation . The contractions that the ready muscle undergoes are slower, are of very low intensity and also have a very long duration, on the other hand, striated muscles have faster or slower contractions, they can also be long or short so their intensity varies markedly.


Some examples of smooth muscle are as follows:

  • The sphincter muscle of the pupil
  • The iris dilator muscle
  • The ciliary muscle which is a muscle located inside the eye
  • Esophagus muscle
  • Uterus muscle
  • Vocal cord muscles
  • Most blood vessels in the human body

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