Smooth Muscle Tissue: Characteristics And Function

The smooth muscle tissue , also known as involuntary or visceral muscle, is a type of muscle striations not as in the case of the skeletal muscle and heart. This type of tissue is what lines most of the organs of the cardiovascular system, the respiratory system, the digestive system and the reproductive system.

This type of muscle is typical of hollow organs, that is, those that are bag-shaped or tube-shaped. Thanks to this, it is possible that they expand or contract according to the movement of the fluids that are inside.

This dilation and contraction is achieved thanks to the shortening and lengthening of smooth muscle cells. These cells are electrically coupled by intercellular connections also known as gap junctions.

Therefore, smooth muscle tissue is responsible for many involuntary functions of the body. For example, its presence in the uterus allows contractions to occur during childbirth and its presence in the iris of the eye, controls the change in diameter of the pupils.

Smooth muscle tissue characteristics

Despite the differences in their functioning, smooth muscles throughout the body share different characteristics with each other.

The cells of this tissue are characterized by their long, narrow, worm-like spindle shape. They are called “smooth” because they do not have the regular striae that characterize skeletal muscle and heart muscle.

In a contracted state, smooth muscle cells can shrink even to half their size. This great flexibility facilitates many vital processes crucial for the body such as breathing.

Once the smooth muscle cells contract, they can be held in this position for variable times. It can occur in a prolonged way as in the case of the blood vessels or rhythmically as in the case of the gastrointestinal tract.

The main stimulus that triggers the contraction of smooth muscle is an increase in the cellular concentration of calcium. This is produced by different chemical signals that come from the brain and depend on the type of smooth muscle involved.

On the other hand, a fundamental difference between smooth muscle and striated muscle is that the former never tires. This occurs because energy is metabolized more efficiently in smooth tissue cells.

The functioning of smooth muscle tissue

Smooth muscle is responsible for very diverse functions within the human body. Therefore, its behavior usually varies according to the activity carried out by each organ.

Most of the smooth muscle tissue in the body is under the control of the autonomic nervous system . However, in some organs it is influenced by the sympathetic component and in others by the parasympathetic component.

On the other hand, the mechanisms that control its expansion and contraction are also different. These depend on the activities carried out by each of the different organs.

In the arteries

In the case of veins and arteries, for example, it is necessary to consider that their work is not limited to serving as conduits to transport blood.

On the contrary, it can be affirmed that there is a dynamic state generated by the contractions of the smooth muscle that makes up its walls.

Smooth muscle cells are arranged in a tough, elastic matrix of connective tissue. They wrap around the blood vessels like spirals in such a way that when they shorten, they also contract the vessel.

This system is essential for the development of vital tasks, for example, preventing blood clotting. In addition, it allows the generation of substances such as nitric oxide, necessary for the growth of cells.

In the intestines

In smooth muscle it is also essential for the work carried out by the intestines. Within these organs it is responsible for various movements such as peristalsis, relaxation, agitation and the expulsion of feces.

Peristalsis is a movement of the smooth muscle that allows the transport of fluids and food throughout the intestine.

It consists of a series of muscular contractions that occur throughout the entire digestive tract, generating a movement within it.

This movement is not unique to the intestines. It is also necessary to carry urine from the kidneys to the bladder and bile from the gallbladder to the duodenum.

In the airways

The smooth muscle that lines the bronchi and bronchioles is a fundamental component of respiratory function. Thanks to them, it is possible to transport air from the trachea to the lungs.

The bronchi are highly sensitive organs that can overreact to certain stimuli. Factors such as exercise, dust, or cigarette smoke can be triggers for reactions that disrupt your normal behavior.

These triggering stimuli cause the bronchi to swell and the smooth muscle around them to contract. Faced with this combination of factors, the airways become narrower and consequently breathing becomes more difficult.

This mechanism causes diseases such as asthma and chronic obstructive pulmonary disease – COPD.

Therefore, the medicines indicated for these diseases have a direct effect on the smooth muscle, relaxing it to allow air to pass through.

In the myometrium

The myometrium is the layer of smooth muscle tissue that lines the walls of the uterus. This wall is located between the endometrium and the perimetrium and in addition to muscle tissue it also contains some connective blood vessels.

This type of smooth muscle tissue is characterized by being the most flexible tissue in the human body. Thanks to him, it is possible that the uterus grows enough to house an embryo throughout its gestation and can return to its original size after delivery.

On the other hand, the myometrium plays a key role during delivery. This muscle is responsible for the contractions that pull the baby out of the uterus.

In addition, once birth has occurred, this tissue continues to undergo contractions that allow the placenta to be expelled.


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