What are involuntary muscles?

Involuntary muscles, also known as “white muscles” or “smooth muscles”, are muscles in the human body whose contraction is controlled by the autonomic nervous system. Involuntary muscles include all muscles whose activity is independent and not affected by voluntary nerve activity.  

From a histological point of view, involuntary muscles differ from voluntary muscles because the fibers from which they are composed do not consist of striations and thus, they are characterized by a structure almost completely uniform, defined as “smooth” musculature.The cells that make the involuntary muscles are crossed by thin filaments, called myofibrils, which are part contractile. Compared to striated muscles, smooth muscles contract and relax more slowly.

Involuntary muscles are present in the walls of the digestive system, blood vessels, bronchi, uterus and bladder. Although they are formed by special striated muscle fibers that are not smooth, they still belong to the smooth muscle group, as well as the heart muscle group.

Involuntary muscles of the heart, respiratory and digestive systems work automatically. These operations are triggered by nerves and the nerves are stimulated by complex chemical fluids (such as acetylcholine and norepinephrine) that surround them and other living cells. The role of the chemical mediators is essential in the operating activities because in addition to giving life to the involuntary contraction, they are also essential in modulating the activity.

Diseases that affect the involuntary muscles include occlusion (blockage) of the coronary arteries caused by atherosclerosis and/ or thrombosis. Problems can lead to myocardial infarction (heart attack), depending on the part of the myocardium that is injured and the severity of the individual’s condition. This occurs because coronary arteries are functional end arteries and if one fails, the others cannot take over the region to make up for the loss, unlike in other tissue activity. Also, there are certain viruses that can lead to myocarditis (inflammation of the myocardium). Seeking the most appropriate form of treatment is essential in order to prevent further complications from arising.


What function do involuntary muscles serve?

The main function of the muscular system is movement. Muscles are the only tissue in the body that have the ability to contract and therefore move the other parts of the body. They also perform different functions such as: enable the body to maintain posture and body position (sitting or standing positions), stabilize and strengthen joints as well as produce heat to maintain normal body temperature.

The function of the involuntary muscles is to cause variations in shape or inner diameter of the organs (movement of substances inside the body, favoring possible passageways or the exclusion of contents). Their contraction is regulated by the autonomic nervous system. An example of an involuntary muscle is the heart.

When the involuntary muscles of the hollow organs (such as blood vessels and intestine) contract, it causes a reduction in the wall surfaces in which the muscle fibers themselves are found in.  When the contraction phase follows distension, the surfaces of the walls of the organs return to their usual dimensions. The repetition of the mechanism is essential to allowing the regulation of blood vessel tone and giving rise to movements vital for the well being of the organism, such as intestinal peristalsis.