Smooth muscles – structure, function & diseases
Smooth musculature
Smooth muscle is a type of muscle found in numerous human hollow organs. She has the ability to work independently.
What is smooth muscle?
Smooth muscle is a type of muscle that, unlike striated muscle, cannot be controlled voluntarily. It influences the form and function of the internal organs. It forms the contractile tissue of numerous hollow organs as well as the blood vessels and lymph vessels in humans.
The term “smooth muscle” is due to the fact that the striations of the muscle tissue are missing on a microscopic image. The heart muscle is one of the exceptions. Although the heart is an internal hollow organ, the cardiac muscle does not consist of smooth muscles. Striated muscle is usually found in skeletal muscle . The striations are visible under polarizing light. It arises from the regular arrangement of the proteins myosin and actin . However, this arrangement does not exist in smooth muscle. For this reason, the muscle cells of the smooth muscles appear homogeneous even under polarizing light.
Anatomy & Structure
A typical feature of smooth muscles are the myocytes. These are narrow, elongated muscle cells that do not have any transverse striations. The smooth muscles are located primarily on the walls of hollow organs such as the urinary tract , the genitals , the respiratory tract , the intestines and in the blood vessels .Smooth muscle myocytes are spindle-shaped. Their diameter is between 5 and 8 µm, which, however, depends on the respective cell condition. For example, the volume of myocytes in a contracted muscle is slightly larger than in a muscle that is relaxed. The length of the myocytes can vary considerably, which also depends on the position of the muscle cell. The myocytes within the blood vessels only reach an average length of 15 to 20 µm. In other organs, however, their length is between 200 and 300 µm.
The cell nuclei of smooth muscles are usually located in the middle of the cells and are relatively elongated. The filaments myosin and actin are found in high concentrations in the cytoplasm of the muscle cells, the structure of which turns out to be less strict than that of the striated muscles. In a disordered form, they pass through the individual muscle cells. They are fixed to anchoring plaques in the cytoplasm and at the cell edge. Due to this arrangement, the cell and thus also the muscle are able to contract more strongly during contractions than in the striated muscles. The single muscle cell surrounds the basal lamina, which is a thin skin.
Smooth muscle can be divided into two different types. These are the single-unit type as well as the multi-unit type. Differences between these two subforms exist in their structure, innervation and function. Sometimes, however, mixed forms also occur, which particularly applies to the vascular muscles.
The individual muscle cells of the single-unit type are able to connect to each other through special gap junctions. In this way, an exchange between second messenger molecules and ions is enabled, resulting in a functional unit because the cells electrically couple. Single-unit types are found particularly in the ureters , the gastrointestinal tract and the uterus . The multi-unit type, on the other hand, is found in the vas deferens , hair muscles, and inner eye muscles .
Function & Tasks
In contrast to striated muscles, smooth muscles cannot be controlled by humans. It is involved in numerous vital processes of the organism. These include the pumping movements of the heart, digestion and the formation of fine hairs on the skin’s surface.
Man is not aware of these processes and cannot control them. Only the vegetative nervous system is capable of influencing the muscles of the hollow organs. This takes place with adrenaline and acetylcholine via the sympathetic and parasympathetic nervous system . In this way at least an indirect influence is possible.
Smooth muscle has the ability to shorten significantly more than skeletal muscle, but this takes more time. After all, this state can be maintained for a long time without being associated with fatigue. This process is also known as sustained tonic contraction or true muscle tone. During the birth process, the smooth muscles ensure that the uterus makes a rhythmic contraction.
Diseases
Functional impairments are more common in smooth muscle than in cardiac muscle or skeletal muscle. It has the disadvantage of being difficult to regenerate, so that a connective tissue scar often forms. Faults in the smooth muscle can lead to various diseases. This includes, among other things, a weak contraction of the uterus.Malignant smooth muscle cells can also develop in the uterus or in the digestive tract . These are called leiomyosarcomas. The tumor originates from the smooth muscle. It accounts for one percent of malignant tumors in the uterus. It usually shows up after the age of 30. The symptoms of leiomyosarcoma are considered non-specific. In most cases, the uterus enlarges rapidly. There is also bleeding . In principle, leiomyosarcomas can occur anywhere in the smooth muscle. Leiomyosarcoma is a rare disease that is usually treated with surgery.
Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.