Smooth muscle is a type of muscle located in numerous hollow human organs. It has the ability to work independently.
What is smooth muscle?
Smooth muscle is a type of muscle that, unlike striated muscle, cannot be controlled at will. It exerts influence on the shape and function of the internal organs. It forms the contractile tissue of numerous hollow organs as well as the blood vessels and lymphatic vessels in humans. The name “smooth muscle” is due to the fact that on a microscopic image the transverse striation of the muscle tissue is absent. One of the exceptions is the heart muscle. Thus, although the heart represents an internal hollow organ, the cardiac muscle is not composed of smooth muscle. The striated muscle is usually found in the skeletal muscles. The transverse striation can be seen under polarizing light. It is formed by the regular arrangement of the proteins myosin and actin. However, this arrangement is not present in smooth muscle. For this reason, smooth muscle cells appear homogeneous even under polarizing light.
Anatomy and structure
Myocytes form a typical feature of smooth muscle. These are narrow, elongated muscle cells that do not have any transverse striations. Smooth muscle is primarily found on the walls of hollow organs such as the urinary tract, genital organs, respiratory tract, intestines and blood vessels. Smooth muscle myocytes are spindle-shaped. Their diameter is between 5 and 8 µm, which, however, depends on the respective cell state. Thus, the circumference of myocytes in a contracted muscle turns out to be slightly larger than in a muscle that is flaccid. The length of the myocytes can vary considerably, which also depends on the position of the muscle cell. For example, within blood vessels, myocytes reach an average length of only 15 to 20 µm. In other organs, however, their length is between 200 and 300 µm. The nuclei of smooth muscle cells are mostly located in the middle of the cells and are relatively elongated. The cytoplasm of the muscle cells contains high concentrations of the filaments myosin and actin, whose structure is less strict than that of the striated muscles. In a disordered form, they pass through the individual muscle cell. In the cytoplasm as well as at the cell edge they are fixed to anchoring plaques. This arrangement enables the cell, and thus the muscle, to contract more strongly during contractions than in striated muscle. The individual 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 and the multi-unit type. The differences between these two subforms lie in their structure, innervation and function. Sometimes, however, mixed forms also occur, which is particularly true of the vascular musculature. The individual muscle cells of the single-unit type are able to connect with each other through special gap junctions. This allows an exchange between second messenger molecules and ions, resulting in a functional unit because the cells couple electrically. Single-unit types are found particularly in the ureter, the gastrointestinal tract and the uterus. Multi-unit type, on the other hand, is found in the vas deferens, hair muscles, and internal eye muscles.
Function and tasks
Smooth muscle, unlike striated muscle, cannot be controlled by humans. It participates in numerous vital processes of the organism. These include the pumping movements of the heart, digestion and the erection of fine hairs on the surface of the skin. Humans are not aware of these processes and cannot control them. Only the autonomic nervous system is capable of influencing the muscles of the hollow organs. This is done with adrenaline and acetylcholine via the sympathetic and parasympathetic nervous systems. In this way, at least an indirect influence is possible. Smooth muscle has the ability to shorten considerably more than skeletal muscle, but this takes more time. After all, this condition can be maintained for a longer time without being associated with fatigue.This process is also known as tonic continuous contraction or true muscle tone. During the birthing process, smooth muscle ensures that the uterus makes a rhythmic contraction.
Diseases
Smooth muscle is more likely to have impaired function than heart muscle or skeletal muscle. It has the disadvantage of regenerating poorly, so a connective tissue scar often forms. Defects in smooth muscle can lead to various diseases. Among them is a weakness of the uterus in labor. The development of malignant smooth muscle cells is also possible in the uterus or digestive tract. These are called leiomyosarcomas. In this case, the tumor originates from the smooth muscle. Its share in the malignant tumors of the uterus is one percent. It usually appears after the age of 30. The symptoms of leiomyosarcoma are considered nonspecific. In most cases, the uterus enlarges rapidly. In addition, bleeding is evident. The occurrence of leiomyosarcoma is basically conceivable at any smooth muscle site. Leiomyosarcoma represents a rare disease whose treatment is mostly surgery.