Smooth muscle , found in the walls of the hollow internal organs such as blood vessels, the gastrointestinal tract , bladder , and uterus , is under control of the autonomic nervous system. Smooth muscle cannot be controlled consciously and thus acts involuntarily. The non-striated smooth muscle cell is spindle-shaped and has one central nucleus. Smooth muscle contracts slowly and rhythmically.
Cardiac muscle , found in the walls of the heart , is also under control of the autonomic nervous system. Also, visceral muscle in the walls of the hollow organs except the heart contains pacesetter cells. A pacesetter cell can spontaneously trigger action potentials and contractions in the muscle. Smooth muscle is organized in two ways: as single-unit smooth muscle, which is much more common; and as multiunit smooth muscle. The two types have different locations in the body and have different characteristics.
Single-unit muscle has its muscle fibers joined by gap junctions so that the muscle contracts as a single unit. This type of smooth muscle is found in the walls of all visceral organs except the heart which has cardiac muscle in its walls , and so it is commonly called visceral muscle. Because the muscle fibers are not constrained by the organization and stretchability limits of sarcomeres, visceral smooth muscle has a stress-relaxation response.
This means that as the muscle of a hollow organ is stretched when it fills, the mechanical stress of the stretching will trigger contraction, but this is immediately followed by relaxation so that the organ does not empty its contents prematurely. This is important for hollow organs, such as the stomach or urinary bladder, which continuously expand as they fill. In general, visceral smooth muscle produces slow, steady contractions that allow substances, such as food in the digestive tract, to move through the body.
Multiunit smooth muscle cells rarely possess gap junctions, and thus are not electrically coupled. As a result, contraction does not spread from one cell to the next, but is instead confined to the cell that was originally stimulated. Stimuli for multiunit smooth muscles come from autonomic nerves or hormones but not from stretching.
This type of tissue is found around large blood vessels, in the respiratory airways, and in the eyes. Similar to skeletal and cardiac muscle cells, smooth muscle can undergo hypertrophy to increase in size. Unlike other muscle, smooth muscle can also divide to produce more cells, a process called hyperplasia. This can most evidently be observed in the uterus at puberty, which responds to increased estrogen levels by producing more uterine smooth muscle fibers, and greatly increases the size of the myometrium.
Smooth muscle is found throughout the body around various organs and tracts. Smooth muscle cells have a single nucleus, and are spindle-shaped.
Smooth muscle cells can undergo hyperplasia, mitotically dividing to produce new cells. Smooth muscle can be stimulated by pacesetter cells, by the autonomic nervous system, by hormones, spontaneously, or by stretching. The fibers in some smooth muscle have latch-bridges, cross-bridges that cycle slowly without the need for ATP; these muscles can maintain low-level contractions for long periods. Single-unit smooth muscle tissue contains gap junctions to synchronize membrane depolarization and contractions so that the muscle contracts as a single unit.
Their cytoplasm includes high concentrations of myosin and actin. Actin and myosin proteins are the primary muscle contraction proteins. The filaments of actin adhere to dense bodies that are scattered across the cell. The filaments of myosin are lying between the filaments of actin. Under an electron microscope, thick bodies may be identified, and they look black. The calcium-containing sarcoplasmic reticulum , which enables contraction, is another significant structure present in smooth muscles.
These cells form the walls of all inner hollow organs of the body. It creates long and uniform contractions that facilitate the passage of substances through the body, such as food digestion in the GIT. These muscles are also present in the blood vessel, which integrate structures and control the vessel diameter by contraction and relaxation of the vessels.
It is a tissue associated with the organs present inside the body, especially those in the abdominal cavity. In other words, we can say that visceral muscle is found near or in the abdomen. In the eyes, they can change iris size and lens shape. It is also present in the skin, allowing the hair to stand straight in reaction to cooler conditions or worries.
Location of multiunit smooth muscle: such muscles are mostly located in the lungs, the arrector pili muscles associated with hair follicles, in the large arteries, and also the internal eye muscles controlling the entry of light and lens formation. Visceral smooth muscle unitary smooth muscle has a gap junction and it is a type of muscle in which all the cells function collectively and simultaneously as a single unit.
In contrast, multiunit smooth muscle ones do not have gap junctions and it is a type of muscle in which all the cells cannot function collectively and work independently from each other. Watch the video below to learn more about the differences between the single-unit and multiple-unit types of smooth muscles.
Smooth muscles are found almost in every part of your body. It is present in the following places of the body. Smooth muscles can function in various ways.
They help the body to perform important and significant jobs, which are very important. Smooth muscles help in the housekeeping tasks of the body. Even many of the other organs are also associated with these muscles.
Questions: What is the function of smooth muscle in the body? Answer: Few important functions are given below:. It is an infrequent vascular hereditary disease that gets active by a cognitional disorder of smooth muscle all over the body.
Anti-smooth muscle antibodies can be a symptom of an autoimmune disorder , such as hepatitis or lupus. Smooth muscle failure can be a reason for many fatal diseases. Many diseases are fatal and will take time to cure, but smooth muscle failure will be a much bigger problem. As many organs are linked with smooth muscles, so damage to these muscles will ultimately cause diseases and defects to other organs and systems of the body.
In smooth muscles, the membrane potential is responsible for initiating or modulating contraction. In comparison to skeletal muscles, the action potential in these cells is rather slower, but they can last for about fifty times more. Sodium channels are responsible for this characteristic of smooth muscles; in contrast to skeletal muscle, smooth muscle has a slower opening of sodium channels s low repolarization. Question: What initiates an action potential on a muscle cell? Answer: When more amount of sodium ions moves across the plasma membrane as compared to the potassium ions, an electrochemical gradient develops.
This gradient leads to depolarization of the membrane, which then initiates an action potential. Smooth muscles can be looked after if we will focus a little on our diet and lifestyle.
We can easily take care by doing the following tips and bits:. Got a question about smooth muscles? Join our Forum: Smooth muscle vs dense regular connective tissue. Our community might be able to help! Try to answer the quiz below and see what you have learned so far about smooth muscles. Muscle cells are specialized to generate force and movement. Learn about the different types of muscle tissues in this tutorial and the molecular mechanisms of contraction Read More. The circulatory system is key to the transport of vital biomolecules and nutrients throughout the body.
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