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There are three types of muscle tissue in the body: skeletal, smooth, and cardiac. Smooth muscle is the only type that is non-striated. Smooth muscle is found in the walls of hollow internal organs such as the stomach, intestines, bladder, and uterus, as well as in the tracts of the respiratory, urinary, and reproductive systems. It is also present in arteries and veins, where it plays a vital role in regulating blood pressure and tissue oxygenation. Smooth muscle is controlled by the autonomic nervous system and contracts involuntarily. It is responsible for a variety of functions in the body, including digestion, nutrient collection, and toxin elimination.
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What You'll Learn
- Smooth muscle is found in the walls of hollow organs like the stomach, intestines, bladder, and uterus
- Smooth muscle is controlled by the autonomic nervous system and acts involuntarily
- Smooth muscle cells are spindle-shaped with a single nucleus
- Smooth muscle contains actin and myosin, which are the main proteins involved in muscle contraction
- Smooth muscle has greater elastic properties than striated muscle
Smooth muscle is found in the walls of hollow organs like the stomach, intestines, bladder, and uterus
Smooth muscle is a type of tissue found in the walls of hollow organs, such as the stomach, intestines, bladder, and uterus. It is also found in the walls of blood vessels, including arteries and veins of the cardiovascular system, and in the walls of passageways such as the gastrointestinal tract. Smooth muscle is under the control of the autonomic nervous system and acts involuntarily. It is described as having a fusiform shape, being round in the centre and tapering at each end.
Smooth muscle differs from skeletal muscle in that it can be contracted and controlled involuntarily. It also has greater elastic properties than striated muscle, which is important in organ systems like the urinary bladder, where contractile tone must be preserved. Smooth muscle consists of thick and thin filaments that are not arranged into sarcomeres, giving it a non-striated pattern. On microscopic examination, it appears homogeneous.
The function of smooth muscle can be expanded to a much larger scale, helping to regulate organ systems. For example, in the gastrointestinal tract, smooth muscle helps with the propulsion of the food bolus, and in the cardiovascular system, it helps regulate blood flow and pressure. Smooth muscle is also found in the urinary system, where it helps rid the body of toxins and works in electrolyte balance.
Smooth muscle is derived from both mesoderm and neural crest cells, which play an important role in the development of smooth muscle throughout the body, especially in the regulation of blood vessels. Smooth muscle can be further categorised into two types: single-unit and multi-unit. Single-unit smooth muscle is much more common and is found in the walls of all visceral organs except the heart, while multi-unit smooth muscle is found in the airways of the lungs, large arteries, and ciliary muscles of the eye.
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Smooth muscle is controlled by the autonomic nervous system and acts involuntarily
There are three types of muscle tissue in the body: skeletal, smooth, and cardiac. Skeletal muscles are attached to bones and are responsible for skeletal movements. They are under conscious, or voluntary, control. The basic unit of skeletal muscle is the muscle fiber, which has many nuclei. These muscle fibers are striated, or striped, and each acts independently of neighboring muscle fibers.
Smooth muscle, on the other hand, is found in the walls of hollow internal organs such as blood vessels, the gastrointestinal tract, bladder, and uterus. Unlike skeletal muscle, smooth muscle is not under conscious control and acts involuntarily. It is controlled by the autonomic nervous system, which means it works without conscious thought. For example, smooth muscles in the urinary system help rid the body of waste and toxins, and those in arteries and veins play a vital role in regulating blood pressure and tissue oxygenation.
Smooth muscle cells have a unique shape described as fusiform or spindle-shaped, with a round center that tapers at each end. They have greater elastic properties than striated muscles, which is important for maintaining contractile tone in organs like the urinary bladder. The function of smooth muscle is essential for regulating many of the body's subsystems, such as digestion and nutrient collection in the stomach and intestines.
The contraction of smooth muscle is a complex process that involves calcium influx. Calcium enters the smooth muscle cell through depolarization, hormones, or neurotransmitters, and stimulates further calcium release from the sarcoplasmic reticulum. This process is known as calcium-induced calcium release. Once activated, calcium binds to calmodulin, which then activates the enzyme myosin light chain kinase (MLCK). This initiates a series of events leading to smooth muscle contraction.
In summary, smooth muscle is a vital component of the body's muscular system, functioning involuntarily under the control of the autonomic nervous system. Its unique properties and functions in various organs contribute to essential physiological processes that maintain overall health and homeostasis.
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Smooth muscle cells are spindle-shaped with a single nucleus
Smooth muscles, also known as smooth muscle cells (SMCs), are non-striated muscles found in the walls of hollow internal organs such as the liver, pancreas, intestines, blood vessels, gastrointestinal tract, bladder, and uterus. They are also present in the eye, where they contract to change the shape of the lens, enabling the eye to focus. Smooth muscles are responsible for involuntary movements, such as the contractions that occur during digestion to propel food through the body (peristalsis).
Smooth muscle cells are typically spindle-shaped, with a single, centrally located nucleus. They vary in size depending on their location, ranging from 20 µm in small blood vessels to 400–500 µm in the uterus. The shape of the smooth muscle has been described as fusiform, round in the center, and tapering at each end. The thin and thick contractile filaments, actin and myosin, form continuous chains within the smooth muscle cell, anchored at the dense bodies. The actin filaments align in an axial direction with a slightly oblique arrangement.
The spindle shape and small size of smooth muscle cells contribute to their unique properties, including slow response and high resistance to fatigue. They contract slowly and rhythmically, and their elastic nature allows them to tense and relax, making them well-suited for maintaining contractile tone in organs like the urinary bladder. Smooth muscle cells are surrounded by a basal lamina and variable amounts of collagen and elastic fibers, which provide additional support and flexibility.
The single nucleus of smooth muscle cells is elongated or elliptical in shape. During contraction and relaxation, the configuration of the nuclear membranes in these cells changes. Smooth muscle cells have poorly developed sarcoplasmic reticulums and do not contain T-tubules due to their restricted size. They are arranged in sheets, enabling them to contract simultaneously and generate powerful contractions under the control of involuntary mechanisms.
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Smooth muscle contains actin and myosin, which are the main proteins involved in muscle contraction
There are three types of muscle tissue: skeletal, smooth, and cardiac. Skeletal muscle is attached to bones and is responsible for skeletal movements. Smooth muscle, on the other hand, is found in the walls of hollow internal organs such as blood vessels, the gastrointestinal tract, bladder, and uterus. It is under the control of the autonomic nervous system and acts involuntarily. Smooth muscle contains thick and thin filaments that do not arrange into sarcomeres, resulting in a non-striated pattern.
Smooth muscle contains large amounts of actin and myosin, which are the main proteins involved in muscle contraction. Actin and myosin form continuous chains within the smooth muscle cell, anchored at the dense bodies. The actin filaments are thin and are attached to these dense bodies, which can be observed under an electron microscope and appear dark. The dense bodies are rich in alpha-actinin (α-actinin) and also attach to intermediate filaments, which consist largely of vimentin and desmin. The intermediate filaments are then connected to other intermediate filaments via dense bodies, which attach to the cell membrane of the smooth muscle cell.
The myosin filaments are thick and, together with actin, have the capability to contract. The ratio of actin to myosin in smooth muscle is between 2:1 and 10:1, whereas, in striated skeletal muscle, myosin is the dominant protein with a ratio of actin to myosin ranging from 1:2 to 1:3. The number of myosin filaments can change between the relaxed and contracted states as the ratio of actin to myosin changes, and the length of the myosin filaments also changes. The type of myosin present in muscle is myosin II, a very large protein consisting of two identical heavy chains and two pairs of light chains.
The regulation of actin-myosin contraction in smooth muscle is primarily mediated by the phosphorylation of one of the myosin light chains, called the regulatory light chain. This phosphorylation is catalysed by the enzyme myosin light-chain kinase, which is itself regulated by the Ca2+-binding protein calmodulin. Increases in cytosolic Ca2+ promote the binding of calmodulin to the kinase, resulting in phosphorylation of the myosin regulatory light chain. Smooth muscle contraction is also dependent on calcium influx, which can be increased through depolarization, hormones, or neurotransmitters.
Smooth muscle is important for various functions in the body, including digestion and nutrient collection in the stomach and intestines, toxin removal and electrolyte balance in the urinary system, and regulation of blood pressure and tissue oxygenation in arteries and veins. It can contract and relax on its own and has greater elastic properties compared to striated muscle, which is important for maintaining contractile tone in organs like the urinary bladder.
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Smooth muscle has greater elastic properties than striated muscle
Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. Smooth muscle is found in the walls of hollow internal organs such as the liver, pancreas, stomach, intestines, bladder, uterus, and blood vessels. It is also present in the tracts of the respiratory, urinary, and reproductive systems. Smooth muscle is non-striated, meaning it has no sarcomeres and therefore no bands or stripes. It consists of thick and thin filaments that are not arranged into sarcomeres, giving it a non-striated pattern.
The ratio of actin to myosin is between 2:1 and 10:1 in smooth muscle. In contrast, myosin is the dominant protein in striated skeletal muscle, with the actin-to-myosin ratio falling in the 1:2 to 1:3 range. Tropomyosin is present in smooth muscle, but its function is unknown. In striated muscle, tropomyosin blocks actin-myosin interactions until calcium is present. Smooth muscle also contains specific elastin and collagen receptors to interact with the proteins of the extracellular matrix. These fibers with their extracellular matrices contribute to the viscoelasticity of these tissues. For example, the great arteries are viscoelastic vessels that act like a Windkessel, smoothing out the pulsatile flow, and the smooth muscle within the tunica media contributes to this property.
Smooth muscle contraction is dependent on calcium influx. Calcium enters the smooth muscle cell through two processes: depolarization, hormones, or neurotransmitters causing calcium to enter through L-type channels, and intracellular calcium stimulating the release of calcium from the sarcoplasmic reticulum through ryanodine receptors and IP3. Once calcium has entered the cell, it binds to calmodulin, which activates the enzyme myosin light chain kinase (MLCK). MLCK then phosphorylates a regulatory light chain on myosin, leading to a conformational change in the myosin head that increases myosin ATPase activity and promotes interaction between the myosin head and actin. This results in cross-bridge cycling and the generation of tension relative to the calcium concentration within the cell.
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Frequently asked questions
Smooth muscle is not striated.
Smooth muscle is one of the three major types of vertebrate muscle tissue, the other two being skeletal and cardiac muscle. Smooth muscle cells are spindle-shaped with a single nucleus in the centre.
Smooth muscle is found in the walls of hollow internal organs such as the stomach, intestines, bladder, and uterus, as well as in the walls of blood vessels and lymph vessels.
