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Smooth muscle is one of the three major types of vertebrate muscle tissue, the other two being skeletal and cardiac muscle. It is present throughout the body and serves a variety of functions. Smooth muscle is found in the stomach, intestines, urinary system, arteries, veins, and in almost all organ systems. It is an involuntary, non-striated muscle, meaning it contracts and relaxes without conscious control and does not show stripes under a microscope. Smooth muscle cells are anchored to the surrounding connective tissue and contain thick and thin filaments that give them their unique properties. Smooth muscle plays a vital role in the regulation of blood pressure and tissue oxygenation, as well as in the disease process throughout the body.
| Characteristics | Values |
|---|---|
| Type of muscle tissue | Smooth muscle |
| Location | Walls of hollow organs, including the stomach, intestines, bladder, and uterus; walls of passageways, including arteries and veins; tracts of the urinary, respiratory, and reproductive systems; skin; eyes |
| Controlled by | Autonomic nervous system |
| Striated | No |
| Type of contraction | Involuntary |
| Structure | Thick and thin filaments that do not arrange into sarcomeres, resulting in a non-striated pattern; actin filaments attach to dense bodies spread throughout the cell; calcium-containing sarcoplasmic reticulum aids in sustaining contraction; fusiform shape |
| Composition | Actin, myosin, calmodulin, caldesmon, calponin, tropomyosin, collagen, elastin |
| Actin-to-myosin ratio | Between 2:1 and 10:1 |
| Cell size | 3-10 µm thick and 20-200 µm long |
| Cell shape | Spindle-shaped with a single, centrally located nucleus |
| Subtypes | Single-unit and multi-unit |
| Function | Digestion and nutrient collection; toxin removal and electrolyte balance in the urinary system; regulation of blood pressure and tissue oxygenation; sealing orifices (e.g. pylorus, uterine os); transport of chyme through wavelike contractions of the intestinal tube (peristalsis); regulation of blood flow and pressure via vascular resistance |
| Role in disease | Multisystemic smooth muscle dysfunction syndrome is a fatal genetic condition; anti-smooth muscle antibodies (ASMA) can be a symptom of autoimmune disorders; smooth muscle tumors are usually benign (leiomyomas) but can also be malignant (leiomyosarcomas) |
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What You'll Learn
- Smooth muscle is one of the three major types of vertebrate muscle tissue
- Smooth muscle is found throughout the body and serves a variety of functions
- Smooth muscle differs from skeletal muscle in a variety of ways
- Smooth muscle consists of thick and thin filaments that are not arranged into sarcomeres
- Smooth muscle is grouped into two types: single-unit and multi-unit
Smooth muscle is one of the three major types of vertebrate muscle tissue
Smooth muscle is an involuntary, non-striated muscle, 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. On microscopic examination, it appears homogeneous. Smooth muscle cytoplasm contains large amounts of actin and myosin, which are the main proteins involved in muscle contraction. Actin filaments attach to dense bodies spread throughout the cell, and these can be observed under an electron microscope, appearing dark. Another important structure is the calcium-containing sarcoplasmic reticulum, which aids in sustaining contraction. The shape of smooth muscle is fusiform, which is round in the centre and tapers at each end.
Smooth muscle can be divided into two subgroups: single-unit and multi-unit smooth muscle. Most smooth muscle is of the single-unit type and is found in the walls of most internal organs, blood vessels (excluding large elastic arteries), the urinary tract, and the digestive tract. In single-unit smooth muscle, a single cell in a bundle is innervated by an autonomic nerve fibre (myogenic). This means that an action potential can be propagated through neighbouring muscle cells due to the presence of many gap junctions between the cells. As a result, single-unit bundles form a syncytium that contracts in a coordinated fashion, causing the whole muscle to contract or relax, such as during childbirth.
Multi-unit smooth muscle, on the other hand, is found in the trachea, the iris of the eye, and lining the large elastic arteries. Smooth muscle differs from skeletal and cardiac muscle in terms of structure, function, regulation of contraction, and excitation-contraction coupling. Smooth muscle tissue demonstrates greater elasticity and function within a larger length-tension curve than striated muscle. This ability to stretch and still maintain contractility is important in certain organs, such as the intestines and urinary bladder.
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Smooth muscle is found throughout the body and serves a variety of functions
Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. Smooth muscle is found throughout the body and serves a variety of functions. It is present in the walls of most internal organs, blood vessels, the urinary tract, and the digestive tract. Smooth muscle differs from skeletal muscle in that it is non-striated, meaning it has no sarcomeres and therefore no bands or stripes. It can be further divided into two subgroups: single-unit and multi-unit smooth muscle.
Single-unit smooth muscle, also known as visceral smooth muscle, is found in the walls of most internal organs, including the stomach, intestines, bladder, and uterus. It is also present in the urinary tract and the digestive tract. In single-unit smooth muscle, a single cell in a bundle is innervated by an autonomic nerve fiber, allowing for cell-to-cell communication and a coordinated response. This type of smooth muscle is usually active, even without neural stimulation, and can contract regularly without input from a motor neuron.
Multi-unit smooth muscle, on the other hand, is found in the trachea, the iris of the eye, and lining the large elastic arteries. While the terms single- and multi-unit smooth muscle are useful distinctions, it is important to note that smooth muscles are generally controlled and influenced by a combination of different neural elements. Smooth muscle plays a vital role in the regulation of blood pressure and tissue oxygenation, as well as in the disease process throughout the body.
For example, in the gastrointestinal tract, smooth muscle is responsible for motility and nutrition collection. Any damage to the smooth muscle of the intestines can have a devastating impact on the body, leading to a condition called gastroparesis, which is a loss of motility. In the cardiovascular system, smooth muscle helps maintain blood pressure and flow, while in the lungs, it opens and closes airways. Smooth muscle is also involved in the treatment of asthma, where bronchodilators are used to relax airway smooth muscle and improve breathing.
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Smooth muscle differs from skeletal muscle in a variety of ways
Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. Smooth muscle is present throughout the body, serving a variety of functions. It is found in the stomach and intestines, where it helps with digestion and nutrient collection. It is also present throughout the urinary system, where it helps rid the body of toxins and works in electrolyte balance. Smooth muscle differs from skeletal muscle in a variety of ways, including structure, function, regulation of contraction, and excitation-contraction coupling.
Structurally, skeletal muscles consist of long, multinucleated fibres, while smooth muscles have shorter, spindle-shaped cells with a single nucleus. Skeletal muscles appear striated or striped under a microscope due to the arrangement of muscle fibres. On the other hand, smooth muscles are non-striated, meaning they do not exhibit stripes or bands. This is because they lack sarcomeres, which are present in skeletal muscles. Smooth muscles have thick and thin filaments that do not arrange into sarcomeres, resulting in a non-striated pattern.
Functionally, skeletal muscles are voluntary muscles, meaning individuals have conscious control over their contractions. These muscles enable movement, such as walking, running, and grasping objects. They work in pairs, with one muscle contracting while the other relaxes, providing the necessary antagonistic action for movement. In contrast, smooth muscles are involuntary muscles, contracting and relaxing without conscious control. Their contractions are slow and sustained, facilitating functions like digestion, blood circulation, and organ processes. Smooth muscles contract rhythmically and continuously, maintaining a steady pressure or flow within organs.
In terms of regulation of contraction, skeletal muscle contractions rely solely on neural input, with signals from the nervous system orchestrating the muscle's movements. Calcium plays a pivotal role in initiating skeletal muscle contractions by binding with troponin. On the other hand, smooth muscle contractions are influenced by neural, hormonal, or chemical changes, allowing them to adapt to various internal and external stimuli. Smooth muscle contains a large amount of actin and myosin, which are the main proteins involved in its contraction. The ratio of actin to myosin is between 2:1 and 10:1 in smooth muscle, while in skeletal muscle, myosin is the dominant protein.
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Smooth muscle consists of thick and thin filaments that are not arranged into sarcomeres
Smooth muscle is one of the three major types of vertebrate muscle tissue, the other two being skeletal and cardiac muscle. Smooth muscle is found in the walls of most internal organs, blood vessels, the urinary tract, the digestive tract, and the respiratory, urinary, and reproductive systems. It is also present in invertebrates and is controlled by the autonomic nervous system. Smooth muscle is unique in that it can be contracted and controlled involuntarily.
At a cellular level, smooth muscle can be described as an involuntary, non-striated muscle. It consists of thick and thin filaments that are not arranged into sarcomeres, giving it a non-striated pattern. A sarcomere is defined as the region of a myofibril contained between two cytoskeletal structures called Z-discs (or Z-lines/bands). The striated appearance of skeletal muscle fibres is due to the arrangement of thick and thin myofilaments within each sarcomere. When a skeletal muscle fibre is activated, cross-bridges form between the thick and thin filaments, and the thin filaments are pulled, sliding past the thick filaments within the sarcomeres.
In contrast, smooth muscle does not contain these striations or bands because it lacks sarcomeres. On microscopic examination, it appears homogenous. The smooth muscle cytoplasm contains a large amount of actin and myosin, which are the main proteins involved in muscle contraction. Actin filaments attach to dense bodies spread throughout the cell, and these can be observed under an electron microscope, appearing dark. Another important structure is the calcium-containing sarcoplasmic reticulum, which aids in sustaining contraction.
The shape of the smooth muscle is described as fusiform, which is round in the centre and tapers at each end. Smooth muscle can tense and relax and has greater elastic properties than striated muscle. This quality is important in organ systems like the urinary bladder, where the preservation of contractile tone is necessary. Actin and myosin form continuous chains within the smooth muscle cell, anchored at the dense bodies. The intermediate and thin filaments formed by these chains can then stretch to dense bodies located on adjacent smooth muscle cells, forming a mesh-like network.
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Smooth muscle is grouped into two types: single-unit and multi-unit
Smooth muscle is one of the three major types of vertebrate muscle tissue, the other two being skeletal and cardiac muscle. Smooth muscle is found throughout the body and serves a variety of functions. It is present in the walls of hollow organs, including the stomach, intestines, bladder, and uterus. It is also found in the walls of blood and lymph vessels (excluding blood and lymph capillaries), where it is known as vascular smooth muscle. Additionally, smooth muscle is present in the tracts of the respiratory, urinary, and reproductive systems, as well as in the eyes, where it forms the ciliary muscles, iris dilator muscle, and iris sphincter muscle.
Smooth muscle can be further categorized into two types: single-unit and multi-unit. Single-unit smooth muscle, also known as visceral smooth muscle, is the most common type and is found in the walls of most internal organs (viscera) and lines the blood vessels (except large elastic arteries), the urinary tract, and the digestive tract. It is characterized by the presence of numerous gap junctions between the cells, allowing for cell-to-cell communication and the propagation of action potentials. This results in synchronous contractions, where the entire bundle or sheet of smooth muscle cells contracts as a single unit. Single-unit smooth muscle can also contract regularly without input from a motor neuron due to its myogenic nature.
Multi-unit smooth muscle, on the other hand, is found in the trachea, the iris of the eye, and lining the large elastic arteries. Unlike single-unit smooth muscle, multi-unit smooth muscle cells rarely possess gap junctions and, therefore, do not exhibit synchronous contractions. Each cell in multi-unit smooth muscle receives its synaptic input, allowing for finer control.
While the terms single-unit and multi-unit provide a basic understanding of smooth muscle types, it is important to note that smooth muscles are primarily controlled and influenced by a combination of different neural elements. There is often some degree of cell-to-cell communication and local production of activators/inhibitors, resulting in a somewhat coordinated response even in multi-unit smooth muscle.
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Frequently asked questions
Soft muscle, or smooth muscle, is one of the three major types of vertebrate muscle tissue. It is found throughout the body and serves a variety of functions. Smooth muscle is an involuntary, non-striated muscle that appears homogenous under a microscope.
Soft muscle is found in the stomach, intestines, bladder, uterus, and the walls of most internal organs. It is also present in the urinary, respiratory, and reproductive systems, as well as in the skin and eyes.
Smooth muscle consists of thick and thin filaments that are not arranged into sarcomeres, giving it a non-striated pattern. Actin and myosin, the main proteins involved in muscle contraction, form continuous chains within the smooth muscle cell. Smooth muscle can contract and relax, and it does so involuntarily.
