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Muscle tissue is composed of specialised cells that can contract to produce movement in the body. There are three types of muscle tissue: skeletal, cardiac, and smooth. Smooth muscle tissue is uninucleated and nonstriated. Smooth muscle tissue is involuntary, meaning its contractions are not consciously controlled. It is found in the walls of hollow visceral organs, like the liver, pancreas, and
| Characteristics | Values |
|---|---|
| Muscle Type | Smooth muscle |
| Muscle Tissue | Involuntary muscle |
| Microscopic Appearance | No cross stripes |
| Cell Shape | Spindle-shaped |
| Cell Thickness | 3-10 micrometers |
| Cell Length | 20-200 micrometers |
| Cell Nucleus | One per cell |
| Cell Branching | No branching |
| Contraction Speed | Slow and spontaneous |
| Location | Walls of hollow visceral organs (e.g. liver, pancreas, intestines) except the heart |
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What You'll Learn
Smooth muscle cells are uninucleate and nonstriated
Muscle tissue is composed of cells that have the ability to contract and produce movement in the body. There are three types of muscle tissue: skeletal, cardiac, and smooth. Smooth muscle tissue is involuntary, meaning its contractions are not consciously controlled. Smooth muscle cells are uninucleate, meaning they have a single nucleus per cell, and nonstriated, meaning they lack striations.
Smooth muscle cells are spindle-shaped, with long tapered ends, and are typically packed together with their long axes parallel to neighbouring cells in an "interdigitating" manner. Each cell is enveloped by a basement membrane and other connective fibres, which bridge the spaces between adjacent cells. The components of the extracellular matrix are produced by the smooth muscle cells themselves, rather than fibroblasts, as seen in skeletal muscle.
Smooth muscle tissue is found in various internal organs, such as the digestive tract, liver, pancreas, and blood vessels, as well as the walls of hollow visceral organs such as the intestines, uterus, and stomach. It is also found in the walls of passages like cardiovascular arteries and veins.
The contractile proteins actin and myosin are present in smooth muscle cells but are not organised into sarcomeres, which is why they lack striations. Smooth muscle tissue contracts slowly and spontaneously compared to striated muscle.
In summary, smooth muscle tissue plays a crucial role in various involuntary bodily functions, characterised by its unique structure and location. Its smooth appearance, spindle-shaped cells, and involuntary nature are key features that define this type of muscle tissue.
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Smooth muscle cells are involuntary
Smooth muscle is one of the three main types of muscle in the human body, the other two being skeletal and cardiac muscle. Smooth muscle is also referred to as involuntary muscle. This is because smooth muscle movements happen without conscious thought. Smooth muscle cells are generally confined to internal organs, such as blood vessels, the gastrointestinal tract, and the respiratory system.
Smooth muscle cells are spindle-shaped and have a single, elongated nucleus. They are unbranched and lack striations. They are controlled by the autonomic nervous system and are capable of responding to local physiological conditions by remodelling their structure. Smooth muscle cells are multifunctional and exhibit spontaneous and agonist-induced contractile properties. They are also involved in the secretion and assembly of a wide variety of extracellular matrix proteins.
The thin (actin) and thick (myosin) contractile filaments of smooth muscle cells are easily visible in electron microscopic preparations. The thin filaments fill most of the cytoplasm and are 4-8 nm thick. They insert into condensations of electron-dense material, known as "dense bodies", located subjacent to the plasma membranes. The external laminae of smooth muscle cells are well-developed, and transport vesicles are numerous along their surfaces. The shape of smooth muscle is fusiform, which is round in the centre and tapering at each end.
Smooth muscle is present throughout the body and serves a variety of functions. It is found in the stomach, intestines, urinary system, arteries, and veins. It helps with digestion, nutrient collection, toxin removal, and the regulation of blood pressure and tissue oxygenation. Smooth muscle is also involved in vascular development and stability. It wraps around larger vessels and influences the regulation of blood flow, endothelial network growth, and vessel stability.
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Smooth muscle cells are spindle-shaped
Smooth muscle can be divided into two subgroups: single-unit and multi-unit. Single-unit smooth muscle contracts as a syncytium, meaning the whole bundle or sheet of cells contracts together. This is due to the presence of many gap junctions between the cells, allowing an action potential to be propagated through neighbouring cells. An example of this is the uterine muscles during childbirth.
Multi-unit smooth muscle, on the other hand, is neurogenic, meaning its contraction must be initiated by an autonomic nervous system neuron. However, it is important to note that the distinction between single- and multi-unit smooth muscle is an oversimplification. Smooth muscle function is often influenced by a combination of different neural elements, and cell-to-cell communication is common even in multi-unit smooth muscle.
Smooth muscle tissue demonstrates greater elasticity and function within a larger length-tension curve than striated muscle. This ability to stretch and maintain contractility is important in organs like the intestines and urinary bladder. Smooth muscle is found in the walls of hollow organs, including the stomach, intestines, bladder, and uterus, as well as in the tracts of the respiratory, urinary, and reproductive systems.
The actin and myosin fibres in smooth muscle cells are analogous to the Z-discs in striated muscle sarcomeres. These fibres are attached to dense bodies, which are rich in alpha-actinin and also attach to intermediate filaments. The dense bodies appear darker under an electron microscope and serve as anchors from which the thin filaments can exert force.
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Smooth muscle cells are found in hollow organs
Smooth muscle cells are found in the walls of hollow organs, such as the urinary bladder, stomach, and intestines, as well as in tubular structures like blood vessels and bile ducts. Smooth muscle is also present in the uterus, eye (including the iris), skin (including the erector pili muscle), and even the brain. Smooth muscle is a type of involuntary muscle that lacks striations, meaning it appears smooth when examined under a microscope. It is composed of spindle-shaped cells with a single, centrally located nucleus. These cells form sheets or chains of tissue and work together in a coordinated fashion due to the presence of gap junctions between them.
Smooth muscle plays a crucial role in various organ systems, such as the gastrointestinal tract, where it aids in the propulsion of food. It is also found in the urinary system, where it helps rid the body of toxins and maintains electrolyte balance. In the cardiovascular system, smooth muscle is essential for regulating blood pressure and tissue oxygenation. Additionally, smooth muscle can be found in the walls of visceral organs, where it can be stretched to trigger its contraction, and it contains pacesetter cells that can spontaneously initiate contractions.
The unique structure of smooth muscle cells, with their actin and myosin filaments arranged in a mesh-like network, enables them to contract uniformly in a spiral corkscrew fashion. This contraction is further enhanced by connexins, which allow for intercellular communication and the flow of calcium and other molecules between neighbouring smooth muscle cells. Smooth muscle can also relax through the process of dephosphorylation of myosin light chains, which is facilitated by myosin light chain phosphatase (MLCP).
Smooth muscle differs from skeletal muscle in several ways. Firstly, it is controlled involuntarily by the nervous system, allowing the body to regulate subsystems without conscious thought. Secondly, smooth muscle cells are uninucleate, with a single nucleus, while skeletal muscle cells are multinucleated. Finally, smooth muscle can contract over a wider range of resting lengths due to the more flexible arrangement of actin and myosin filaments compared to skeletal muscle.
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Smooth muscle tissue contracts slowly
Smooth muscle tissue is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. Smooth muscle tissue is uninucleated and nonstriated. It is composed of small, spindle-shaped cells with a single, central nucleus. Smooth muscle tissue is present in the walls of hollow organs, such as the stomach, intestines, bladder, and uterus, and in the walls of blood vessels and lymph vessels. It is also found in the eye, the skin, the tracts of the respiratory, urinary, and reproductive systems, and throughout arteries and veins.
The primary function of smooth muscle is contraction. Smooth muscle differs from skeletal muscle in that it can be contracted and controlled involuntarily. Smooth muscle contraction is initiated when calcium ions (Ca++) bind to intracellular calmodulin, which then activates an enzyme called myosin kinase. This enzyme phosphorylates myosin heads so they can form cross-bridges with actin and pull on the thin filaments. Actin and myosin are the main proteins involved in muscle contraction.
Smooth muscle-containing tissue needs to be stretched often, so elasticity is an important attribute. Smooth muscle cells secrete a complex extracellular matrix containing collagen, elastin, glycoproteins, and proteoglycans. These fibers contribute to the viscoelasticity of these tissues, allowing them to act like a Windkessel, propagating ventricular contraction, and smoothing out the pulsatile flow.
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Frequently asked questions
Smooth muscle tissue is uninucleated and non-striated.
Smooth muscle tissue is a type of muscle tissue that is involuntary, meaning its contractions are not consciously controlled. Smooth muscle cells are spindle-shaped and unbranched, with a single nucleus per cell. They are found in the walls of hollow visceral organs, such as the liver, pancreas, intestines, and uterus, as well as in blood vessels and the walls of passages like arteries and veins.
In addition to smooth muscle tissue, there are two other types of muscle tissue: skeletal muscle tissue and cardiac muscle tissue. Skeletal muscle tissue is voluntary, striated, and multinucleated, while cardiac muscle tissue is involuntary, striated, and typically uninucleate.
