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There are three types of muscle tissue in the body: skeletal, smooth, and cardiac. Skeletal and cardiac muscles are striated, meaning they have a striped appearance under a microscope. Smooth muscle is non-striated and lacks these characteristic stripes. 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. It is controlled by the autonomic nervous system and acts involuntarily. In this paragraph, we will focus on smooth muscle, exploring its structure, function, and unique characteristics that set it apart from striated muscles.
| Characteristics | Values |
|---|---|
| Muscle Type | Smooth Muscle |
| Striated | No |
| Location | Walls of hollow visceral organs (liver, pancreas, intestines, etc.), except the heart |
| Shape | Spindle-shaped |
| Control | Involuntary |
| Contraction | Slow and rhythmic |
| Nucleus | One central nucleus |
| Subgroups | Single-unit and multi-unit smooth muscle |
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What You'll Learn
Smooth muscle is found in the walls of hollow organs
Smooth muscle is a type of tissue found in the walls of hollow organs, such as the stomach, bladder, liver, pancreas, uterus, and intestines. It is also present in the walls of passageways, including arteries, veins, and bile ducts, as well as in sphincters and ducts of exocrine glands. Smooth muscle is a non-striated muscle, meaning it does not exhibit the transverse streaks seen in skeletal muscle. Instead, it appears homogeneous under microscopic examination.
Smooth muscle plays a crucial role in various organ systems, including the gastrointestinal tract, where it propels food through wavelike contractions. In the cardiovascular system, smooth muscle regulates blood flow and pressure by controlling the diameter of blood vessels. This regulation is achieved through the contraction and relaxation of smooth muscle cells, which is facilitated by the presence of thick and thin filaments composed primarily of the proteins actin and myosin.
The shape of smooth muscle cells is described as fusiform, with a rounded center that tapers at each end. These cells are smaller than skeletal muscle cells, typically measuring 3-10 µm in thickness and 20-200 µm in length. Smooth muscle is functionally divided into two types: single-unit and multi-unit. Single-unit smooth muscle cells are electrically connected by gap junctions, allowing them to contract uniformly. This type is commonly found in the walls of hollow visceral organs, such as the stomach and urinary bladder, where it exhibits a unique stress-relaxation response. As the organ stretches when filling, the mechanical stress triggers a contraction, which is then followed by relaxation, preventing premature emptying.
Multi-unit smooth muscle cells, on the other hand, are independent of each other and require individual innervation, allowing for more precise control. These cells are found in the airways of the lungs, large arteries, ciliary muscles of the eye, and hair erector muscles. Smooth muscle is an involuntary muscle, meaning it cannot be controlled consciously. Its contraction is triggered by hormones, neural stimulation, and local factors, and it plays a vital role in maintaining muscle tone in various organ systems.
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Smooth muscle is controlled by the autonomic nervous system
There are three types of muscles in the body: skeletal, cardiac, and smooth muscle. Skeletal muscles are attached to bones and are responsible for skeletal movements. They are under conscious, or voluntary, control.
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 muscles, smooth muscle is under the control of the autonomic nervous system. This means that smooth muscle contracts and relaxes involuntarily, without conscious thought.
The autonomic nervous system regulates smooth muscle cells through nonadrenergic noncholinergic (NANC) nerve fibers. NANC neurotransmitters play a crucial role in the contraction and relaxation of smooth muscle cells, influencing important physiological processes such as blood pressure regulation and the motility of the digestive, respiratory, and urinary tracts.
Research has identified various neurotransmitters that may be involved in NANC neurotransmission in smooth muscle. For example, ATP and nitric oxide (NO) have been implicated in the digestive, respiratory, and urinary tracts. Additionally, peptides such as VIP and substance P are also candidates for excitatory and inhibitory NANC neurotransmission.
The contraction and relaxation of smooth muscle through the autonomic nervous system's NANC nerve fibers are essential for maintaining homeostasis and proper organ function. This involuntary control ensures that vital physiological processes occur without conscious effort, contributing to the overall health and stability of the body.
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Smooth muscle has no sarcomeres
Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. Smooth muscle is non-striated, meaning it has no sarcomeres and therefore no striations (bands or stripes). Smooth muscle can be found in the walls of hollow organs, including the stomach, intestines, bladder, and uterus, as well as in the walls of blood vessels and lymph vessels (excluding blood and lymph capillaries). It is also found in the tracts of the respiratory, urinary, and reproductive systems, such as the trachea and the iris of the eye.
Smooth muscle is controlled by the autonomic nervous system and acts involuntarily. It cannot be controlled consciously. Smooth muscle contracts slowly and rhythmically, generating force and changing length to control the organ's dimension and mechanical function. Smooth muscle differs from skeletal and cardiac muscle in terms of structure, function, and regulation of contraction.
Skeletal muscle, on the other hand, is attached to bones and is responsible for skeletal movements. It is under voluntary control, meaning it can be controlled consciously. Skeletal muscle fibres are striated in appearance, having transverse streaks. Cardiac muscle, found in the walls of the heart, is also under the control of the autonomic nervous system. Cardiac muscle cells have one central nucleus and are rectangular in shape. The contraction of cardiac muscle is involuntary, strong, and rhythmical.
The lack of sarcomeres in smooth muscle is due to the arrangement of filaments. In smooth muscle, the filaments are not arranged in repeated patterns of light and dark bands, resulting in a non-striated appearance. However, smooth muscle still contains the proteins actin and myosin, which are essential for muscle contraction. Actin filaments are attached to dense bodies, which serve as anchors for the thin filaments and coordinate tensions from both the contractile machinery and the cytoskeleton.
While significant progress has been made in understanding the actomyosin interaction in smooth muscle, the sarcomeric structure that allows for the sliding of contractile filaments and cell shortening has not yet been fully elucidated. Evidence suggests a model of a malleable sarcomeric structure composed of contractile units assembled in series and in parallel, with the ability to alter the geometric organization and dimensions of individual contractile units when adapting to different lengths.
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Smooth muscle is usually active, even without neural stimulation
Smooth muscle is one of the three major types of vertebrate muscle tissue, the other two being skeletal and cardiac muscle. Smooth muscle is non-striated, meaning it has no sarcomeres and therefore no striations (bands or stripes). It is found in the walls of hollow organs, including the stomach, intestines, bladder, and uterus, as well as in the walls of blood vessels and lymph vessels. Smooth muscle is controlled by the autonomic nervous system and acts involuntarily.
Smooth muscle contraction is essential for maintaining vital functions in the body. For example, smooth muscle contraction in the digestive tract helps propel food through the system. Smooth muscle is also involved in regulating blood pressure and blood flow by contracting and relaxing the walls of blood vessels. This regulation is achieved through the release of neurotransmitters and hormones that stimulate smooth muscle contraction or relaxation.
The contraction of smooth muscle is also regulated by the regulatory protein calmodulin. Calcium binds to calmodulin, forming a calcium-calmodulin complex that activates myosin light-chain kinase (MLCK). MLCK phosphorylates the light chains of myosin, making them active and allowing contraction to occur. This process is essential for the formation of cross-bridges between myosin and actin, which leads to muscle contraction.
Smooth muscle plays a crucial role in various physiological processes, and its ability to remain active even without neural stimulation ensures the continuous regulation of vital functions in the body. However, it is important to note that while smooth muscle can be active without neural stimulation, it can also be influenced by neural and hormonal factors, depending on its location and function.
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Smooth muscle is involuntary
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 hollow visceral organs such as the liver, pancreas, stomach, intestines, bladder, and uterus, as well as in the walls of blood and lymph vessels. It is also present in the tracts of the respiratory, urinary, and reproductive systems.
Smooth muscle is unique in that it is an involuntary, non-striated muscle. This means that it cannot be consciously controlled and acts involuntarily. Smooth muscle contains thick and thin filaments that do not arrange into sarcomeres, resulting in a non-striated pattern. On microscopic examination, it appears homogeneous.
The primary function of smooth muscle is contraction. Smooth muscle consists of two types: single-unit and multi-unit. Single-unit smooth muscle consists of multiple cells connected through connexins that can become stimulated in a synchronous pattern from a single synaptic input. Single-unit smooth muscle can contract regularly without input from a motor neuron, while multi-unit smooth muscle requires initiation by an autonomic nervous system neuron.
Smooth muscle plays a vital role in various bodily functions, 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. Smooth muscle also contributes to the stability and regulation of blood flow in blood vessels. Without these functions, the body would not be able to maintain its basic functions.
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Frequently asked questions
Smooth muscle is the only type of muscle tissue that is non-striated.
Striated muscles have a striped appearance under a microscope due to the regular arrangement of contractile proteins (actin and myosin).
Smooth muscle is found in the walls of hollow organs such as the liver, pancreas, stomach, intestines, bladder, uterus, and heart. It is also found in the tracts of the respiratory, urinary, and reproductive systems, as well as in the eyes.
