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Circular muscles, also known as skeletal sphincter muscles, are a type of muscle tissue that plays a crucial role in various bodily functions. These muscles are arranged in concentric rings, forming either inner or outer layers of muscular walls in different parts of the body. They are commonly found in the walls of tubular structures such as vessels, the gut, ducts, and bronchi, as well as hollow organs like the urinary bladder and uterus. The unique arrangement of circular muscles allows them to generate radial closure pressure, contributing to functions like peristalsis in the esophagus and facilitating movement in certain animals. Understanding the mechanics and physiology of circular muscle tone is essential for studying their role in maintaining body stature, producing body heat, and supporting overall health.
| Characteristics | Values |
|---|---|
| Structure | Arranged in concentric rings or fascicles |
| Location | External body openings, hollow organs, and the walls of tubular structures |
| Function | Contraction, radial closure pressure, and modification of the size of tubular structures |
| Types | Skeletal sphincter muscles, orbicular muscles, outer and inner layers |
| Composition | Muscle cells or myocytes, actin and myosin filaments |
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What You'll Learn
Circular muscle walls are part of the muscular layer
Circular muscle walls are indeed part of the muscular layer. The muscular layer (myosalpinx or tunica muscularis) consists primarily of an inner layer of circularly arranged muscle fibres and an outer layer of longitudinally oriented fibres. The muscular wall proper consists of smooth muscles arranged in an inner circular layer and an outer longitudinal layer. The thin smooth muscle layer, named muscularis mucosae (MM), penetrates the mucosa (M) and submucosa (SM) from the side of the inner circular layer of the muscular wall.
The muscularis mucosae contributes to the mucosa and produces local movement and folding of the mucosa. The smooth muscle cells (SMCs) of both muscle layers are functionally connected. The typical arrangement of the two layers of the muscular wall proper (MP) is seen in the micrographs showing the cross-section of the duodenum, jejunum, and ileum. The inner and outer layers of the smooth muscle cells adhere tightly and firmly to each other.
The contractile proteins, i.e., actin and myosin, are present in thin and thick filaments, respectively, and the ratio of the number of actin filaments to the number of myosin filaments is important. The actin filaments are anchored by dense bodies, analogous to the Z-discs of the skeletal and cardiac muscle fibres, which are the basic elements of the SMC cytoskeleton. Calcium ions, necessary for smooth muscle fibre contractions, are supplied by the sarcoplasmic reticulum in the fibres and by sequestration from the extracellular fluid through membrane indentations called caveolae.
The circular muscle layer is located between the submucosa and the outer longitudinal muscular layer. The structure of the circular layer impairs the luminal contents travelling backward along the intestinal tract. It works with the longitudinal layer to provide peristaltic contractions, which facilitate the passage of intestinal content to the anus for excretion. The circumferential "hoop" stress in the esophageal wall is a summation of neurologically induced active stress (or "tone") in the circular muscle. Changes in hoop stress are directly reflected in changes in intraluminal "closure" pressure, so manometric measurement of intraluminal pressure can be used to study the mechanics and physiology of circular muscle tone.
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They are found in the walls of hollow organs
Circular muscle walls are found in the walls of hollow organs such as the liver, pancreas, and intestines. They are also present in the esophagus, where they play a crucial role in peristalsis, the wave-like motion responsible for moving food and liquid through the digestive tract. The circular muscles in the esophagus generate radial closure pressure, contributing to the local peristaltic closure wave.
In the muscular wall of the tubal gut, the inner layer of muscles has a more circular orientation, while the outer layer has a longitudinal orientation. This arrangement is also observed in the duodenum, jejunum, and ileum segments of the small intestine. The circular muscle layer contributes to the peristaltic contractions that facilitate digestion and the movement of food.
The smooth muscle cells in the circular layer are spindle-shaped, with fibers that are wide in the middle and tapered at the ends. These cells are functionally connected to the longitudinal muscle layer, and their coordinated contractions regulate the closure pressure and maintain the necessary tension.
The presence of circular muscle walls in hollow organs is essential for their respective functions, whether it is in the digestive system, where they aid in peristalsis, or in other organ systems like the urinary bladder, where they help regulate contractile tone and maintain the body's basic functions.
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They are also found in the walls of tubular structures
Circular muscle walls are found in the walls of tubular structures such as vessels, the gut, ducts, and bronchi. They are also found in the muscularis mucosae, which is a thin layer of smooth muscle that separates the mucosa from the submucosa.
In the case of the gut, the muscularis propria is the thickest part of the gut wall and is composed of bundles of smooth muscle cells with a spiral orientation forming two distinct layers: the inner layer with a more circular orientation and the outer layer with a more longitudinal orientation.
In larger arteries, there is a thick, distinct layer of elastic fibres known as the internal elastic membrane, which provides structure and allows the vessel to stretch. The internal elastic membrane is permeated with small openings that allow for the exchange of materials between the tunics. The tunica media is the substantial middle layer of the vessel wall and is generally the thickest layer in arteries. It consists of layers of smooth muscle supported by connective tissue, with most of the elastic fibres arranged in circular sheets.
The contraction and relaxation of the circular muscles in the tunica media decrease and increase the diameter of the vessel lumen, respectively. Vasoconstriction decreases blood flow as the smooth muscle in the walls of the tunica media contracts, making the lumen narrower and increasing blood pressure. Conversely, vasodilation increases blood flow as the smooth muscle relaxes, allowing the lumen to widen and blood pressure to drop.
Circular muscles are also found in the walls of hollow organs, such as the intestines, uterus, and stomach. They surround external body openings, which they close by contracting.
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Circular muscles are also known as skeletal sphincter muscles
The iris sphincter, also known as the pupillary sphincter or sphincter pupillae, is an involuntary sphincter that regulates the constriction of the pupil in response to changes in light. The anal sphincter, located at the opening of the rectum, has both inner and outer muscles. The inner sphincter is involuntary and prevents stool from leaking out, while the outer sphincter is voluntary and allows the passing or holding of stool on demand.
The urethral sphincter, or sphincter urethrae, controls the passing of urine from the body and has inner and outer muscles that regulate urine flow through the urethra. Sphincter ani externus is another type of anal sphincter that keeps the anal opening closed by its normal contraction. The sphincter pylori is a thickening of the middle layer of stomach muscle that holds food in the stomach until it is thoroughly mixed with gastric juices.
In addition to their role in regulating bodily functions, sphincters also play a crucial part in sensory mechanisms. For example, the muscular wall of the tubal gut, called the muscularis propria, is composed of bundles of smooth muscle cells forming two distinct layers: an inner circular layer and an outer longitudinal layer. The action of these two layers creates peristaltic contractions, which are essential for the movement of substances through the body.
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They are involved in the sensory mechanism
Circular muscle walls are composed of smooth muscles arranged in an inner circular layer and an outer longitudinal layer. The muscular wall of the gut, also known as the muscularis propria, is the thickest part of the gut wall. It is made up of bundles of smooth muscle cells with a spiral orientation, forming two distinct layers. The inner layer has a more circular orientation, while the outer layer is more longitudinal.
The circular muscle layer plays a crucial role in generating radial closure pressure, leading to the formation of a local peristaltic closure wave. This mechanism is essential for the movement of food or liquid through the digestive system. The circular muscle fibres contract, reducing the width of the tube to maintain the constant volume of the body contents.
The sensory mechanism involves the reception and transmission of sensory information to the brain. This process is facilitated by sensory receptors located in various parts of the body, including the skin, muscles, organs, eyes, ears, nose, mouth, and joints. These receptors detect and respond to different stimuli, such as touch, pressure, temperature, pain, and itch.
In the context of circular muscle walls, the sensory mechanism is influenced by the contractile activity of the circular muscle fibres. The contractile proteins actin and myosin play a key role in this process. The actin filaments are anchored by dense bodies, similar to the Z-discs found in skeletal and cardiac muscle fibres. Calcium ions, essential for smooth muscle fibre contractions, are supplied by the sarcoplasmic reticulum and through membrane indentations called caveolae.
The sensory mechanism associated with circular muscle walls is also evident in the primary cilia found in certain cells. These primary cilia act as antennae for sensory reception, providing information about the oviduct lumen. This demonstrates the involvement of circular muscle walls in sensory mechanisms, particularly in relation to the body's internal processes and the maintenance of homeostasis.
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Frequently asked questions
Circular muscles, also known as skeletal sphincter muscles, are muscles with a fascicular pattern where the fascicles are arranged in concentric rings. They surround external body openings and close them by contracting.
Some examples of circular muscles include the orbicularis oculi, which covers the eye, and the orbicularis oris, which surrounds the mouth.
The function of circular muscle fibers is to generate radial closure pressure to create a local peristaltic closure wave. They also help to maintain body stature and position, joint stability, and body heat.
