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Circular muscles are those that encircle the body, running perpendicular to longitudinal muscles. They are found in the colon, oesophagus, and intestines, and are responsible for gut movements such as peristalsis. A sphincter is a type of circular muscle that controls the opening and closing of natural body passages or orifices, such as the anal sphincter, which controls the exit of faeces from the body.
| Characteristics | Values |
|---|---|
| Definition | A circular muscle is a muscle layer that encircles the body between the epidermis and longitudinal muscle layer. |
| Location | Circular muscles are found in the colon, esophagus, stomach, vas deferens of the spermatic cord, ureter, and intestines. |
| Types | Anatomical sphincters have localized circular muscle thickening, while functional sphincters achieve their function through muscle contraction around or within the structure. |
| Function | The circular muscle layer works with the longitudinal layer to provide peristaltic contractions, facilitating the passage of intestinal content for excretion. It also helps maintain constriction of natural body passages or orifices and relaxes as required for physiological functioning. |
| Control | Sphincters, a type of circular muscle, can be controlled voluntarily or involuntarily. Voluntary sphincters are supplied by somatic nerves, while involuntary sphincters are stimulated by autonomic nerves. |
| Number in the Human Body | There are over 60 types of sphincters in the human body, some of which are microscopically small. |
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What You'll Learn
Sphincters: muscles that control body openings
A sphincter is a type of circular muscle that controls the body's natural openings and passages. They are found in many animals, including humans, where there are over 60 types in the human body. Sphincters can be anatomical or functional. Anatomical sphincters have a localised and often circular muscle thickening, while functional sphincters do not have this thickening and achieve their sphincteric action through muscle contraction.
Sphincters can be voluntarily or involuntarily controlled. Voluntary sphincters are supplied by somatic nerves, while involuntary sphincters are stimulated by autonomic nerves. An example of a voluntary sphincter is the orbicularis oris muscle, a complex of muscles in the lips that encircles the mouth. The orbicularis oculi muscle, a muscle around the eye, is also under voluntary control.
Involuntary sphincters include the internal anal sphincter, which is one of two sphincters that control the exit of faeces from the body. The other sphincter controlling the exit of faeces is the external anal sphincter, which is under voluntary control. Another example of an involuntary sphincter is the lower oesophageal sphincter (or cardiac sphincter), which is closed most of the time to prevent the acidic contents of the stomach from moving upwards into the oesophagus.
Sphincters work by maintaining constriction of a body passage or orifice and relaxing as required by normal physiological functioning. When a sphincter muscle is contracted, its length is shortened and the lumen (opening) is constricted and closed. Relaxation of the muscle causes it to lengthen, opening the lumen and allowing the passage of liquids, solids, or gases. This mechanism is evident in the blowholes of numerous marine mammals.
The circular muscle layer is found in various parts of the body, including the colon, where it forms the inner layer of circularly coursing fibres in the muscular coat (tunica muscularis). The circular layer extends the width of the colon and is thicker than the outer longitudinal fibres, which are continuous along the length of the colon. The circular and longitudinal layers work together to provide peristaltic contractions, facilitating the passage of intestinal content to the anus for excretion.
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Circular fibres: encircling the body
Circular muscles are those that encircle the body and are present in many animals, including humans. They are found between the epidermis and longitudinal muscle layers. The gastrointestinal system breaks down ingested food particles into molecular forms through digestion, and the circular muscles then work with the longitudinal muscles to facilitate the passage of intestinal contents to the anus for excretion.
The muscular layer, or muscular coat, is a region of muscle in many organs in the vertebrate body, adjacent to the submucosa. The muscularis propria is comprised of a layer of circularly aligned muscle fibres within, and a layer of longitudinally aligned muscle fibres without, surrounding mucosal layers of loosely connected tissue. The muscular layer receives its arterial blood supply from permeating capillaries coming from the submucosal arteriolar network and is innervated by the myenteric plexus.
The colon provides a good example of the circular muscular layer. Here, the muscular layer is composed of smooth muscle organised into outer longitudinal and inner circular layers. These layers run perpendicular to each other. The inner circular layer extends the width of the colon, while the outer longitudinal fibres are continuous along its length. The circular layer of smooth muscle is thicker than the longitudinal layer, and it is divided by connective tissue clefts. Small fascicles of muscle pass between the circular and longitudinal layers, especially in the region of the teniae coli.
The circular muscle layer of the esophageal muscularis propria has a clear function. It generates radial closure pressure to create a local peristaltic closure wave. Peristalsis implies that the circular muscle contraction travels as a wave along the esophageal lumen to create a travelling contraction wave and a moving point of luminal closure that forces the bolus fluid ahead, ultimately into the stomach.
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Esophageal peristalsis: contraction waves in the oesophagus
A circular muscle is a muscle that normally maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning. Sphincters are an example of circular muscles.
Esophageal peristalsis is a highly sophisticated function that involves the coordinated contraction and relaxation of striated and smooth muscles in a cephalocaudal fashion. It is determined by the balance of the intrinsic excitatory cholinergic, inhibitory nitrergic, and post-inhibitory rebound excitatory output to the oesophageal musculature.
The oesophagus is made up of a layer of circularly aligned muscle fibres within, and a layer of longitudinally aligned muscle fibres without, surrounded by mucosal layers of loosely connected tissue. The muscle mass of the circular and longitudinal muscle layers is nearly the same.
The sequential contraction of the circular muscle of the oesophageal body results in a peristaltic wave that pushes food toward the stomach. Peristalsis occurs in both the smooth muscle oesophagus and the skeletal muscle oesophagus. Peristalsis in the smooth muscle oesophagus is mediated by the vagus nerve at the dorsomotor nucleus and myenteric plexus level. Peristalsis in the skeletal muscle oesophagus, on the other hand, results from the activation of neurons at the level of the vagal nucleus (nucleus ambiguus).
The smooth muscle cells responsible for peristalsis include the inner circular and outer longitudinal layers of muscle, collectively called the muscularis propria. The reflexive stimulation of afferent nerves is initiated when a food bolus causes stretch within the intestines. The efferent signal causes the inner circular muscles just before the bolus to contract and push the bolus forward, while also causing the outer longitudinal muscles to contract and shorten the tube. Descending inhibitory reflexes cause the circular muscles just beyond the bolus to relax, allowing for forward movement. The bolus moves a few centimetres during each peristalsis wave, and the process starts again.
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Muscular layers: the colon has inner circular and outer longitudinal layers
The colon is a part of the large intestine in the intestinal tract. The muscular layer of the colon consists of an inner circular layer and an outer longitudinal layer of smooth muscle. The inner circular layer is made up of circularly aligned muscle fibres, while the outer longitudinal layer is composed of longitudinally aligned muscle fibres. These layers of muscles are responsible for producing peristaltic contractions of the gut, propelling ingested substances through the lumen of the gut.
The inner circular muscle layer generates radial closure pressure to create a local peristaltic closure wave, which forces the ingested substances forward. This is known as peristalsis, a process that involves a travelling contraction wave and a moving point of luminal closure. The outer longitudinal muscle layer has both physiological and mechanical functions. It reduces the tension of individual circular muscle fibres, maintaining closure as the longitudinal muscle shortens and the circular muscle tone increases.
The muscularis propria, or muscular layer, is found in many organs in the vertebrate body and is responsible for gut movements such as peristalsis. In the colon, the longitudinal layer is incomplete and forms three distinct bands called the teniae coli. These bands are named according to their position along the colon: mesocolic, omental, and free teniae coli.
The muscularis propria contains smooth muscle cells organised into a tightly coiled, inner circular layer and an outer longitudinal layer. Between these layers are autonomic neural fibres and ganglionic clusters that form a myenteric plexus. The major function of the muscularis propria is to initiate and regulate contractile peristaltic waves, propelling food through the gut.
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Functions: radial closure pressure and reduced muscle tension
Circular muscles are a type of muscle layer that encircles the body between the epidermis and longitudinal muscle layer. They are found in the colon, oesophagus, and other parts of the gastrointestinal system. The circular muscle layer is also known as the muscularis propria, and it works with the longitudinal muscle layer to facilitate peristalsis and the passage of intestinal content.
The function of the circular muscle layer is to generate radial closure pressure, creating a local peristaltic closure wave. This radial closure pressure is essential in the oesophagus, where it prevents the acidic contents of the stomach from moving upward into the oesophagus. The lower oesophageal sphincter, for example, resides at the top of the stomach and remains closed most of the time, keeping acids and other contents in the stomach.
The radial closure pressure generated by the circular muscle layer is also important in the intestines, where it creates a travelling contraction wave, forcing the bolus fluid into the stomach. This process is known as peristalsis and is facilitated by the longitudinal muscle layer. Peristalsis also occurs in the colon, where the circular muscle layer is thicker than the longitudinal layer.
The physiological function of the circular muscle layer is to maintain closure against the resisting forces within a viscous bolus. This closure is achieved through the generation of "hoop" stresses, which create applied pressures sufficient to squeeze the lumen closed. The mechanical function of the circular muscle layer, on the other hand, is to reduce the level of pressure required to maintain closure. This reduction in pressure is achieved through the longitudinal shortening (LLS) of the longitudinal muscle layer, which reduces the tension of individual circular muscle fibres.
In summary, the circular muscle layer plays a crucial role in generating radial closure pressure and reducing muscle tension, particularly in the oesophagus, intestines, and colon. This layer works in conjunction with the longitudinal muscle layer to facilitate peristalsis and maintain the proper flow of bodily contents.
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Frequently asked questions
A circular muscle is a muscle layer that encircles the body between the epidermis and longitudinal muscle layer.
The lower oesophageal sphincter (or cardiac sphincter) is a circular muscle that sits at the top of the stomach. The orbicularis oris muscle is a complex of muscles in the lips that encircles the mouth.
Circular muscles maintain constriction of a natural body passage or orifice and relax as required by normal physiological functioning. For example, the lower oesophageal sphincter is closed most of the time, keeping the contents of the stomach from moving upwards into the oesophagus.
Circular muscles generate radial closure pressure to create a local peristaltic closure wave. Longitudinal muscles have one physiological function with mechanical implications and one purely mechanical function.
