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Circular muscles are a type of muscle layer that encircles the body between the epidermis and the longitudinal muscle layer. They are also known as sphincters, which are found in many animals, including humans. Sphincters are circular muscles that maintain constriction of natural body passages or orifices, such as the lower oesophageal sphincter, and relax as required by normal physiological functioning. The body contents can be deformed into different shapes, but they maintain a constant volume.
| Characteristics | Values |
|---|---|
| Definition | A circular muscle is a muscle layer that encircles the body between the epidermis and the longitudinal muscle layer. |
| Types | There are over 60 types of circular muscles in the human body, including the orbicularis oris muscle, orbicularis oculi muscle, and sphincters. |
| Function | Sphincters, a type of circular muscle, maintain constriction of a natural body passage or orifice and relax as required by normal physiological functioning. |
| Location | Circular muscles are located in various parts of the body, including the gastrointestinal system, eyes, and lips. |
| Examples | Examples of sphincters include the lower oesophageal sphincter, pyloric sphincter, and anal sphincters. |
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What You'll Learn
Sphincters
A sphincter is a circular muscle that maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning. There are over 60 types of sphincters in the human body, some of which are microscopically small, such as the millions of precapillary sphincters. Sphincters can be categorised into functional and anatomical sphincters. Anatomical sphincters have a localised and often circular muscle thickening to facilitate their action as a sphincter. Functional sphincters, on the other hand, do not have this localised muscle thickening and achieve their sphincteric action through muscle contraction around or within the structure.
Many sphincters are used every day in the normal course of digestion. For instance, the lower oesophageal sphincter, or cardiac sphincter, resides at the top of the stomach and is closed most of the time, preventing acids and other contents of the stomach from moving upward into the oesophagus. The pyloric sphincter, located at the lower end of the stomach, is another important digestive sphincter. The sphincter of Oddi, or Glisson's sphincter, controls secretions from the liver, pancreas, and gallbladder into the duodenum.
In addition to the gastrointestinal tract, sphincters are also found in the urinary tract, blood vessels, and eyes. The precapillary sphincters are the most numerous type of sphincter in the human body, regulating the flow of blood into the capillaries. The sphincter pupillae, or pupillary sphincter, belonging to the iris in the eye, is another example of a sphincter that regulates the flow of light into the eye by controlling the size of the pupil.
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Anal sphincters
A sphincter is a circular muscle that maintains constriction of a natural body passage or orifice and relaxes as required by normal physiological functioning. There are over 60 types of sphincters in the human body, some of which are microscopically small. Sphincters can be categorised as anatomical or functional. Anatomical sphincters have a localised and often circular muscle thickening to facilitate their action as a sphincter. Functional sphincters do not have this localised muscle thickening and achieve their sphincteric action through muscle contraction around or within the structure. Sphincters can also be categorised as voluntary or involuntary. Voluntary sphincters are supplied by somatic nerves, whereas involuntary sphincters are stimulated by autonomic nerves.
The anal sphincter is a group of muscles at the end of the rectum, which is located at the last few inches of the large intestine (colon). The anal sphincter surrounds the anus and controls the release of stool, thereby maintaining continence. There are two types of anal sphincters: the internal anal sphincter and the external anal sphincter. The internal anal sphincter is a smooth muscle, measuring about 30mm in height and 3mm in thickness, and originates from extensions of the circular layers of the rectum into the anal canal. The external anal sphincter is a skeletal muscle made up of striated fibres. Its exact morphology is much more complex as it is composed of three different parts that loop around the anal canal one above the other. The internal anal sphincter is involuntary, whereas the external anal sphincter is voluntary.
The anal sphincter reflex is a process that occurs when stool moving through the digestive tract triggers the internal anal sphincter to relax, which in turn makes the external anal sphincter contract. This response is also called the defecation reflex. The internal and external anal sphincters work together to produce a bowel movement. If there is a loss of muscle control in the sphincter muscles, faecal incontinence may occur. Obstetric anal sphincter injury (OASIS) is associated with third- and fourth-degree perineal tears, which involve the anal sphincters. Pelvic floor physical therapy (PFPT) is a non-invasive treatment that can be used to treat faecal incontinence.
Several diseases, conditions, and injuries can affect the anal sphincter. Anal stenosis is a condition in which the internal anal sphincter becomes narrowed to the point where it is difficult to have a bowel movement. Anismus is a functional disorder of the pelvic floor muscles, including the external anal sphincter, caused by a problem with the way these muscles contract and relax during defecation. An anal fissure is a tear in the lining of the anus that often extends into the internal anal sphincter.
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Orbicularis oris muscle
The orbicularis oris muscle is a complex of muscles in the lips that encircles the mouth. It is composed of four independent quadrants that interlace, giving the appearance of circularity. The muscle closes the mouth and puckers the lips when it contracts.
The orbicularis oris is made up of numerous strata of muscular fibres surrounding the orifice of the mouth. It is formed of two parts: a larger peripheral part and a smaller marginal part. The peripheral portion, or extrinsic portion, originates from the modiolus, arising from the fibres of five of the many buccolabial muscles that attach to it. These include the levator anguli oris, depressor anguli oris, buccinator, risorius, and zygomaticus major muscles. The marginal portion, or intrinsic portion, consists of a single or double band of muscle fibres that stem from the modiolus toward the midline, passing through the connective tissue of the lips.
The orbicularis oris is part of a large group of muscles of facial expression called the buccolabial group. The function of the orbicularis oris is to control the shape and movements of the lips. It is also one of the muscles used in the playing of all brass instruments and some woodwind instruments.
The orbicularis oris muscle develops from two embryonic laminae: the mandibular lamina and the infraorbital lamina. The main blood supply to the muscle is from the facial artery, specifically the superior and inferior labial branches. It also receives supply from the maxillary artery and the superficial temporal artery.
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Oesophageal sphincter
The oesophagus is a fibromuscular tube that is approximately 25 cm in length and transports food from the throat to the stomach. It has two functional sphincters: the upper and lower oesophageal sphincters. The upper oesophageal sphincter (UES) is a high-pressure zone at the transition between the pharynx and the cervical oesophagus. It is a round group of striated muscles that opens and closes the oesophagus. It is usually closed when a person is not eating to prevent air from entering the oesophagus, such as when a person is talking. It also prevents the contents of the oesophagus from flowing back up into the throat.
The lower oesophageal sphincter (LES) is a high-pressure zone located where the oesophagus meets the stomach. It is composed of intrinsic and extrinsic components. The intrinsic component of the LES consists of smooth muscle that keeps the sphincter tightly contracted. The smooth muscles in the LES relax, opening the sphincter and enabling the passage of food. The LES does not possess dilator muscles, and its opening relies solely on the relaxation of the smooth muscles. The extrinsic component consists of the diaphragmatic crura and the phrenoesophageal ligament, which provide anatomical support to the LES and further protection against gastric reflux.
The UES and LES create rhythmic contractions that move food downward and out of the oesophagus. Any malfunction in either of these sphincters can lead to pathologies. For example, weakness in or pressure on the LES may cause food to flow back up the oesophagus, causing reflux and the sensation of heartburn. Conversely, when the LES does not relax to allow food to pass through, food may remain in the oesophagus. Dysfunction of the UES can make swallowing food difficult or painful, a phenomenon called dysphagia.
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Circular muscle fibres
Circular muscles are those that encircle the body between the epidermis and the longitudinal muscle layer. They are also known as sphincters, which are muscles that maintain constriction of a natural body passage or orifice and relax as required by normal physiological functioning. There are over 60 types of sphincters in the human body, some of which are microscopic in size. Sphincters can be voluntarily or involuntarily controlled. An example of a voluntary sphincter is the orbicularis oris muscle, which is a complex of muscles in the lips that encircle the mouth. An example of an involuntary sphincter is the internal anal sphincter, which controls the exit of faeces from the body.
The human uterus also contains circular muscle fibres. The uterus is a hollow, thick-walled and contractile organ that is composed of three layers or strata: an outer layer of predominantly horizontal and longitudinal fibres; a middle layer of circular fibres arranged in multiple directions containing blood vessels; and an inner layer of similar structure to the outer layer.
The electrical activity of circular muscle fibres differs from that of longitudinal muscle fibres. For example, the membrane potential of the electrically quiescent circular muscle fibre is higher than that of the spontaneously active longitudinal muscle fibre. Furthermore, the length constant of circular muscle fibres is longer than that of longitudinal muscles.
The gastrointestinal system breaks down ingested food particles through digestion and then transfers them to the internal environment through absorption.
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Frequently asked questions
Circular muscles are a layer of muscles that encircle the body between the epidermis and longitudinal muscle layer. They are also known as skeletal sphincter muscles.
Circular muscles surround external body openings, such as the mouth and eyes. They are also found in the oesophagus, stomach, and intestines.
Circular muscles close body openings by contracting. They are essential for digestive tract function, maintaining constriction of natural body passages or orifices, and relaxing to allow the passage of liquids, solids, or gases.
Circular muscles generate radial closure pressure to create a local peristaltic closure wave. They work in conjunction with longitudinal muscles to facilitate processes like digestion and blood flow regulation.
