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Circular muscles are a muscle layer that encircles the body between the epidermis and longitudinal muscle layers. They are found in the gastrointestinal system, which is responsible for breaking down ingested food into molecular forms through digestion and absorption. Sphincters, which are also circular muscles, 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 used every day in the normal course of digestion.
| Characteristics | Values |
|---|---|
| Definition | A muscle layer encircling the body between the epidermis and longitudinal muscle layer |
| Function | Generates radial closure pressure to create a local peristaltic closure wave |
| Location | Throughout the body, including the gastrointestinal system, oesophagus, and anus |
| Types | Anatomical and functional; voluntary and involuntary |
| Examples | Sphincters, orbicularis oculi muscle, orbicularis oris muscle |
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What You'll Learn
Sphincters are a type of circular muscle
Circular muscles are those that encircle the body and can be deformed into different shapes while maintaining a constant volume. Sphincters are a type of circular muscle. They 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. Voluntary sphincters are supplied by somatic nerves, while involuntary sphincters are stimulated by autonomic nerves.
Sphincters can be found in the gastrointestinal (GI) tract, where they help regulate the flow of food from the oesophagus to the anus, as well as the flow of bile and digestive enzymes into the intestine. They can also be found in the urinary tract, blood vessels, and eyes. For example, the lower oesophageal sphincter, or cardiac sphincter, is a sphincter at the upper portion of the stomach that prevents the acidic contents of the stomach from moving upward into the oesophagus. The pyloric sphincter, on the other hand, is located at the lower end of the stomach.
The urethral sphincter, which controls the flow of urine, and the iris sphincters, which narrow the pupils, are examples of sphincters in the urinary tract and eyes, respectively. The microscopic precapillary sphincters are another type of sphincter that controls blood flow into the capillaries in response to local metabolic activity. They are the most common type of sphincter in the human body, with millions located throughout.
The function of gastrointestinal sphincters is to act as one-way valves by contracting and closing to regulate and coordinate the caudal flow of gastrointestinal contents. Sphincters include both skeletal and smooth muscles located in specific sites throughout the gut. Smooth muscle sphincters include the lower oesophageal sphincter, pyloric sphincter, sphincter of Oddi, ileocecal sphincter, and internal anal sphincter.
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Circular muscles and longitudinal muscles work together
In the esophagus, the circular and longitudinal muscles contract together during peristalsis, a process that involves the movement of food or liquid through the digestive system. The coordination between these muscle layers ensures the efficient transport of food or liquid. The longitudinal muscle layer contracts first, bringing together the rings of circular muscles, which then contract to propel the contents forward. This process is known as a "milking action" and is essential for bolus propulsion.
The functions of circular and longitudinal muscles in esophageal peristalsis have been studied using mathematical modelling, experimental data, and medical imaging techniques such as ultrasound and manometry. These studies have helped understand the mechanical and physiological implications of their contractions. For example, longitudinal muscle tone has two functions: one physiological with mechanical implications and one purely mechanical. The mechanical function of longitudinal muscle shortening (LLS) is to reduce the pressure required to maintain closure in the esophagus. This, in turn, reduces the tension and power of the circular muscle fibres, making it easier for them to work together in creating a peristaltic closure wave.
Additionally, the presence of longitudinal muscles in the gut offers several advantages. The contraction of longitudinal muscles helps reduce the tension in individual circular muscle fibres, which is particularly important in processes like esophageal emptying, reflux, and pathology. The two layers of muscles work in a precisely coordinated manner, with the longitudinal muscles also helping to prevent an outward bulging effect during contractions.
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Circular muscles are involved in the digestive tract
Circular muscles play a crucial role in the digestive tract, also known as the gastrointestinal (GI) tract. The GI tract is a series of hollow organs, including the oesophagus, stomach, and intestines, that work together to facilitate digestion and the absorption of nutrients.
The circular muscles in the GI tract contract and relax in a wave-like pattern, known as peristalsis, to move food and fluids forward through the system. This process begins in the throat when we swallow and continues through the oesophagus, stomach, and intestines. Peristalsis involves both the circular muscles that encircle the tubes of the digestive tract and the longitudinal muscles that span the walls of these tubes. The circular muscles squeeze and expand in a synchronised manner to push food through, while the longitudinal muscles aid in propelling it forward.
In addition to peristalsis, another type of involuntary muscle movement called segmentation occurs in the intestines. Segmentation activates the circular muscles in the intestines, causing them to contract and move food back and forth, similar to the churning action of a washing machine. This churning allows food to mix with gastric juices and helps break it down into smaller pieces to facilitate digestion.
The GI tract is composed of multiple layers, including the muscularis propria, which contains both circular and longitudinal muscle layers. The circular muscle layer within the muscularis propria is made up of smooth muscle cells arranged in a tightly coiled pattern. This layer is essential for propelling food through the gut by generating peristaltic waves.
Sphincters, which are circular muscles that control the constriction and relaxation of natural body passages or orifices, are also found throughout the digestive tract. For example, the lower oesophageal sphincter, or cardiac sphincter, located at the top of the stomach, prevents the backward flow of acidic stomach contents into the oesophagus. At the other end of the digestive tract, the internal and external anal sphincters work together to control the exit of feces from the body.
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The body shortens or lengthens depending on circular muscle contraction
Circular muscles are those that encircle the body and are involved in the movement of the body's contents, which can be liquids or tissues. They work in conjunction with longitudinal muscles, which run along the length of the body. The two types of muscles have different functions and work together to maintain the body's shape and transport contents within the body.
The body shortens or lengthens depending on the contraction of its circular and longitudinal muscles. When circular muscles contract, the body shortens and widens to accommodate its volume. Conversely, when longitudinal muscles contract and the body shortens, it lengthens to maintain its volume. This relationship between muscle contraction and body length is particularly evident in the esophagus during peristalsis, the process of transporting food and liquids through the digestive system.
Peristalsis involves the coordination of circular and longitudinal muscle contractions to create a wave-like motion that propels contents through the esophagus. The specific functions of these muscle types differ, with circular muscles generating radial closure pressure to create a local peristaltic closure wave, and longitudinal muscles having both physiological and mechanical implications. The physiological function of longitudinal muscles involves reducing the tension of individual circular muscle fibres to maintain closure during shortening, while the mechanical function is to reduce the pressure required to maintain this closure.
The interaction between circular and longitudinal muscles is further influenced by their sliding patterns and morphological characteristics. Studies have shown that during peristalsis, circular and longitudinal muscles contract together to propel the bolus in the aboral direction, and they relax together to receive the bolus. Additionally, the spiral morphology of circular muscles in certain areas, such as the distal esophagus, contributes to axial shortening.
In summary, the body's lengthening or shortening depends on the contraction of its circular and longitudinal muscles, with each muscle type playing specific roles in maintaining body shape and facilitating the transport of contents within the body.
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Circular muscles are found in the eye
Circular muscles, also known as sphincters, are found throughout the human body. They maintain constriction of a natural body passage or orifice and relax as required by normal physiological functioning.
One type of circular muscle, the ciliary muscle, is found in the eye. It is formed as a ring of smooth muscle in the eye's middle layer, known as the uvea or vascular tunic. This layer is composed of the choroid, ciliary body, and iris. The ciliary muscle controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal.
The ciliary muscle is innervated by short ciliary muscles that arise from the ciliary ganglion, a parasympathetic ganglion located behind the eye. Activation of the M3 muscarinic receptors triggers the contraction of the ciliary muscle, reducing its diameter. This contraction allows the zonular fibres, which are in contact with the ciliary muscle, to relax. Zonular fibres are suspensory ligaments that hold the lens in position. When these fibres relax, the lens becomes more spherical, improving near vision.
The ciliary muscle also has longitudinal fibres that insert into the trabecular meshwork in the anterior chamber of the eye. Contraction and relaxation of these fibres cause an increase and decrease in the meshwork pore size, respectively, facilitating or impeding the flow of aqueous humour into Schlemm's canal. This process is important in the treatment of glaucoma, a disease that damages the optic nerve due to increased intraocular pressure.
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Frequently asked questions
Circular muscles are muscle layers that encircle the body between the epidermis and longitudinal muscle layers.
Some examples of circular muscles include the orbicularis oris muscle, a complex of muscles in the lips that encircle the mouth, and the orbicularis oculi muscle, a muscle around the eye.
Circular muscles have various functions, including maintaining constriction of natural body passages or orifices and relaxing as required by normal physiological functioning. For example, the lower oesophageal sphincter, or cardiac sphincter, is a circular muscle that prevents the acidic contents of the stomach from moving up into the oesophagus.
Circular muscles differ from longitudinal muscles in their function and structure. While circular muscles generate radial closure pressure to create a local peristaltic closure wave, longitudinal muscles have both physiological and mechanical functions.
Yes, sphincters are a type of circular muscle. Sphincters are circular muscles that maintain constriction of natural body passages or orifices and relax as needed for physiological functions. There are over 60 types of sphincters in the human body, some of which are microscopically small, such as the precapillary sphincters.
