Logan Steele
The human body is a complex system, and its muscular composition is no exception. One type of muscle, the circular muscle, is found in various parts of the body, including the colon and esophagus. Circular muscles work in conjunction with longitudinal muscles to facilitate peristalsis, a process that involves the propulsion of food through the gastrointestinal tract. This process is made possible by the contraction and relaxation of both muscle types, with longitudinal muscles reducing the tension of individual circular muscle fibres. Interestingly, the circular muscle layer is located between the submucosa and the outer longitudinal muscular layer, and its structure impairs the luminal contents from travelling backward along the intestinal tract. The focus of this discussion is to delve into the nature of circular muscles and understand whether they fall under the category of smooth muscles.
| Characteristics | Values |
|---|---|
| Function | Generates radial closure pressure to create a local peristaltic closure wave |
| Layers | Inner circular and outer longitudinal |
| Peristalsis | Involves both circular and longitudinal muscle layers of the esophagus, distal stomach, and small and large intestines |
| Peristaltic movement | Contraction of the circular muscle behind the bolus and relaxation of the circular muscle in front of the bolus propels the object along the GI system toward the anus |
| Peristaltic contractions | Works with the longitudinal layer to facilitate the passage of intestinal content to the anus for excretion |
| Sphincteric smooth muscle | Maintains tone in the resting contracted state |
| Texture | Smooth muscle nuclei appear small and round when cut in cross section, very long in longitudinal section, and somewhere in between in oblique section |
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What You'll Learn
Circular muscle fibres are cut longitudinally
Smooth muscle is a type of muscle that is not under conscious control. It is found in various parts of the body, including the gastrointestinal tract, where it plays a crucial role in the propulsion of food and waste. The circular muscle fibres are one component of this system, working in conjunction with the longitudinal muscle fibres to facilitate the movement of contents through the intestines.
The circular muscle fibres are arranged in a circularly aligned inner layer within the muscular coat of the colon, also known as the tunica muscularis. This layer is composed of smooth muscle fibres that run perpendicular to the outer longitudinal layer. The circular layer is thicker than the longitudinal layer, but it is not a uniform sheet of muscle. Instead, it is divided by connective tissue clefts that penetrate the entire muscle, creating superficial creases on its outer surface.
When examining the structure of smooth muscle, it is important to consider the appearance of the muscle nuclei. In cross-sectioned smooth muscle, the nuclei appear small and round, while in longitudinal sections, they appear long and sometimes wiggly. In the case of circular muscle fibres, when they are cut longitudinally, the muscle nuclei will exhibit this elongated appearance. This distinction is important for understanding the orientation and characteristics of the muscle fibres.
The function of the circular muscle fibres is closely related to their anatomical arrangement. The circular layer generates radial closure pressure, which creates a local peristaltic closure wave. This contraction of the circular muscle behind a bolus and the simultaneous relaxation in front of it propel the contents along the gastrointestinal tract toward the anus. The coordination of circular and longitudinal muscle contractions ensures efficient propulsion and prevents backflow.
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Peristaltic esophageal transport
Peristalsis is the wave-like movement of muscles that line the gastrointestinal tract, allowing for the passage of food and fluids through the throat, esophagus, stomach, and intestines. Peristaltic esophageal transport, therefore, refers to the process by which food is moved through the esophagus.
The esophagus is composed of striated muscle, smooth muscle, and longitudinal muscle fibres. The upper esophageal sphincter (UES) and proximal one-third of the esophageal body are composed of striated muscle. The lower esophageal sphincter (LES) and the distal one-half to two-thirds of the esophageal body are composed of smooth muscle. The outer muscular coat, or muscularis propria, consists of an inner layer of circular muscle fibres and an outer layer of longitudinal fibres.
The sequential contraction of the circular muscle of the esophageal body results in a peristaltic wave that pushes food toward the stomach. This occurs in conjunction with the appropriately timed relaxation of the upper and lower esophageal sphincters. The circular muscle tone generates radial closure pressure to create a local peristaltic closure wave. Meanwhile, the longitudinal muscle tone has both physiological and mechanical functions. The physiological function involves reducing the tension of individual circular muscle fibres to maintain closure as a consequence of the shortening of the longitudinal muscle. The mechanical function is to reduce the level of pressure required to maintain closure.
Peristaltic movements are usually directed toward the anus due to the organisation of the myenteric plexus, which resides between the circular and longitudinal muscle layers. During peristalsis, the circular smooth muscle contracts behind the bolus and relaxes in front of it. At the same time, the longitudinal muscle relaxes in front of the bolus and contracts behind it.
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The role of longitudinal muscle in esophageal emptying
The longitudinal muscle plays a key role in the physiology and pathophysiology of esophageal sensory and motor function. During peristalsis, the two muscle layers of the oesophagus—the circular and the longitudinal muscles—contract in perfect synchrony. However, during transient lower oesophageal sphincter (LES) relaxation, the longitudinal muscle contracts independently of the circular muscle.
The longitudinal muscle has two functions: a physiological one with mechanical implications, and a purely mechanical one. The physiological function involves reducing the tension of individual circular muscle fibres to maintain closure as a consequence of the shortening of the longitudinal muscle locally coordinated with increasing circular muscle tone. The mechanical function of the longitudinal muscle is to reduce the level of pressure required to maintain closure. The combined physiological and mechanical consequences of the longitudinal muscle are to reduce circular muscle fibre tension and power by as much as one-tenth of what would be required for peristalsis without the longitudinal muscle layer. This benefit may explain the existence of longitudinal muscle fibre in the gut.
In certain diseases, such as some motility disorders of the oesophagus, there is a lack of coordination between the two muscle layers. This discoordination likely plays a role in the genesis of dysphagia and delayed oesophageal emptying. In achalasia oesophagus, a condition characterised by the loss of peristalsis and incomplete oesophagogastric junction (EGJ) relaxation, longitudinal muscle contraction of the distal oesophagus is the cause of pan-oesophageal pressurization in type 2 achalasia. Type 2 achalasia is characterised by simultaneous esophageal pressure and EGJ contraction following swallows. In type 1 and 3 achalasia, patients had no emptying or relatively normal emptying during most swallows, respectively.
The hypothesis is that the active suppression of LES tone ceases not when the LES returns to its resting state after oesophageal emptying, but rather before emptying begins as the LES is pulled orad and placed over the ampulla surface by the contraction of the longitudinal muscle in the oesophageal body. In this way, the LES contributes to the opening of the hiatus by increasing intrabolus pressure during ampulla formation, and to transsphincteric flow by the maintenance of high ampullary pressure.
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The appearance of smooth muscle nuclei
Smooth muscle consists of discrete individual cells (leiomyocytes), each with its own nucleus. The nucleus occupies only a small proportion of the cell's total length, and in a small bundle of smooth muscle, it can be particularly inconspicuous when cut in cross-section. The cell nuclei appear as small round dots, with a size of about 3-4µm, unlike the 5-6µm diameter of lymphocytes, the smallest cells commonly found in connective tissue. When cut obliquely, the nuclei may appear irregular, neither small and round nor long and cigar-shaped.
Smooth muscle fibres usually occur in bundles with uniform size and orientation, unlike collagen fibres, which vary in size and orientation. In histological specimens, smooth muscle can often be found in the walls of arteries and larger veins. To distinguish smooth muscle from collagen, texture (size, shape, orientation of fibres and associated cell nuclei) is a more reliable indicator than colour. However, a selective stain for collagen can be used when a quick and reliable distinction is required.
The smooth muscle cell is 3-10 µm thick and 20-200 µm long. The nucleus is located in the centre and takes on a cigar-like shape during contraction. The cytoplasm is homogeneously eosinophilic and consists mainly of myofilaments. The cell membrane forms small pouch-like invaginations into the cytoplasm (caveolae), which are functionally equivalent to the T-tubules of skeletal musculature. The smooth muscle fibres group into branching bundles, which, unlike skeletal muscle fibres, do not run strictly parallel but consist of a complex system, allowing for stronger contractions.
Smooth muscle is found in almost all organ systems, including hollow organs (e.g. stomach, bladder), tubular structures (e.g. vessels, bile ducts), sphincters, the uterus, and the eye. It plays a crucial role in sealing orifices (e.g. pylorus, uterine os) and transporting chyme through wavelike contractions of the intestinal tube. Smooth muscle cells contract slower than skeletal muscle cells but are stronger, more sustained, and require less energy.
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The circular muscular layer in the colon
The gastrointestinal (GI) tract is composed of four layers of tissue, known as tunics. Each layer has different structures and functions. From the inside out, they are called the mucosa, submucosa, muscularis externa, and serosa. The muscularis externa is responsible for segmental contractions and peristaltic movement in the GI tract. These muscles cause food to move and churn together with digestive enzymes down the GI tract.
The muscularis externa consists of an inner circular layer and a longitudinal outer muscular layer. The inner circular layer is helical with a steep pitch, and the outer longitudinal layer is helical with a much shallower pitch. The coordinated contractions of these layers are called peristalsis. The circular muscular layer in the colon is part of the muscularis externa. It is thicker in the colon than in other parts of the tract because feces are large and heavy and require more force to push along.
The circular muscle layer prevents food from traveling backward, while the longitudinal layer shortens the tract. Peristaltic movements are usually directed toward the anus and are due to the organization of the myenteric plexus, which resides between the circular and longitudinal muscle layers. During peristalsis, the circular smooth muscle contracts behind (on the orad side) the bolus and relaxes in front (on the aborad side) of the bolus. At the same time, the longitudinal muscle relaxes orad to the bolus and contracts on the aborad side of the bolus.
The pylorus of the stomach has a thickened portion of the inner circular layer: the pyloric sphincter. The outer longitudinal layer of the colon thins out into three discontinuous longitudinal bands known as tiniae coli (bands of the colon). These are three separate longitudinal ribbons of smooth muscle on the outside of the ascending, transverse, descending, and sigmoid colons.
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Frequently asked questions
The circular muscle is a layer of muscle fibres that encircle the body. It is located between the submucosa and the outer longitudinal muscular layer.
The circular muscle works with the longitudinal layer to provide peristaltic contractions, facilitating the passage of intestinal content to the anus for excretion.
The circular muscle generates radial closure pressure to create a local peristaltic closure wave, while the longitudinal muscle has both a physiological function with mechanical implications and a purely mechanical function.
The circular muscle contracts behind the bolus and relaxes in front of it, propelling the object along the gastrointestinal system toward the anus.
