Logan Steele
Longitudinal muscles are those that run lengthwise along the body, as opposed to circular muscles, which encircle it. They are present in the esophagus, distal stomach, and small and large intestines, and play a role in peristalsis, a propulsive activity that involves the contraction and relaxation of both circular and longitudinal muscles to move objects along the gastrointestinal (GI) tract. The contraction of longitudinal muscles on the aborad side of the bolus, or mass of food, promotes propulsion by shortening the segment and widening the lumen, reducing resistance to propulsion.
| Characteristics | Values |
|---|---|
| Definition | Longitudinal muscle fibres run lengthwise along the body |
| Function | Contraction of longitudinal muscle promotes propulsion |
| Location | Longitudinal muscles are present in the trunk, oesophagus, superior longitudinal muscle, tongue, tentacles, GI tract, distal stomach, small and large intestines |
| Structure | Longitudinal muscle bands are composed of a single layer of cells; the fibres are arranged in radiating plates |
| Peristalsis | Peristalsis involves the contraction of both circular and longitudinal muscle layers of the oesophagus |
| Contraction | When longitudinal muscles contract, the body shortens and widens |
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What You'll Learn
Longitudinal muscle fibres run lengthwise along the body
Longitudinal muscles play a crucial role in the human body, particularly in the oesophagus, stomach, and intestines. During peristalsis, the circular and longitudinal muscle layers contract together, with the longitudinal muscles contracting independently during transient LES relaxation. Peristalsis is a propulsive activity that helps move food and liquids through the digestive system.
In the oesophagus, longitudinal muscle contraction causes oesophageal shortening. Studies have shown that at a specific site in the oesophagus, longitudinal muscles contract before and outlast circular muscle contraction by 2-4 seconds. This coordination between the two muscle layers ensures the smooth passage of food and liquids.
The role of longitudinal muscles in the upper oesophagus has been observed in studies where metal markers were sewn into the wall of the lower oesophagus of cats and tracked during swallows. The results indicated that shortly after the initiation of the swallow, the markers shifted, implying longitudinal shortening of the upper oesophagus due to the contraction of longitudinal muscle fibres.
Additionally, the contraction of longitudinal muscles promotes propulsion. When the longitudinal muscle contracts, the segment shortens and widens the lumen, reducing resistance to propulsion. This behaviour can be demonstrated using a "finger trap" or "Chinese finger puzzle," where pushing the ends of the trap together widens the lumen, allowing a marble to pass through easily.
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Contraction of longitudinal muscle causes esophageal shortening
Longitudinal muscle fibres run lengthwise along the body, with circular fibres encircling it. The contraction of longitudinal muscles causes the body to shorten and widen. This is true for the longitudinal muscles in the esophagus, which also contract during peristalsis. Peristalsis is a propulsive activity that involves both circular and longitudinal muscle layers of the esophagus, distal stomach, and small and large intestines.
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. The longitudinal muscle relaxes orad to the bolus and contracts on the aborad side. This contraction of the longitudinal muscle on the aborad side of the bolus promotes propulsion. This can be visualised using a ""finger trap" model, where pushing the ends of the trap together causes the trap to widen, allowing a marble to fall through.
The mass of the esophageal muscularis propria is split evenly between circular and longitudinal muscle fibres. The two layers contract in a precisely coordinated way, with the longitudinal muscles contracting prior to and outlasting the circular muscle contraction by 2-4 seconds. The coordination between the two layers has been observed in studies using radio-opaque markers implanted on the esophageal wall.
The contraction of longitudinal muscle in the esophagus causes local esophageal shortening, which has been known to radiologists since the 1950s. This shortening of the upper esophagus is due to the active tension generated by longitudinally aligned muscle fibres.
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Longitudinal muscles contract prior to circular muscles
The human body is composed of various muscles, including longitudinal and circular muscles, which play a crucial role in our motor functions. Longitudinal muscles run lengthwise along the body, while circular muscles encircle it. These two types of muscles work together to produce contrasting movements and maintain the body's shape while allowing for deformations.
The longitudinal muscles are responsible for the axial shortening of the esophagus, which is essential for the descending relaxation of the peristaltic reflex. This shortening action brings together the rings of circular muscles, increasing the overall muscle mass and efficiency of contraction. The contraction of longitudinal muscles is also believed to induce Lower Esophageal Sphincter (LES) relaxation through the activation of stretch-sensitive motor neurons.
The coordination between the longitudinal and circular muscles is crucial for proper body functions. Ultrasound imaging and manometry studies have revealed that the two layers of muscles contract in a precisely coordinated manner. The onset, peak, and termination of their contractions are synchronized. This coordination ensures the efficient propulsion of swallowed contents, either from the mouth to the stomach or vice versa during vomiting, belching, or reflux.
Several studies have shown that, at any given location in the esophagus, longitudinal muscles contract first and continue contracting for 2-4 seconds after the circular muscles have stopped. This temporal difference suggests a specific function for the longitudinal muscles in the overall process of peristalsis. The longitudinal muscles' initial contraction increases the efficiency of the subsequent circular muscle contraction by bringing their rings together and increasing the muscle thickness at the site of contraction.
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Contraction of longitudinal muscle promotes propulsion
The human body is composed of various muscles, including longitudinal muscles and circular muscles. The longitudinal muscle fibres run lengthwise along the body, while the circular fibres encircle it.
The contraction of longitudinal muscles plays a crucial role in propulsion within the body, specifically in the oesophagus, distal stomach, and small and large intestines. This process, known as peristalsis, involves the coordination of both circular and longitudinal muscle layers. During peristalsis, the circular muscle contracts behind a food bolus (on the orad side) and relaxes in front of it (on the aborad side). Simultaneously, the longitudinal muscle relaxes orad to the bolus and contracts on the aborad side. This sequential contraction and relaxation of muscles propel the bolus along the gastrointestinal (GI) tract towards the anus.
The understanding of how contraction of the longitudinal muscle promotes propulsion can be challenging to grasp. To address this, educators have employed a simple and inexpensive model using a "finger trap" or "Chinese finger puzzle". In this model, students are given a finger trap and a marble. They observe that when the ends of the finger trap are pushed together, the trap shortens and widens, similar to the contraction of a longitudinal muscle. When a marble is dropped into the trap, it initially gets stuck. However, by pushing the ends of the trap towards the middle, the trap widens, causing the marble to fall out, demonstrating the concept of reduced resistance to propulsion due to muscle contraction.
The behaviour of the finger trap model aligns with the mechanics of the longitudinal muscle. When the longitudinal muscle contracts, the segment shortens, resulting in the widening of the lumen, which is the internal cavity of the body. This widening reduces resistance to propulsion, facilitating the movement of substances through the GI tract.
In summary, the contraction of longitudinal muscles is an essential component of peristalsis, working in conjunction with circular muscles to promote propulsion in the GI tract. The finger trap model effectively illustrates how the contraction of longitudinal muscles reduces resistance and facilitates propulsion, enhancing our understanding of this crucial physiological process.
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Longitudinal muscles help earthworms move
Longitudinal muscles play a crucial role in enabling earthworms to move and burrow through soil. This movement is achieved through the coordination of longitudinal and circular muscle fibres, which act in antagonism to produce contrasting movements.
Earthworms possess a liquid-filled body cavity called the coelom, which functions as a pressurizable hydrostatic skeleton. By contracting its circular muscles, an earthworm becomes thinner, and the incompressible liquid within its body is forced outwards, lengthening the worm. Conversely, when the longitudinal muscles contract, the worm becomes shorter and thicker.
The ability to manipulate its body shape allows the earthworm to navigate through tight spaces. This movement can be understood through the following process: the earthworm protrudes setae, which are short stiff hairs, from the front of its body to anchor itself to the soil. Subsequently, the longitudinal muscles in the front of the body contract, making the segments shorter and fatter. This contraction pulls the rest of the body forward. The setae at the front then retract, and the process repeats as the earthworm moves through the soil in a wave-like motion.
The coordination of longitudinal and circular muscle fibres in earthworms grants them fine control over their hydrostatic skeletons. This control allows earthworms to adjust their body shape and navigate their environment effectively, showcasing the essential role of longitudinal muscles in the locomotion of these invertebrates.
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Frequently asked questions
Longitudinal muscle fibres run lengthwise along the body.
When longitudinal muscles contract, the body shortens and widens.
The circular and longitudinal muscle layers of the esophagus contract together during peristalsis, and longitudinal muscle contracts independently of circular muscle during transient LES relaxation.
When the longitudinal muscle contracts, the segment shortens and widens the lumen, reducing the resistance to propulsion.
