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Longitudinal muscles are sheets of muscle fibres that work in conjunction with circular muscles to produce contrasting movements. They are found in the oesophagus, where they contract during peristalsis, and are also present in the gut. The contraction of longitudinal muscles in the distal oesophagus is believed to induce LES relaxation through the activation of stretch-sensitive motor neurons. The two layers of muscles in the oesophagus work together in a coordinated manner, with longitudinal muscles contracting before and lasting longer than circular muscles.
| Characteristics | Values |
|---|---|
| Function | Two functions, one physiological with mechanical implications, and one purely mechanical |
| Control Mechanisms | Stimulation of the cervical vagus nerve induces contraction |
| Peristaltic Contraction | Coordinated with circular muscles |
| Peristalsis | Reduces circular muscle fiber tension and power |
| Layers | Two layers of the esophagus |
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What You'll Learn
Longitudinal muscle contraction
Longitudinal muscles are simple sheets of muscle fibres that run lengthwise along the body. They work in conjunction with circular muscle fibres to produce contrasting movements. For example, if the longitudinal and circular muscles of the trunk contract with the mouth slightly open, fluid flows out of the body's large digestive cavity.
Several studies have shown that longitudinal muscle contraction begins before and outlasts circular muscle contraction during peristalsis. This has been observed through various techniques, including synchronized high-frequency ultrasound imaging, manometry, and HFIUS imaging. The coordination between the two types of muscle layers ensures the smooth passage of food through the oesophagus.
The control mechanisms for longitudinal muscle contraction differ from those of circular muscles. Stimulation of the cervical vagus nerve induces contraction of the longitudinal muscle oesophagus, whereas circular muscles exhibit "on" and "off" responses. Additionally, longitudinal muscles show depolarization during swallowing, suggesting the absence of a peripheral inhibitory mechanism. However, studies have observed relaxation of the longitudinal muscle distal to the site of oesophageal distension, indicating the possibility of a peripheral mechanism of inhibition.
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Peristaltic esophageal transport
Peristalsis is the involuntary muscle movement that moves food through your gastrointestinal tract. It is an automatic wave-like movement of the muscles that line the gastrointestinal tract, beginning in the throat when we swallow and continuing through the esophagus, stomach, and intestines. Peristaltic movement of smooth muscle can also be found in other parts of the body, including the ureters, vas deferens, bile ducts, and glandular ducts.
The esophageal phase of deglutition involves the sequential contraction of circular muscle in the esophageal body, resulting in a peristaltic wave that pushes food toward the stomach. 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 LES relaxes and opens to allow food to pass through.
The outer muscular coat of the esophagus, or muscularis propria, consists of an inner layer of circularly oriented muscle fibres and an outer layer of longitudinally oriented fibres. The longitudinal muscle tone has two functions: a physiological function with mechanical implications, and a purely mechanical function. The physiological function involves reducing the tension of individual circular muscle fibres to maintain closure as a consequence of the shortening of longitudinal muscle locally coordinated with increasing circular muscle tone. The mechanical function is to reduce the level of pressure required to maintain closure.
The combined physiological and mechanical consequences of longitudinal muscle tone 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.
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Skeletomusculature
The oesophagus is a muscular tube that connects the pharynx to the stomach and is part of the digestive system. During the process of swallowing, the oesophagus propels food towards the stomach through a series of coordinated contractions called peristalsis. This involves the contraction of both the longitudinal and circular muscles.
The contraction of longitudinal muscles in the oesophagus has two main functions: a physiological function with mechanical implications, and a purely mechanical function. The physiological function helps to reduce the tension of individual circular muscle fibres, which maintains closure as a consequence of the shortening of the longitudinal muscle. This is coordinated with an increase in circular muscle tone. The mechanical function of longitudinal muscle contraction is to reduce the pressure required to maintain this closure.
The circular and longitudinal muscles of the oesophagus contract together during peristalsis. However, the control mechanisms for these two types of muscles differ. The longitudinal muscles of the oesophagus are controlled by the stimulation of the cervical vagus nerve, which induces contraction for as long as the stimulus is present. In contrast, circular muscles exhibit "on" and "off" responses. Additionally, the longitudinal muscles show depolarization during swallowing, indicating the absence of a peripheral inhibitory mechanism.
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Neural control
One example of neural control of longitudinal muscles is observed during swallowing, a complex motor process involving the coordination of multiple cranial nerves and neural structures. The pharyngeal phase of swallowing is initiated by the pharyngeal plexus, which includes the glossopharyngeal, vagus, and accessory nerves, along with the trigeminal, facial, glossopharyngeal, and hypoglossal nerves. The cervical plexus and the hypoglossal nerve form the ansa cervicalis, which contributes to the elevation of the hyoid-laryngeal complex. Additionally, the swallowing process involves the longitudinal and circular muscular layers of the oesophagus, with their distinct spiral fibre arrangements, allowing for the widening of the oesophagus during swallowing.
The neural control of longitudinal muscle contraction involves excitation-contraction coupling (ECC), where depolarization of skeletal muscles through neural innervation results in muscle action potentials. These action potentials spread across the muscle surface and into the muscle fibre network, leading to the activation of specific receptors and the release of calcium ions, which are essential for muscle contraction.
Nitric oxide also plays a role in the neural control of longitudinal muscles. It induces relaxation of circular muscles while causing contraction of longitudinal muscles through an excitation-contraction coupling mechanism. This process requires extracellular Ca2+ entry through the activation of L-type Ca2+ channels. In vivo studies in cats have shown that blocking nitric oxide reduces longitudinal muscle contraction.
Furthermore, electrical field stimulation of mouse small intestinal longitudinal muscles has revealed contractile responses mediated by acetylcholine and neurokinin A. Inflammatory mediators, such as histamine and leukotriene D4, can also influence the contractile activity of these muscles. These interactions between neural signals and inflammatory mediators highlight the complex neural control of longitudinal muscles.
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Longitudinal vs circular muscles
Longitudinal and circular muscles are types of muscle fibres that produce contrasting movements. The longitudinal muscle fibres run lengthwise along the body, while the circular fibres encircle it.
In the human esophagus, longitudinal muscles contract during peristalsis, which is the process of pushing food down the throat and into the stomach. The circular muscle fibres generate radial closure pressure to create a local peristaltic closure wave, while the longitudinal muscles have two functions: a physiological one with mechanical implications, and a purely mechanical one. The physiological function of the longitudinal muscles is to reduce the tension of individual circular muscle fibres to maintain closure as a consequence of their own shortening, which is locally coordinated with increasing circular muscle tone. The mechanical function is to reduce the level of pressure required to maintain closure.
The combined physiological and mechanical consequences of the longitudinal muscles' functions 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.
Mathematical modelling has been used to study the functions of longitudinal and circular muscle fibres in esophageal peristalsis, integrating experimental data, the conservation laws of mechanics, and endoluminal ultrasound.
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Frequently asked questions
Longitudinal muscles are those that contract independently of circular muscles during transient LES relaxation.
Longitudinal muscles have two functions: a physiological function with mechanical implications and a purely mechanical function.
The physiological function of longitudinal muscles is to reduce the tension of individual circular muscle fibres to maintain closure.
The mechanical function of longitudinal muscles is to reduce the level of pressure required to maintain closure.
