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Muscle fascicles are bundles of muscle cells, also known as muscle fibres, that are surrounded by a layer of connective tissue called the perimysium. Each muscle fibre within a fascicle is enveloped by a membrane called the endomysium that strongly adheres to the surrounding tissues. Fascicles are arranged in various anatomical positions within a muscle, producing different movements. The arrangement of fascicles in a skeletal muscle determines the force generated by the muscle and affects its range of motion.
| Characteristics | Values |
|---|---|
| Definition | Muscle fascicles are bundles of muscle cells (muscle fibres) enveloped by a membrane called the perimysium. |
| Structure | Each muscle cell within a fascicle is surrounded by a thin connective layer called the endomysium. |
| The perimysium contains capillaries, nerve endings, and neuromuscular spindles. | |
| The perimysium divides muscle fibres into groups (fascicles) and is itself enclosed within the epimysium. | |
| The epimysium is a connective tissue sheath that surrounds each individual muscle. | |
| The epimysium, perimysium, and endomysium extend beyond the fleshy part of the muscle to form a thick ropelike tendon or a broad, flat sheet-like aponeurosis. | |
| Types | Parallel, circular, convergent, pennate, fusiform, and triangular. |
| Functions | The arrangement of fascicles in skeletal muscle contributes to the force generated by the muscle and affects its range of motion. |
| Fascicle orientation contributes to ultrasound architecture. | |
| Fascicle structure is a useful diagnostic tool for dermatomyositis. |
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What You'll Learn
Muscle fascicle structure is a diagnostic tool for dermatomyositis
A muscle fascicle is a bundle of skeletal muscle fibres enveloped by a membrane called the perimysium, which is a type of connective tissue. These bundles of muscle cells, or fascicles, are then grouped together to form an individual muscle, which is enclosed within another layer of connective tissue called the epimysium.
The structure of muscle fascicles is a useful diagnostic tool for dermatomyositis, a rare condition characterised by muscle inflammation, weakness, and skin rash. Dermatomyositis is an idiopathic inflammatory myopathy, meaning it affects the skin and muscles, but can also impact joints, the oesophagus, and the lungs. The disease can also affect other organ systems, including the pulmonary, cardiovascular, and gastrointestinal systems.
The examination of muscle fascicle structure through ultrasound imaging plays an important role in evaluating normal and pathological states. Ultrasound assessment of the muscle fascicle structure can reveal perifascicular atrophy, which is a hallmark histopathological feature of dermatomyositis. This atrophy of muscle fibres, especially around the periphery of the fascicles, can be observed through ultrasound as a diffuse hyperechoic texture in intense cases of muscle soreness.
Additionally, the arrangement of fascicles in skeletal muscles contributes to the force generated and the range of motion. The organisation of fascicles can be parallel, circular, convergent, pennate, fusiform, or triangular, with each arrangement having its own range of motion and ability to generate force. This knowledge of fascicle arrangement and its impact on muscle function can aid in the diagnosis and understanding of dermatomyositis, particularly regarding muscle weakness and atrophy associated with the condition.
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Ultrasound architecture is affected by muscle fascicle orientation
Muscle fascicles are bundles of muscle cells, enveloped by a membrane called the perimysium. Fascicles are formed when muscle fibres are grouped into bundles of various sizes. The perimysium contains capillaries, nerve endings, and neuromuscular spindles.
Ultrasound imaging is a valuable tool for assessing skeletal muscle architecture during movements. It is often used to quantify muscle architecture and diagnose, prognose, and treat muscle injuries. The ultrasound appearance of muscle is affected by several factors, including the angulation of the ultrasound beam, the state of contraction of the muscle, and the ratio of perimysium to fascicles.
The orientation of the ultrasound probe relative to the skin or muscle can affect the measured architectural parameters, such as fascicle length and pennation angle. Changes in probe orientation can introduce measurement errors, so it is important to standardise the orientation to ensure reliable and valid data. The best estimate of actual fascicle architecture is achieved by orienting the probe perpendicular to the skin or visualising both aponeuroses and fascicles with the shortest possible fascicle length.
Deep-learning approaches have been developed to automate the quantification of fascicle architecture, but they require large, manually analysed training data sets. The geometric layout of fascicles within a skeletal muscle, or muscle architecture, is defined by two parameters: fascicle length and pennation angle. These parameters impact the force, velocity, excursion, and power-generating capabilities of the muscle.
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Fascicles are bundles of muscle cells
The arrangement of fascicles in a skeletal muscle affects the force generated by the muscle and its range of motion. Fascicles can be arranged in a variety of anatomical positions within a muscle, producing different movements. For example, in a unipennate muscle, the fascicles are located on one side of the tendon, while in a bipennate muscle, the rectus femurs, for example, has fascicles on both sides of the tendon, similar to the arrangement of a single feather.
The perimysium, in turn, is enclosed within the epimysium, which is a connective tissue covering that surrounds the entire muscle. The epimysium is contiguous with the tendon on long-axis views. The ultrasound appearance of muscle is influenced by several factors, including the state of contraction of the muscle, which alters size and echogenicity, and any factor that affects the ratio of perimysium to fascicles.
The structure of muscle fascicles is a useful diagnostic tool for dermatomyositis. Myocytes towards the edges of the muscle fascicle are typically narrower, while those at the centre of the muscle fascicle are a normal thickness.
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Fascicles are surrounded by perimysium
Fascicles are bundles of muscle fibres that are surrounded by a layer of connective tissue called the perimysium. This tissue separates the fascicles from other fascicles within the skeletal muscle. The perimysium is made up of dense irregular connective tissue, which contains mainly type I and type III collagen. It is continuous with the endomysium, which envelops each individual muscle fibre, and the epimysium, which encloses the entire muscle.
The perimysium contains capillaries, nerve endings, and neuromuscular spindles. It also contains a rich network of blood vessels and nerves, known as neurovascular bundles, that branch out to supply muscle fibres of each fascicle with nutrients and oxygen, while also facilitating signal transmission. During muscle contraction, the perimysium transmits the force produced by individual muscle fibres across the fascicles, generating smooth, coordinated muscle contraction and movement.
The tensile properties of the perimysium are similar to those of the endomysium, and it is possible that the perimysium could act to transmit the forces generated in fascicles to their adjacent neighbours by translaminar shear. However, this mechanism is unlikely under normal working conditions in living muscle, as the perimysium is much thicker than the endomysium, and would therefore be an inefficient force transmission pathway.
The perimysium is easily deformed in tension and does not exhibit high tensile stiffness unless it has been stretched far beyond the range of working lengths in living muscle. The ultrasound appearance of muscle is affected by the ratio of perimysium to fascicles, as well as other factors such as the angulation of the ultrasound beam and the state of contraction of the muscle.
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Fascicle arrangement affects the force generated by a muscle
Skeletal muscle is enclosed in connective tissue scaffolding at three levels. Each muscle fibre (cell) is covered by endomysium, and the entire muscle is covered by epimysium. When a group of muscle fibres is "bundled" as a unit within the whole muscle, it is called a fascicle. Fascicles are covered by a layer of connective tissue called perimysium.
Fascicle arrangement is correlated to the force generated by a muscle and affects the muscle's range of motion. Based on the patterns of fascicle arrangement, skeletal muscles can be classified in several ways. Parallel muscles have fascicles that are arranged in the same direction as the long axis of the muscle. The majority of skeletal muscles in the body have this type of organisation. Some parallel muscles are flat sheets that expand at the ends to make broad attachments, such as the sartorius.
When a muscle has a widespread expansion over a sizable area and the fascicles come to a single, common attachment point, the muscle is called convergent. The attachment point for a convergent muscle could be a tendon, an aponeurosis (a flat, broad tendon), or a raphe (a very slender tendon). The pectoralis major is an example of a convergent muscle.
In a unipennate muscle, the fascicles are located on one side of the tendon. An example of a unipennate muscle is the extensor digitorum of the forearm. A bipennate muscle, such as the rectus femurs, has fascicles on both sides of the tendon, similar to the arrangement of a single feather. Multipennate muscles have fascicles that insert on multiple tendons, converging on a common tendon, like multiple feathers coming together at a central point. An example is the deltoid muscle of the shoulder.
The organisation of connective tissue throughout and around a muscle provides strength and flexibility while distributing the force evenly. It also maintains the close association of the vascular and nervous systems with the muscle, which is required to deliver necessary metabolites and nerve impulses.
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Frequently asked questions
A fascicle is a bundle of muscle fibres surrounded by a layer of connective tissue called the perimysium.
A muscle fibre is an individual muscle cell, whereas a fascicle is a bundle of muscle fibres.
The connective tissue surrounding a fascicle is called the perimysium.
The connective tissue surrounding a muscle fibre is called the endomysium.
The connective tissue surrounding a muscle is called the epimysium.
