Logan Steele
Muscle fascicles are bundles of muscle fibres in a skeletal muscle. They are enveloped by a layer of connective tissue called perimysium. The outermost connective tissue sheath surrounding the entire muscle is known as epimysium. The individual muscle fibres are surrounded by endomysium. The arrangement of muscle fascicles determines the force a muscle can generate and affects its range of motion. The four primary types of muscle based on fascicle arrangement are parallel, convergent, pennate, and circular.
| Characteristics | Values |
|---|---|
| Definition | A group of muscle cells (fibers) that are grouped together in parallel within a connective tissue sheath |
| Other names | Fasciculus, muscle bundle |
| Location | Skeletal muscles |
| Function | Responsible for the voluntary movements of bones |
| Arrangement | Parallel, circular, convergent, pennate, fusiform, or triangular |
| Ultrasound appearance | Hypoechoic fascicles surrounded by hyperechoic perimysium |
| Associated conditions | Dermatomyositis, myokymia |
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What You'll Learn
- Muscle fascicles are bundles of muscle fibres
- Fascicles are covered by a layer of connective tissue called perimysium
- Fascicle arrangement affects the force generated by a muscle
- Skeletal muscles have four major classes based on fascicle arrangement: parallel, convergent, pennate, and circular
- Muscle fascicle structure is a useful diagnostic tool for dermatomyositis
Muscle fascicles are bundles of muscle fibres
The outermost connective tissue sheath surrounding the entire muscle is called epimysium. Endomysial, perimysial, and epimysial tissues unite into tendons or tendinous layers. The ultrasound assessment of muscle requires multiple focal zones and adjustments of depth, depending on the size and location of the muscle. The fine echo structure of muscle is demonstrated by hypoechoic fascicles surrounded by hyperechoic perimysium, best viewed as parallel bands on long-axis ultrasound.
The arrangement of fascicles in skeletal muscle determines the force that a muscle can generate. There are four primary types of muscle based on fascicle arrangement: parallel, convergent, pennate, and circular. Parallel muscles have fascicles that run parallel to the long axis of the muscle. When these muscles contract, they shorten in length and increase in diameter, enabling movement. Convergent muscles have fascicles that extend over a broad area but converge on a common attachment site, such as a tendon or aponeurosis. Pennate muscles have fascicles that attach obliquely to a central tendon, resembling the pattern of feathers along a quill. Circular muscles, also known as sphincters, have fascicles arranged concentrically around an opening.
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Fascicles are covered by a layer of connective tissue called perimysium
A muscle fascicle is a bundle of muscle fibres in a skeletal muscle. Fascicles are covered by a layer of connective tissue called perimysium. This connective tissue sheath surrounds each fasciculus (the bundle of muscle fibres). The perimysium is the outermost layer of the fascicle, with the individual muscle fibres being surrounded by a membrane called the endomysium.
The perimysium contains capillaries, nerve endings, and neuromuscular spindles. It is the densest connective tissue in the muscle. The primary artery supplying blood to the skeletal muscle runs parallel to the longitudinal axis of the muscle fibre. Tributaries from this primary artery, known as feed arteries, run perpendicular to the perimysium. The feed arteries branch into primary arterioles, which eventually give rise to terminal arterioles. These terminal arterioles are the final vascular branches and they perfuse the capillaries present within the endomysium.
The perimysium is an important part of the muscle's structure, as it contributes to the ultrasound architecture of the muscle. It is best demonstrated as parallel bands on long-axis views. The perimysium is hyperechoic, meaning it reflects sound waves, and this is useful for diagnostic imaging.
The arrangement of fascicles within a muscle determines the force that a muscle can generate and affects its range of motion. There are four primary types of muscle based on fascicle arrangement: parallel, convergent, pennate, and circular. Parallel muscles have fascicles that run parallel to the long axis of the muscle, and the majority of skeletal muscles in the body have this type of organisation. Convergent muscles have fascicles that extend over a broad area but converge on a common attachment site, such as a tendon or aponeurosis. Pennate muscles have fascicles that attach obliquely to a central tendon, resembling the pattern of feathers along a quill. This arrangement allows for a higher density of muscle fibres and greater force production. Circular muscles, also known as sphincters, have fascicles arranged concentrically around an opening, controlling the passage of substances through the body's pathways.
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Fascicle arrangement affects the force generated by a muscle
A muscle fascicle is a group of muscle cells (fibres) that are bundled together in parallel within a connective tissue sheath called the perimysium. The whole muscle belly is then composed of all the muscle fascicles grouped together by an outer layer of connective tissue called the epimysium. The fascicles of skeletal muscles are visible to the naked eye and based on their orientation, skeletal muscles can be arranged into four basic structural patterns: circular, parallel, convergent, and pennate.
The arrangement of fascicles in a skeletal muscle determines the type of movement the muscle can make and the force generated by the muscle. For example, in parallel and fusiform muscles, the long fibres allow for greater shortening but generate less force. In contrast, convergent muscles balance force and contraction length due to their broad origin and narrow insertion. Pennate muscles, on the other hand, optimise strength by packing more fibres at an angle to the tendon, enhancing force at the expense of contraction length.
The different arrangements of fascicles result in different ranges of motion and abilities to perform work. For instance, circular muscles control openings by contracting to close them, acting as sphincters. Unipennate muscles have fibres running at an angle from one side of a tendon, like the extensor digitorum longus. Bipennate muscles, such as the rectus femoris, have fibres extending from both sides of a central tendon. Multipennate muscles, like the deltoid, feature fibres extending from multiple tendons.
The arrangement of fascicles also determines the direction of the pull of a muscle. For example, when the deltoid muscle contracts, the arm abducts, but when only the anterior fascicle is stimulated, the arm will abduct and flex. This demonstrates how the arrangement of fascicles within a muscle can affect the force generated and the subsequent movement of the body.
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Skeletal muscles have four major classes based on fascicle arrangement: parallel, convergent, pennate, and circular
A muscle fascicle is a group of muscle cells (fibres) that are bundled together within a connective tissue sheath called the perimysium. The whole muscle belly is then composed of all the muscle fascicles grouped together by an outer layer of connective tissue called the epimysium. The arrangement of fascicles in skeletal muscles influences their contraction capabilities and range of motion. Skeletal muscles can be classified into four major classes based on fascicle arrangement: parallel, convergent, pennate, and circular.
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. Other parallel muscles have a larger central region called a muscle belly tapering to tendons on each end. This arrangement is called fusiform, such as the biceps brachii.
Convergent muscles have fascicles that extend over a broad area but converge on a common attachment site, such as a tendon or aponeurosis (a flat, broad tendon). An example of a convergent muscle is the pectoralis major, which converges on the intertubercular groove and greater tubercle of the humerus via a tendon.
Pennate muscles have fibres at angles to tendons, with shorter muscle fibres than parallel muscles. This arrangement allows for a higher density of muscle fibres in a given area, resulting in greater force production at the expense of reduced range of motion and speed. There are three subtypes of pennate muscles: unipennate, bipennate, and multipennate. An example of a unipennate muscle is the extensor digitorum of the forearm, while the rectus femurs is an example of a bipennate muscle. The deltoid muscle of the shoulder is an example of a multipennate muscle.
Circular muscles, also known as sphincters, have fascicles arranged concentrically around an opening. This arrangement forms a ring-like structure that can constrict or relax to control the passage of substances through the body's various pathways. An example of a circular muscle is the orbicularis oculi, which surrounds the eye.
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Muscle fascicle structure is a useful diagnostic tool for dermatomyositis
A muscle fascicle is a bundle of skeletal muscle fibres enveloped by perimysium, a type of connective tissue. Each muscle fibre (cell) is covered by endomysium, and the entire muscle is covered by epimysium. The muscle fascicles are then grouped together by the epimysium connective tissue.
The structure of muscle fascicles is particularly useful in diagnosing dermatomyositis, a rare immune-mediated muscle disease. Dermatomyositis is characterised by muscle weakness and skin rash, and it can affect other organ systems such as the pulmonary, cardiovascular, and gastrointestinal systems. Examination of muscle fascicle structures can reveal perifascicular atrophy, predominantly at the periphery, along the connective-tissue border. This atrophy is a hallmark histopathological feature of dermatomyositis.
In addition, myocytes towards the edges of the muscle fascicle are typically narrower, while those at the centre are of normal thickness. This variation in thickness can be a useful indicator in diagnosing dermatomyositis. Initial testing for dermatomyositis should include muscle enzyme levels, such as creatine kinase and lactate dehydrogenase. Further diagnostic studies include imaging to screen for underlying malignancy, as dermatomyositis is strongly associated with malignancy in adults.
The structure of muscle fascicles can also be indicative of the force generated by a muscle and its range of motion. The arrangement of fascicles in a skeletal muscle can be parallel, circular, convergent, pennate, fusiform, or triangular, each with its own range of motion and ability to generate force. For example, in a bipennate muscle, the fascicles are arranged on both sides of the tendon, similar to the arrangement of a single feather.
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Frequently asked questions
A muscle fascicle is a group of muscle cells (fibres) that are bundled together in parallel within a connective tissue sheath called the perimysium.
A muscle fibre is a single muscle cell enveloped by a membrane called the endomysium. Muscle fibres form bundles of various sizes, which are the muscle fascicles.
Based on the arrangement of fascicles, skeletal muscles can be classified into four major classes: parallel, convergent, pennate, and circular.
