Victor Forge
Muscle fascicles are bundles of skeletal muscle fibres that are held together by a connective tissue called perimysium. Each muscle fibre is covered by endomysium, and the entire muscle is covered by epimysium. The arrangement of fascicles in a skeletal muscle determines the type of movement the muscle can make and the force it can generate.
| Characteristics | Values |
|---|---|
| Muscle fascicle definition | A bundle of skeletal muscle fibers surrounded by perimysium, a type of connective tissue |
| Muscle fascicle composition | Muscle cells |
| Connective tissue sheath around each fasciculus | Perimysium |
| Connective tissue sheath around each muscle fiber | Endomysium |
| Connective tissue sheath around the entire muscle | Epimysium |
| Fascicle arrangement patterns | Circular, parallel, convergent, and pennate |
| Fascicle orientation | Parallel to the long axis of the muscle in fusiform muscles or oblique in pennate muscles |
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What You'll Learn
Connective tissue sheath called perimysium
A muscle fascicle is a bundle or group of skeletal muscle fibres. Each muscle fibre is a single cylindrical muscle cell. Each muscle cell is surrounded by a connective tissue called the endomysium. A bundle of these muscle fibres is known as a fasciculus, and each fasciculus is surrounded by a layer of connective tissue called the perimysium.
The perimysium is a sheath of dense irregular connective tissue that groups muscle fibres into bundles or fascicles. It is an extension of the epimysium, which is the connective tissue sheath that surrounds the entire muscle. The perimysium wraps around each fascicle, separating the skeletal muscle tissue into muscle fascicles.
The perimysium is composed of collagenous connective tissue and contains type I, III, VI, and XII collagen. It plays a role in transmitting lateral contractile movements. The amount and spatial distribution of perimysium vary between muscles in the body. For example, thick amounts of perimysium enclosing large fascicles of myofibers form tubes in a honeycomb arrangement in the direction of myofibers.
The perimysium, along with the endomysium and epimysium, provides support and protection for the delicate muscle cells and allows them to withstand the forces of contraction. It also provides a pathway for blood vessels, lymphatics, and nerve fibres.
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Muscle fascicles and muscle fibres
Muscle fascicles are bundles of skeletal muscle fibres surrounded by a layer of connective tissue called the perimysium. Each muscle fibre is covered by a thin layer of connective tissue called the endomysium. The perimysium holds the muscle fascicles together.
Muscle fibres are cylindrical muscle cells that are bundled together and wrapped in a connective tissue covering. Each muscle fibre is surrounded by endomysium, and the entire muscle is covered by epimysium. The epimysium is the outermost connective tissue sheath surrounding the entire muscle. The perimysium is the connective tissue sheath covering each bundle of muscle fibres.
The arrangement of muscle fascicles in skeletal muscle determines the force a muscle can generate and affects its range of motion. Fascicles are arranged in four basic structural patterns: circular, parallel, convergent, and pennate. For example, circular muscles act as sphincters, closing orifices. The arrangement of fascicles also determines what type of movement a muscle can make and how much force it can produce.
Multipennate muscles have fascicles that insert on multiple tendons, converging on a common tendon. An example is the deltoid muscle of the shoulder, which covers the shoulder and has a single tendon that inserts on the deltoid tuberosity of the humerus.
Skeletal muscles are attached to bones by tendons and work in pairs to facilitate movement of the bones around the joints. The end of the muscle attached to a fixed or stabilized bone is called the origin, and the end of the muscle that attaches to the bone being pulled is called the insertion. The muscle responsible for a movement is called the prime mover, and muscles that assist in this action are called synergists.
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Types of muscle cells
Muscle fascicles are bundles of skeletal muscle fibres surrounded by perimysium, a type of connective tissue. Fascicles are typically composed of a single type of muscle cell, either type I or type II fibres, but they can contain a mixture of both. The arrangement of fascicles in skeletal muscle determines the type of movement a muscle can make and its range of motion. For example, circular muscles act as sphincters, closing orifices, while parallel muscles have fascicles arranged in the same direction as the long axis of the muscle.
There are three types of muscle cells in the human body: skeletal, smooth, and cardiac muscle cells. Skeletal muscle cells, also known as muscle fibres, are the individual contractile cells within a muscle. They are multinucleated, with nuclei usually referred to as myonuclei, and they contain myofibrils consisting of long protein chains of myofilaments. The three types of myofilaments are thin (actin), thick (myosin), and elastic (titin) filaments that work together to produce muscle contraction. Skeletal muscles have an origin and an insertion point on bones, and they work in pairs to facilitate movement.
Smooth muscle cells, on the other hand, lack sarcomeres and myofibrils but contain large amounts of the contractile proteins actin and myosin. They form three types of muscle: multi-unit smooth muscle, which contracts as separate units and is found in the iris of the eye or the vas deferens; single-unit smooth muscle, where cells contract together and are found in the gastrointestinal tract, bladder, and uterus; and vascular smooth muscle, which helps regulate blood pressure.
Cardiac muscle cells, found in the walls of the heart, are controlled by the autonomic nervous system. They have one central nucleus and are rectangular and striated in shape. Cardiac muscle cells receive electrical impulses from the atrioventricular node (AV node) and transmit them to the Purkinje fibres. The contraction of cardiac muscle is involuntary, strong, and rhythmical, with intracellular calcium concentrations directly influencing contraction force.
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Fascicles and tendons
A muscle fascicle is a bundle or group of muscle cells (fibers) that are grouped together in parallel within a connective tissue sheath called the perimysium. Each individual muscle cell or fibre is surrounded by another connective tissue called the endomysium. The perimysium is a dense connective tissue.
Fascicles are then bundled together by another connective tissue called the epimysium, which is the outermost connective tissue sheath surrounding the entire muscle. The epimysium is hyperechoic and is contiguous with its tendon on long-axis views. The endomysial, perimysial, and epimysial tissues unite into tendons or tendinous layers (aponeurosis) or may insert directly into the periosteum or dermis.
The arrangement of fascicles in the skeletal muscle determines the force generated by the muscle and affects its range of motion. Fascicles are arranged in four basic structural patterns: circular, parallel, convergent, and pennate. For instance, circular muscles act as sphincters, closing orifices. The deltoid muscle of the shoulder, for example, has a single tendon that inserts into the deltoid tuberosity of the humerus.
Tendons are what connect muscles to bones. The muscle-tendon junction is the weakest area of the musculature, making it vulnerable to injuries.
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Ultrasound assessment of muscle
Muscle fascicles are bundles of skeletal muscle fibres surrounded by perimysium, a type of connective tissue. Ultrasound imaging, also known as ultrasonography or sonography, is a safe, non-invasive, and painless medical procedure that uses sound waves to produce images of muscles, tendons, ligaments, nerves, joints, and soft tissues throughout the body. It is a valuable tool for assessing muscle health and diagnosing various conditions.
Ultrasound imaging provides real-time visualization of soft tissue structures, allowing for the assessment of their movement and function. This capability is beneficial for examining tendons, joints, and extremities. Additionally, ultrasound can detect muscle inflammation and has been used to study myositis and inflammatory myopathies. For example, Nodera et al. observed differences in echo intensity between the gastrocnemius and soleus muscles, aiding in the sensitivity and specificity of ultrasound in diagnosing inflammatory conditions.
While ultrasound is a valuable tool, it has certain limitations. One significant disadvantage is its dependence on examiner expertise, which can lead to variability in results. Muscle echo intensity, a critical parameter in muscle quality assessment, is influenced by machine settings, making it challenging to compare results across different centres. However, advancements in ultrasound technology, such as the addition of Doppler and elastography modalities and machine learning, hold promise for improving the consistency and accuracy of ultrasound assessments.
Overall, ultrasound assessment of muscles is a valuable clinical tool that offers a safe, cost-effective, and non-invasive approach to evaluating muscle health and diagnosing various muscle-related conditions. Its real-time imaging capabilities and soft tissue visualization make it a powerful complement to other diagnostic modalities.
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Frequently asked questions
A muscle fascicle is a bundle or group of skeletal muscle fibres.
Muscle fascicles are held together by a layer of connective tissue called perimysium.
Perimysium is a dense connective tissue that contains capillaries, nerve endings, and neuromuscular spindles. It is the outermost connective tissue sheath surrounding the muscle fascicle.
