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A muscle fascicle is a group of muscle cells (fibres) that are bundled together in parallel within a connective tissue sheath called the perimysium. Each muscle fibre is surrounded by a thin layer of connective tissue called the endomysium. The perimysium contains capillaries, nerve endings, and neuromuscular spindles. The muscle fascicle is then surrounded by an outer layer of connective tissue called the epimysium.
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What You'll Learn
Connective tissue sheath called the epimysium
The epimysium is a connective tissue sheath that surrounds an entire muscle. It is the outermost layer of three layers of connective tissue that protect skeletal muscle. The other two layers are the perimysium and the endomysium. The perimysium surrounds groups of 10 to 100 muscle fibres, separating them into bundles called fascicles. The endomysium separates individual muscle fibres from one another.
The epimysium is a dense, irregular connective tissue, predominantly composed of type I collagen fibres. It helps to define the muscle's volume and prevents friction between neighbouring muscles. It is continuous with the perimysium and endomysium deep to it, and all three layers converge and blend with the connective tissue of the muscle's tendon.
During muscle contractions, muscle fibres pull on these connective tissue sheaths, transmitting force to the bone that the tendon is inserting into, thus producing movement. If the muscle does not have a tendon of insertion and instead inserts directly onto bone, the epimysium will blend into the periosteum of the bone. The epimysium can sometimes also blend with the deep fascia between adjacent muscles, or even the superficial fascia.
The epimysium is hyperechoic and is contiguous with its tendon on long-axis views. It is also contiguous with the perimysial septa separating the fascicles. Endomysial, perimysial, and epimysial tissues unite into tendons or tendinous layers (i.e., aponeurosis) or may insert directly into the periosteum or dermis.
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Perimysium
The perimysium is a connective tissue sheath that surrounds individual muscle fascicles (bundles of muscle fibres) and separates them from other fascicles within the skeletal muscle. It is made up of dense irregular connective tissue, which contains mainly type I and type III collagen. The perimysium is continuous with the endomysium, which wraps around individual muscle fibres, and the epimysium, which encloses the entire muscle.
The perimysium plays a crucial role in force transmission within the muscle. It connects various muscular fibres and transmits forces between adjacent synergistic muscular fibre bundles, whether or not they belong to the same motor unit. This allows the perimysium to generate smooth, coordinated muscle contractions and movements. During muscle contraction, the perimysium transmits the force produced by individual muscle fibres across the fascicles.
The perimysium is rich in hyaluronic acid (HA), which helps to assure the autonomy and gliding of the various muscular fibres. It also forms the intramuscular neurovascular tracts, which are reinforced by collagen fibres and protect blood vessels, lymph vessels, nerves, and their branches. The perimysium 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.
The perimysium can be divided into three layers of collagen fibres: superficial, intermediate, and deep. The superficial layer consists of straight fibres with smaller diameters that spread out without a definite direction, intersecting to form a disorganized network. The intermediate layer is made up of larger diameter fibres that are flattened and curved, intersecting at various angles depending on the state of the muscle fibres. The deep layer is in direct contact with the endomysium and has a significantly increased space between the collagen fibres, giving it a discreet laxity.
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Endomysium
The endomysium is a thin layer of connective tissue that surrounds each muscle cell (myocyte). It is a sheath of reticular fibrils that envelops each individual skeletal muscle fibre. Endomysium is formed by fibroblasts and is composed of collagen and elastin fibres, with collagen being the main structural protein. Endomysium is highly deformable, adapting to the changes in volume that occur during muscle fibre contraction.
The endomysium is a key element that separates single muscle fibres from one another. Studies have shown that immobilization of the endomysium results in a substantial increase in the number of perpendicularly oriented collagen fibres that make contact with two adjacent muscle fibres.
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Tendons or tendinous layers
The endomysium is a thin connective tissue layer that surrounds each individual muscle fiber within a fascicle. It contains terminal axons and a rich capillary network. The perimysium is a layer of connective tissue that surrounds the fascicle, which is a bundle of muscle fibers. It contains capillaries, nerve endings, and neuromuscular spindles. The epimysium is the outermost connective tissue layer that surrounds the entire muscle. It is a thick sheath of fibrous connective tissue that is contiguous with the muscle tendon.
The epimysium, perimysium, and endomysium layers unite to form tendons or tendinous layers. These layers provide structural support and protection and act as pathways for blood vessels and nerves. They contribute to the functional capabilities of skeletal muscles, including the types of movements and the force generated.
Tendons are fibrous connective tissues that attach muscles to bones. They are composed primarily of collagen and are responsible for transmitting the force generated by muscles to the bones, enabling movement. Tendinous layers, also known as aponeuroses, are similar to tendons but have a broader, flat shape. They serve as attachment sites for muscles and help transmit muscular force.
The arrangement of fascicles within a muscle can vary, and this arrangement influences the formation of tendons or tendinous layers. The orientation of fascicles can be circular, parallel, convergent, or pennate, with the latter including patterns such as unipennate, bipennate, multipennate, and circumpennate. The arrangement of fascicles contributes to the overall structure and function of the muscle, including the formation and orientation of tendons or tendinous layers.
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Fascial compartments
The anterior compartment of the forearm, for example, contains muscles that flex the wrist and digits and pronate the forearm. In contrast, the posterior component contains muscles that extend the wrist and digits and supinate the forearm. The thigh, or upper leg, has three compartments: anterior, medial, and posterior. The anterior compartment contains the sartorius, articularis genu, and the four muscles that make up the quadriceps femoris group. These muscles are primarily involved in the extension of the knee joint. The medial compartment contains the pectineus, external obturator, gracilis, adductor magnus, adductor longus, and adductor brevis, most of which participate in the adduction of the hip.
There are four fascial compartments in each of the upper limbs: the anterior and posterior compartments of the arm and the anterior and posterior compartments of the forearm. The lower leg is divided into four compartments by the interosseous membrane of the leg, the anterior intermuscular septum, the transverse intermuscular septum, and the posterior intermuscular septum.
Compartment syndrome is a pathology specific to fascial compartments and can be acute or chronic. It occurs when there is bleeding or swelling inside a compartment, increasing pressure on the surrounding fascia, nerves, and blood vessels. Acute compartment syndrome usually results from a severe injury, such as a broken bone or crush injury, and requires immediate medical attention. Prolonged pressure within a compartment can lead to tissue death and nerve damage.
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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.
A muscle fasciculus is surrounded by a layer of connective tissue called the perimysium.
Perimysium is a fibrous septum that contains capillaries, nerve endings, and neuromuscular spindles. It divides muscle fibres into groups called fascicles.
The perimysium is surrounded by the epimysium, a thick sheath of fibrous connective tissue that is contiguous with the muscle tendon.
The fascia, a connective tissue outside the epimysium, surrounds and separates the muscles.
