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Skeletal muscle is enclosed in connective tissue scaffolding at three levels. Each muscle fibre (cell) is covered by endomysium, a thin, delicate network of strands between the fibres. Each bundle of muscle fibres, or fascicle, is surrounded by a layer of connective tissue called the perimysium. The entire muscle is then covered by the epimysium, a sheath of dense, irregular connective tissue that separates muscle from other tissues and organs in the area. The fascia, connective tissue outside the epimysium, surrounds and separates the muscles.
| Characteristics | Values |
|---|---|
| Connective tissue layer | Endomysium |
| Connective tissue layer | Perimysium |
| Connective tissue layer | Epimysium |
| Connective tissue layer | Fascia |
| Connective tissue layer | Tendon |
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Connective tissue compartments
Skeletal muscles have three layers of connective tissue (mysia) that enclose them, providing structure and compartmentalizing the muscle fibres within. Each muscle fibre (cell) is covered by a layer of connective tissue called the endomysium. The endomysium is a thin, delicate network of strands between the fibres, also containing the microcirculation network.
The next layer is the perimysium, which encloses the fascicles, or bundles of muscle fibres. The perimysium is made up of concentric strands of connective tissue. It is continuous with the endomysium and carries larger vessels, nerve trunks, and sensory neuromuscular spindles.
The outermost layer is the epimysium, which surrounds the whole muscle. The epimysium contains adipose tissue, the arterial and venous supply to the fascicles, and larger peripheral nerve branches. It separates the muscle from other tissues and organs in the area, allowing the muscle to move independently. It also allows the muscle to contract and move powerfully while maintaining its structural integrity.
Outside the epimysium is the fascia, a layer of connective tissue that surrounds and separates the muscles. The fascia is composed of cross-linked collagen and elastin fibres oriented parallel to the direction of muscle force, allowing it to resist high-tension forces while remaining elastic. The fascia can project beyond the end of the muscle and attach to bones, other muscles, and other tissues.
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Perimysium
The perimysium, also called interfascicular connective tissue, is a connective tissue sheath that surrounds each bundle of muscle fibres, known as muscle fascicles. It separates them 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, as well as many elastic fibres. It is continuous with the endomysium, which wraps around individual muscle fibres, and the epimysium, which encloses the entire muscle.
The perimysium is a thin fibrous layer that is closely connected with the epimysial fasciae. It divides the muscle belly into fascicles of different dimensions. The perimysium is similar to the epimysium in its multilayered organisation of collagen fibres. Rowe (1981) divides the perimysium into three layers: superficial, intermediate, and deep. The superficial layer consists of straight fibres with a smaller diameter that spread out without a definite direction, intersecting with each other to form a disorganised network. The intermediate layer is made up of larger diameter fibres that are flattened and curved, intersecting with an average angle of 55° with respect to the resting muscular fibres. The deep layer comprises soft lamina that is in direct contact with the endomysium, with significantly increased space between the collagen fibres, giving it a discreet laxity.
The perimysium plays a crucial role in force transmission. It connects various muscular fibres and transmits forces between adjacent synergistic muscular fibre bundles, whether or not they belong to the same motor unit. During muscle contraction, the perimysium transmits the force produced by individual muscle fibres across the fascicles, resulting in smooth and coordinated muscle contraction and movements. The direction of the collagen fibres in the perimysium changes with the state of the muscle, demonstrating its dynamic nature.
Additionally, the perimysium is rich in hyaluronic acid (HA), which helps assure the autonomy and gliding among the various muscular fibres. It also forms the intramuscular neurovascular tracts, which are reinforced by collagen fibre bundles that envelop and protect blood vessels, lymph vessels, nerves, and their branches. These neurovascular tracts supply the muscle fibres of each fascicle with nutrients and oxygen while also facilitating signal transmission, contributing to the overall functioning and coordination of the muscular system.
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Epimysium
The epimysium is one of three collagenous sheaths that surround skeletal muscles, the other two being the perimysium and the endomysium. The perimysium surrounds each bundle of muscle fibres, while the endomysium covers each single muscle fibre or muscle cell. The epimysium usually contains many bundles (fascicles) of muscle fibres.
The epimysium separates the muscle from adjacent structures and organs and carries blood vessels and nerves that supply the muscle tissue. An extensive vascular network is necessary to deliver the energy required for muscle contraction and to carry away the by-products of this contraction, such as metabolic waste. The epimysial nerves innervate the skeletal muscle fascicles and the muscle fibres within them, carrying the signal required to initiate contraction of the muscle tissue.
The epimysium also contributes to the resistance of a muscle to tensile forces. It protects the muscle and indirectly participates in force generation during muscle contraction.
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Fascial compartments
There are four fascial compartments in each of the upper limbs: two in the arm (anterior and posterior) and two in the forearm (anterior and posterior). The anterior compartment of the arm contains the muscles that flex the elbow joint, while the posterior compartment contains the muscles that extend it. The anterior compartment of the forearm contains muscles that flex the wrist and digits and pronate the forearm, while the posterior compartment 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, which are involved in extending the knee joint. The medial compartment contains the pectineus, external obturator, gracilis, adductor magnus, adductor longus, and adductor brevis, which are involved in the adduction of the hip.
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. The muscles in each compartment are usually supplied by the same nerve and have separate blood supplies from their neighbours.
Compartment syndrome is a pathology specific to fascial compartments, which can be either acute or chronic. It occurs when there is bleeding or swelling inside a compartment, putting pressure on the surrounding fascia, nerves, and blood vessels. Acute compartment syndrome is usually the result of a severe injury, such as a broken bone or crush injury, and requires emergency medical treatment.
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Endomysium
The endomysium electrically isolates each muscle fibre, reducing the risk that depolarization is transmitted to other muscle fibres. It also helps transfer the force generated by individual muscle fibres to additional connective tissue layers, which connect to tendons, fascial sheaths, and bone.
The endomysium is a highly deformable tissue that adapts to changes in volume during muscle fibre contraction, allowing their autonomous gliding during contraction.
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Frequently asked questions
Each muscle fascicle is surrounded by a layer of connective tissue called the perimysium.
Perimysium is a layer of dense irregular connective tissue that is continuous with the endomysium.
Perimysium encloses the fascicle and runs larger vessels, nerve trunks, and sensory neuromuscular spindles.
