Victor Forge
Skeletal muscle is made up of muscle cells, also known as muscle fibres. Each muscle fibre is surrounded by connective tissue called the endomysium. A bundle of these fibres, or a fasciculus, is surrounded by a layer of connective tissue called the perimysium. Finally, the whole muscle is surrounded by the epimysium, which contains adipose tissue, the arterial and venous supply to the fascicle, and larger peripheral nerve branches.
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What You'll Learn
Connective tissue
Skeletal muscle is surrounded by a connective tissue sheath called the epimysium. The epimysium is surrounded by fascia, another form of connective tissue. The epimysium projects inward to divide the muscle into compartments, each of which contains a bundle of muscle fibres called a fasciculus. Each fasciculus is surrounded by a layer of connective tissue called the perimysium. Within the fasciculus, each individual muscle cell or fibre is surrounded by a thin, delicate network of connective tissue strands called the endomysium. The connective tissue coverings provide support and protection for the muscle cells, allowing them to withstand the forces of contraction. They also provide pathways for the passage of blood vessels and nerves.
The perimysium is made up of concentric strands of connective tissue that enclose the fascicle. It is more abundant than the endomysium, which is scant. Larger vessels, nerve trunks, and sensory neuromuscular spindles run through the perimysium. The endomysium, on the other hand, carries the capillary network with its longitudinal orientation.
The perimysium and endomysium layers of connective tissue are important components of the muscle structure. They provide structural support and facilitate the necessary vascular and neural connections for the muscle fibres. These connective tissues are also involved in muscle regeneration, as they contain satellite cells that play a role in early muscle fibre growth.
In summary, the connective tissue surrounding a muscle fascicle consists of three layers: the epimysium, the perimysium, and the endomysium. These layers of connective tissue work together to support, protect, and facilitate the functioning of the muscle fascicle.
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Perimysium
The perimysium is a dense connective tissue sheath that surrounds individual muscle fascicles (bundles of muscle fibres) and separates them from other fascicles within the skeletal muscle. It is composed of concentric strands of connective tissue, primarily collagen, that enclose the fascicle. 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 muscle function. It contains a rich network of blood vessels and nerves, known as neurovascular bundles, that branch out to supply muscle fibres of each fascicle with essential nutrients and oxygen. This network also facilitates signal transmission between the muscle fibres. During muscle contraction, the perimysium transmits the force produced by individual muscle fibres across the fascicles, resulting in smooth and coordinated muscle contractions and movements.
The tensile properties of the perimysium are similar to those of the endomysium. However, due to its thicker composition, the perimysium may not be an efficient force transmission pathway under normal working conditions. Instead, it demonstrates high tensile stiffness and can carry large loads in tension, but only at very large extensions beyond the range of working lengths in living muscle.
The perimysial layers separating two fascicles are composed of two or more crossed-plies of wavy collagen fibres in a proteoglycan matrix. The arrangement of these collagen fibres changes with muscle length, exhibiting maximal waviness when the muscle is at its relaxed length. As the muscle shortens, the angle between the long axis of the collagen fibres and the longitudinal axis of the fascicle increases, and this angle decreases when the muscle is passively stretched out.
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Endomysium
The endomysium is a sheath of connective tissue that surrounds each individual muscle fibre within a fascicle. It is a thin, delicate network of strands that envelops the muscle fibres, creating their immediate internal environment.
The endomysium is composed primarily of type III and type IV collagen, with type I and type V collagen also present in smaller amounts. It is the site of metabolic exchange between blood and the skeletal muscle fibre, as it contains a capillary network that supplies the muscle fibre. This capillary network has a longitudinal orientation and is carried by the endomysium.
The endomysium is one of three collagenous sheaths that unite and fuse where muscles connect to adjoining structures such as tendons. The other two sheaths are the epimysium and the perimysium. The epimysium surrounds the entire muscle and carries tendon organ sensory endings and, sometimes, prominent tendinous bands. The perimysium surrounds each primary bundle or fascicle of muscle fibres and contains larger vessels, nerve trunks, and sensory neuromuscular spindles.
These three sheaths extend beyond the fleshy part of the muscle to form a thick, ropelike tendon or a broad, flat, sheet-like aponeurosis. The tendon and aponeurosis form indirect attachments from muscles to the periosteum of bones or to the connective tissue of other muscles.
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Epimysium
The epimysium is one of three collagenous sheaths that unite and fuse where muscles connect to adjoining structures such as tendons. The other two sheaths are the perimysium and the endomysium. The perimysium surrounds each primary bundle or fascicle of 40-150 fibres, while the endomysium surrounds each individual muscle fibre. The perimysium is a continuous network of connective tissue that divides the muscle into fascicles or muscle fibre bundles. It is a well-ordered structure that lies throughout the muscles, with thick amounts enclosing large fascicles of myofibers to form tubes in a honeycomb arrangement in the direction of myofibers.
The epimysium carries tendon organ sensory endings and sometimes prominent tendinous bands. It also carries blood vessels and nerves that supply the muscle tissue. Blood vessels travel through the collagenous sheath around the outside of the whole skeletal muscle, providing the muscle with blood supply. Nerves travel through the epimysium around the outside of the whole skeletal muscle, innervating the skeletal muscle.
The epimysium is important for protecting the muscle and also indirectly participates in force generation during muscle contraction. It separates the muscle from adjacent structures and other fascicles, and contributes to the muscle's resistance to tensile forces.
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Neuromuscular spindles
The neuromuscular spindles are stretch receptors that respond to changes in muscle length and velocity. They are supplied with a γ-efferent neuron and are activated by gamma motor neurons, which change the resting firing rate and stretch-sensitivity of the afferents. The gamma motor neurons induce contractions of sarcomeres in the polar region to exert tension on the central region of intrafusal fibres, preventing their slackening during muscle shortenings.
The sensory information from the neuromuscular spindles is transmitted to the higher brain centres, where it is unconsciously integrated with other sensory information. This information is essential for the central nervous system (CNS) to compute the position and movement of our extremities in space, which is necessary for motor control, posture maintenance, and a stable gait.
Impaired neuromuscular spindle function can lead to problems in motor control, unstable gait, frequent falls, and ataxic behaviour. Therapeutic strategies for neuromuscular diseases should therefore aim to restore and maintain proprioception and muscle spindle function.
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Frequently asked questions
A muscle fascicle is surrounded by the perimysium, which is made up of connective tissue.
The perimysium surrounds a primary bundle, or fascicle, of 40-150 muscle fibres. Larger vessels, nerve trunks, and sensory neuromuscular spindles run through the perimysium.
The epimysium, another layer of connective tissue, surrounds the perimysium and separates the fascicles from each other. The fascia and tendons are contiguous to the epimysium.
