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Muscle fascicles are groups of muscle cells (fibers) that are bundled together within a connective tissue sheath called the perimysium. Each muscle fiber is covered by endomysium, and the entire muscle is covered by epimysium. The perimysium may surround anywhere from 10 to 100 fascicles, which are then grouped together to form a whole muscle belly, encased by an outer layer of connective tissue called the epimysium.
| Characteristics | Values |
|---|---|
| Type of tissue | Skeletal muscle |
| Composition | Group of muscle cells (fibers) |
| Arrangement | Parallel, circular, convergent, unipennate, bipennate, multipennate, circumpennate |
| Covering | Connective tissue sheath called perimysium |
| Whole muscle composition | Group of fascicles covered by epimysium |
| Connective tissue | Endomysium, perimysium, epimysium |
| Connective tissue function | Support, protection, blood vessel and nerve pathways |
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Connective tissue sheath perimysium
A muscle fascicle is a group of muscle cells or fibres that are grouped together in parallel within a connective tissue sheath called the perimysium. The perimysium is also called interfascicular connective tissue. It is a sheath of dense irregular connective tissue that surrounds individual muscle fascicles (bundles of muscle fibres), separating 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. It is continuous with the endomysium, which wraps around individual muscle fibres, and the epimysium, which encloses the entire muscle. The perimysium contains a rich network of blood vessels and nerves, known as the neurovascular bundles, that branch out to supply muscle fibres of each fascicle with nutrients and oxygen, while also facilitating signal transmission.
The perimysium is a well-ordered structure that lies throughout the muscles. Thick amounts of perimysium enclosing large fascicles of myofibers form tubes in a honeycomb arrangement in the direction of myofibers. The walls of these tubes are in continuity with tendons at their ends and with the epimysium at the outer surface of the muscle. The perimysium also plays a role in transmitting lateral contractile movements.
Fascicle arrangements determine the type of movement a muscle can make. For example, circular muscles act as sphincters, closing orifices. 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.
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Endomysium
The endomysium is a sheath of reticular fibrils that surrounds each muscle fibre. It is a delicate connective tissue that envelops individual skeletal muscle fibres within the fascicles and forms their immediate internal environment. Endomysium is formed by fibroblasts and is composed of collagen and elastin fibres, mainly type III and type IV collagen. The collagen fibres are flexible and adapt to changes in volume during muscle fibre contraction.
The endomysium is the key element that separates single muscle fibres from one another. It allows their autonomous gliding during muscle contraction and contributes to the resistance of a muscle to tensile forces. It also has a role in regulating the metabolic exchange between muscle and blood, containing capillaries which supply the muscle fibre.
Studies have shown that immobilisation of the endomysium results in a marked increase in the number of perpendicularly oriented collagen fibres that make contact with two adjacent muscle fibres. This disturbs the normal structure of the endomysium, making it impossible to distinguish the various networks of fibres from one another.
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Epimysium
The epimysium is one of three collagenous sheaths that unite and fuse where the muscles connect to adjoining structures such as tendons. The other two sheaths are the perimysium and the endomysium. The perimysium surrounds each bundle of muscle fibres, while the endomysium covers each single muscle fibre.
The epimysium carries blood vessels and nerves that supply the muscle tissue and innervate the skeletal muscle fascicles and the muscle fibres within them. This vascular network delivers the energy required for muscle contraction and carries away metabolic waste. The epimysium also separates the muscle from adjacent structures and organs.
The epimysium does not only protect the muscle, but it also indirectly participates in force generation during muscle contraction (passive force).
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Fascicles
A muscle fascicle is a group of muscle cells (fibres) that are bundled together within a connective tissue sheath called the perimysium. Each muscle fibre is a single cylindrical muscle cell, and an individual skeletal muscle may be made up of hundreds or even thousands of muscle fibres.
The perimysium contains capillaries, nerve endings, and neuromuscular spindles. It may surround anywhere from 10 to 100 fascicles, and these fascicles are then grouped together to form a whole muscle belly, encased by a fibrous tissue envelope called the epimysium. The perimysium and epimysium are both connective tissues that provide support and protection for the muscle cells, allowing them to withstand the forces of contraction.
Each muscle fibre within a fascicle is surrounded by another connective tissue called the endomysium, which also contains capillaries and nerve tissue to supply the individual muscle fibres. The endomysium, perimysium, and epimysium extend beyond the fleshy part of the muscle to form a thick, ropelike tendon or a broad, flat sheet-like aponeurosis. These connective tissues unite to form indirect attachments from muscles to bones or to the connective tissue of other muscles.
The arrangement of fascicles within a muscle determines the type of movement the muscle can make. 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. The majority of skeletal muscles in the body have this parallel arrangement. Other types of fascicle arrangements include convergent, unipennate, bipennate, and multipennate.
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Tendons
The structure of tendons is important for their function. The tendon is surrounded by the epitenon, a thin layer of connective tissue. Outside the epitenon is the paratenon, a loose layer of connective tissue that enables the tendon to move against neighbouring tissues. Sharpey fibres, composed of collagen, attach the tendon to the bone. The endotenon is connective tissue that surrounds the fibre bundles within the tendon, facilitating their smooth gliding. Some tendons, particularly in the hands and feet, have a protective outer covering called the sheath or synovium, which produces a lubricating fluid for smooth movement.
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Frequently asked questions
A muscle fascicle is covered by a layer of connective tissue called perimysium.
Perimysium is a connective tissue sheath that surrounds a muscle fascicle. It contains capillaries, nerve endings, and neuromuscular spindles.
Inside the perimysium are muscle fascicles, which are bundles of muscle fibers or cells.
