Dominic Stone
Skeletal muscles, the most common type of muscle in the body, are made up of muscle fibres. Each muscle fibre is surrounded by a thin layer of connective tissue known as the endomysium. This layer of connective tissue supplies the muscle fibres with capillaries and nerve tissue. The endomysium is surrounded by the perimysium, which is another layer of connective tissue that encases multiple muscle fibres. The outermost layer of connective tissue is called the epimysium.
| Characteristics | Values |
|---|---|
| Outer covering | Epimysium |
| Middle covering | Perimysium |
| Inner covering | Endomysium |
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Connective tissue layer of fascia
Skeletal muscles are covered by a connective tissue layer of fascia. Fascia is a band of thin, fibrous connective tissue that wraps around and supports every structure in the body. It is a continuous layer of tissue that is flexible and able to resist tension. Fascia is a stringy, white substance composed primarily of collagen, a protein that provides strength and flexibility.
There are four main layers of fascia in the body: superficial, deep, visceral, and parietal. The superficial layer is located directly beneath the skin and varies in thickness depending on its location. Deep fascia surrounds the musculoskeletal system, covering muscles, bones, tendons, cartilage, nerves, and blood vessels. This layer is thicker than the superficial layer and has two subtypes: aponeurotic fascia and epimysial fascia. Aponeurotic fascia is a thick, pearly-white tissue that separates easily from muscles, while epimysial fascia is thinner and more tightly connected to the muscles.
Visceral fascia surrounds organs in the abdomen, lungs, and heart, while parietal fascia lines the walls of certain body cavities, such as the pelvis. Fascia provides structure and support to the body, allowing muscles to contract and stretch smoothly without creating friction or tears. It also stabilizes body structures, improves circulation, and enables all body systems to work together.
The connective tissue layer of fascia surrounding individual muscle fibers is called the endomysium. Each muscle fiber is composed of several hundred to several thousand myofibrils, which are made up of actin and myosin filaments. These filaments form functional units called sarcomeres, which are necessary for muscle contraction. The endomysium contains capillaries and nerve tissue that supply the individual muscle fibers.
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Endomysium
The endomysium, along with the perimysium and epimysium, covers the sarcolemma, or muscle cell membrane. Capillaries and nerve tissue are present within the endomysium to supply the individual muscle fibres. The endomysium contains the capillaries that supply the muscle fibre with blood, contributing to the muscle's resistance to tensile forces. The collagen fibres of the endomysium provide strength to the surrounding connective tissue and transfer the force generated by muscle cells.
The endomysium is a delicate structure that can be disturbed by immobilisation. Studies have shown that immobilisation results in a marked increase in endomysial collagen, which is deposited directly on the sarcolemma of the muscle cells. This leads to an increase in the number of perpendicularly oriented collagen fibres that make contact with two adjacent muscle fibres, disrupting the normal structure of the endomysium.
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Perimysium
The perimysium, also known as interfascicular connective tissue, is a connective tissue sheath that surrounds bundles of muscle fibres, known as fascicles. It is the middle layer of connective tissue covering muscle fibres, with the endomysium forming the innermost layer and the epimysium forming the outermost layer.
The perimysium surrounds anywhere from 10 to 100 fascicles, delineating the muscle bundles within a muscle. It is made up of dense irregular connective tissue, containing mainly type I and type III collagen. The collagen fibres are arranged at an angle of ±55º to the longitudinal axis of the fascicle when the muscle is at rest. This angle increases as the muscle shortens and decreases when it is stretched out.
The perimysium is continuous with the endomysium and the epimysium. It contains a rich network of blood vessels and nerves, known as neurovascular bundles, that branch out to supply muscle fibres with nutrients and oxygen, as well as facilitating signal transmission. During muscle contraction, the perimysium transmits the force produced by individual muscle fibres across the fascicles, resulting in smooth and coordinated muscle movements.
The tensile properties of the perimysium are similar to those of the endomysium. However, due to its greater thickness, the perimysium may not be an efficient pathway for force transmission under normal working conditions. Instead, it may come into play during extreme circumstances, such as muscle damage or surgical disconnection of tendinous attachments.
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Epimysium
The epimysium is one of three collagenous sheaths that surround and protect muscle fibres. The other two sheaths are the perimysium and the endomysium. The perimysium is the middle layer of connective tissue that surrounds bundles of muscle fibres, known as fascicles. Each fascicle contains multiple muscle fibres, and the perimysium serves to delineate and hold these bundles together. The endomysium is the innermost layer of connective tissue, covering each single muscle fibre or myofibre.
Blood vessels and nerves travel through the epimysium, supplying the muscle tissue with blood and innervating the skeletal muscle fascicles and muscle fibres within them. This extensive vascular network delivers the large amounts of energy required for muscle contraction and carries away metabolic waste. The epimysium also contributes to the muscle's resistance to tensile forces.
The three collagenous sheaths unite and fuse where the muscles connect to adjoining structures such as tendons. Together, these layers are called mysia.
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Sarcolemma
The sarcolemma is a membrane that surrounds individual muscle fibres. It is a lipid bilayer with a thin outer coat of polysaccharide material (glycocalyx) that contacts the basement membrane. The basement membrane contains thin collagen fibrils and proteins such as laminin, which provide a scaffold for the muscle fibre to adhere to.
The sarcolemma is approximately 75 Å thick and is a highly convoluted structure in the region of the innervation zone (motor end-plate) due to the presence of junctional folds. Elsewhere along the surface of the fibre, the membrane displays much shallower folds resulting from the slackness of the membrane when the fibre is in its resting or contracted state.
The sarcolemma has the special property of excitability, enabling electrical impulses to be transmitted down the length of the cell. It contains proteins for various functions, including transport and enzyme activity. For example, ATPase enzymes pump cations across the membrane, with one type responsible for sodium and potassium and another for calcium and magnesium.
The sarcolemma also invaginates into the sarcoplasm of the muscle cell, forming membranous tubules called T-tubules or transverse tubules. These tubules are connected to the surface membrane by narrow necks, and their function is uncertain, although they can act as reserve sources of membrane during fibre stretching.
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Frequently asked questions
Individual muscle fibers are covered by a thin layer of connective tissue called the endomysium.
The endomysium surrounds the extracellular matrix of the cells and plays a role in transferring force produced by the muscle fibers to the tendons. It also contains capillaries and nerve tissue to supply the individual muscle fibers.
The outermost layer of tissue covering the entire muscle is called the epimysium.
The epimysium provides pathways for the passage of blood vessels and nerves. It also projects inward to divide the muscle into compartments.
The middle layer of connective tissue surrounding bundles of muscle fibers is called the perimysium.
