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Skeletal Muscle Structure


Background Information

skeletal muscle is an organ composed mainly of muscle fibers (cells), blood vessels, nerves, and three layers of connective tissue. The muscle fibers are the contractile units of a muscle, and the blood vessels and nerves sustain and regulate the activity of the fibers. The collagen fibers in the connective tissues bind the muscle components together and maintain their structural organization.

The thick outermost layer of connective tissue is called the epimysium. It consists primarily of irregularly arranged collagen fibers and covers the whole muscle, defining its volume. The epimysium layer helps stabilize a muscle’s structure so forceful contractions can occur. It also separates a muscle from other nearby tissues and organs, allowing it to move smoothly and independently.

Connected to the inner epimysium is a second layer of connective tissue called the perimysium, which spans the interior of a muscle. The collagen fibers in this connective tissue layer surround groups of muscle fibers (10 – 80) and organize them into bundles called fascicles that run the length of a muscle. Arteries, veins, and nerves are also embedded in the perimysium and can supply individual fascicles to create graded contractions.

Skeletal muscle structure

The innermost layer of connective tissue, the endomysium, is an extension of the perimysium that spans the interior of each fascicular bundle. The endomysium comprises a loose network of collagen and reticular fibers that connects the individual muscle fibers. Nerve cells (neurons) in the endomysium trigger groups of muscle fibers to contract (motor unit), and nearby capillaries supply nutrients and gases.

To move a bone, the collagen in the connective tissue layers of a muscle must first merge with the collagen in a tendon. The tendon then attaches to a bone by joining with its collagenous outer layer, the periosteum. When muscle fibers contract, the generated tension transfers to the muscle’s connective tissues, then to the tendon and the bone periosteum. The force pulls on the bone and moves the skeleton.

Most skeletal muscles attach to bones through tendons, which are cord-like bands of dense connective tissue. The collagen fibers in a tendon attach to the outer collagenous layer of a bone, called the periosteum. When muscle fibers contract, the tension transfers to the tendon and the periosteum, which moves the bone.

Other skeletal muscles connect to wide, flat sheets of connective tissue called aponeuroses. Like tendons, aponeuroses are composed of collagen and attach to bones. However, they also envelop muscles and organs, bind muscles together, and bind muscles to other tissues, such as the skin. They support muscles, give the body strength and stability, and absorb energy when muscles move. The ventral abdomen and the dorsal lumbar regions are two areas of the body that contain prominent layers of aponeuroses. For example, the thoracolumbar fascia consists of several aponeurotic fascial layers and joins the latissimus dorsi muscles in the lower back.

Example of an aponeurosis

Structure Identification

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References and Attributions

Cleveland Clinic – “Skeletal Muscle.”

Frontiers in Pathology – “The Structure and Role of Intramuscular Connective Tissue in Muscle Function.”

NIH: National Library of Medicine – “Anatomy, Fascia Layers.”

NIH: National Library of Medicine – “Anatomy of Skeletal Muscle and Its Vascular Supply.”

NIH: National Library of Medicine – “Anatomy, Tendons.”

NIH: National Library of Medicine – “Connecting muscles to tendons: tendons and musculoskeletal development in flies and vertebrates.”

NIH: National Library of Medicine – “The development of the myotendinous junction. A review.”

NIH: National Library of Medicine – “The Structure and Role of Intramuscular Connective Tissue in Muscle Function.”

NIH: National Library of Medicine – “The Myotendinous Junction—A Vulnerable Companion in Sports. A Narrative Review.”

Science Direct – “Muscle Fascicle.”

Science Direct – “Perimysium.”