The attachment of skeletal muscles to the skeleton is demonstrated in Fig. 12.1, resulting in the extension or flexion of the arm. The basic structure of skeletal muscle is depicted in Fig. 12.2, with each muscle comprising numerous cells known as muscle fibers. Tendons connect the skeletal muscle to the skeleton, and the points of attachment are referred to as origin and insertion, respectively. The insertion is located close to the joint, allowing for a wide range of motion. Skeletal muscles span both sides of the joint, enabling both flexion and extension of the forearm.

Skeletal muscle is composed of bundles of muscle fibers called fasciculi, each of which represents an individual muscle cell containing bundles of myofibrils. The arrangement of thick and thin filaments within these myofibrils leads to striations, giving skeletal muscle its striated appearance. The endomysium, a connective tissue layer, surrounds each muscle fiber, while the perimysium encloses the fascicles. Blood vessels and nerves supplying the muscle fibers are found within the perimysium. The fascicles are joined together to form the muscle, which is enveloped by the epimysium, a connective tissue sheath. The muscle is attached to the skeleton through tendons, and at the myotendinous junction, the ends of the muscle fibers interlock with the tendon to transmit the force of muscle contraction and facilitate movement of the skeleton.

The connective tissue layers, including the tendon, primarily consist of elastin and collagen fibers, contributing to passive tension of the muscle and protecting against damage caused by overstretching or contraction. Individual skeletal muscle cells are elongated and can reach lengths of up to 25 cm. Within each skeletal muscle fiber, myofibrils run along the axis of the cell, containing bundles of filaments. The organized alignment of adjacent myofibrils and the arrangement of thick and thin filaments give rise to the striated pattern observed in skeletal muscle at both the fiber and myofibril levels.
Fig. 12.1 ) results in the reverse motion (extending the arm).
The basic structure of skeletal muscle is shown in
Fig.
12.2 . Each muscle is composed of numerous cells called • Fig. 12.1. Skeletal Muscle Attaches to the Skeleton by Way of Tendons and Typically Spans a Joint. The proximal and distal points of attachment of the tendon are termed origin and insertion, respectively. Note that the insertion is close to the joint, which allows a broad range of motion. Also note that skeletal muscles span both sides of the joint, which allows both flexion and extension of the forearm.
• Fig. 12.2. Skeletal muscle is composed of bundles of muscle fibers; each such bundle is called a fasciculus. A muscle fiber represents an individual muscle cell and contains bundles of myofibrils. The striations are due to the arrangement of thick and thin filaments. See text for details. (Redrawn from Bloom W, Fawcett DW. A Textbook of Histology. 10th ed. Philadelphia: Saunders; 1975.) muscle fibers. A connective tissue layer called the endomysium surrounds each of these fibers. Individual muscle fibers are then grouped together into fascicles, which are surrounded by another connective tissue layer called the perimysium. Within the perimysium are the blood vessels and nerves that supply the individual muscle fibers. The fascicles are joined together to form the muscle. The connective tissue sheath that surrounds the muscle is called the epimysium. At the ends of the muscle, the connective tissue layers come together to form a tendon, which attaches the muscle to the skeleton. The myotendinous junction is a specialized region of the tendon where the ends of the muscle fibers interdigitate with the tendon for the transmission of the force of contraction of the muscle to the tendon to effect movement of the skeleton (discussed later in this section). The tendon and the connective tissue layers are composed mainly of elastin and collagen fibers, and thus they also contribute to passive tension of muscle and prevent damage to the muscle fibers as a result of overstretching or contraction.
Individual skeletal muscle cells are narrow (≈10 to 80 µm in diameter), but they can be extremely long (up to 25 cm in length). Each skeletal muscle fiber contains bundles of filaments, called myofibrils, running along the axis of the cell. The gross striation pattern of the cell results from a repeating pattern in the myofibrils. Specifically, it is the regular arrangement of the thick and thin filaments within these myofibrils, coupled with the highly organized alignment of adjacent myofibrils, that gives rise to the striated appearance of skeletal muscle. Striations can be observed in intact muscle fibers and in the underlying  myofibrils.