Skeletal muscle is composed of bundles of muscle fibers, which are individual muscle cells. These fibers contain numerous smaller, rod-like structures known as myofibrils. Myofibrils are the fundamental structures that organize contractile proteins within the muscle cell. They are arranged in parallel throughout the muscle fiber, running its entire length. The myofibril’s primary role is to house the apparatus responsible for generating force and movement.
Locating the Fundamental Unit
The functional contractile unit of the myofibril is the sarcomere, the smallest segment of the muscle capable of shortening. Myofibrils are long, repetitive chains of sarcomeres linked end-to-end. This repeating arrangement gives skeletal and cardiac muscle its characteristic striated appearance. The physical boundaries of a single sarcomere are defined by two dense, sheet-like structures called Z-discs (or Z-lines).
The Structural Components of the Sarcomere
The sarcomere’s architecture relies on the precise arrangement of two main types of protein filaments: thick and thin filaments. Thick filaments are primarily composed of myosin and occupy the central region of the sarcomere. Thin filaments are mainly made of actin and are anchored to the Z-discs at the ends of the sarcomere. The orderly overlap and spacing of these filaments create the visible bands and zones.
Sarcomere Bands and Zones
The central region containing the entire length of the thick filaments is called the A-band, which appears dark due to the density of myosin. Within the A-band is the lighter H-zone, a central area containing only thick filaments in a relaxed muscle. Bisecting the H-zone is the M-line, a protein structure that anchors and stabilizes the thick filaments.
The area containing only thin filaments is known as the I-band, which appears lighter than the A-band. Thin filaments include actin and the regulatory proteins tropomyosin and troponin. Tropomyosin is a strand-like protein that spirals around the actin chain. Troponin is a complex of three globular proteins attached to tropomyosin, which controls access to the binding sites on the actin.
How Muscles Contract
Muscle contraction is explained by the Sliding Filament Theory, where the thick and thin filaments slide past one another without shortening themselves. The process begins when a neural signal triggers the release of calcium ions from the sarcoplasmic reticulum. These calcium ions flood the myofibril and bind specifically to the Troponin-C subunit of the troponin complex. This binding causes a conformational change in the troponin molecule.
The change in troponin’s shape pulls the associated tropomyosin strand away from the thin filament’s surface. This movement exposes the active binding sites on the actin molecules, which were previously blocked by the tropomyosin. With the binding sites exposed, the globular heads of the myosin molecules, which extend from the thick filaments, are now free to attach to the actin. This attachment forms a cross-bridge.
The attachment of the myosin head to the actin binding site triggers the release of inorganic phosphate, causing the myosin head to execute a mechanical movement called the power stroke. During the power stroke, the myosin head pivots and pulls the attached thin filament toward the center of the sarcomere, shortening the distance between the two Z-discs. A molecule of adenosine triphosphate (ATP) is required to detach the myosin head from the actin binding site.
After detachment, the ATP molecule is hydrolyzed into adenosine diphosphate (ADP) and inorganic phosphate, a reaction that “re-cocks” the myosin head back to its ready-to-bind position. This cycle of attachment, power stroke, detachment, and re-cocking repeats rapidly as long as calcium ions remain present. Each cycle progressively pulls the thin filaments further inward.
The result of this molecular sliding is a change in the banding pattern of the sarcomere. As the thin filaments slide toward the M-line, the I-band (the region containing only thin filaments) visibly shortens. Similarly, the H-zone, containing only thick filaments, also shortens and may even disappear at full contraction. The length of the A-band remains unchanged because the thick filaments themselves do not shorten during the process. The coordinated shortening of millions of sarcomeres generates the force that results in muscle contraction.