Myofibrils are long, cylindrical organelles found within muscle cells, also known as muscle fibers. These specialized structures run parallel along the entire length of the muscle fiber, making up a large percentage of its volume. Their purpose is to convert chemical energy into mechanical force, driving muscle movement and generating tension. The organized arrangement of proteins within the myofibrils makes them the contractile unit of all striated muscle tissue.
The Architecture of Myofibrils
The structural organization of the myofibril is defined by repeating segments called sarcomeres, which function as the smallest contractile units of the muscle. Each sarcomere is delineated by Z-discs, which anchor the thin protein filaments at their ends. The sequential arrangement of these sarcomeres, placed end-to-end, gives skeletal and cardiac muscle their characteristic striped or striated appearance.
The striations are the result of the precise overlap of two primary types of protein filaments, creating alternating light and dark bands. The dark region, known as the A-band, encompasses the entire length of the thick protein filaments and includes the areas where the thick and thin filaments overlap. Positioned in the center of the A-band is the H-zone, which contains only thick filaments in a relaxed muscle.
The lighter regions, referred to as I-bands, contain only the thin protein filaments and are bisected by the Z-disc. Running down the center of the H-zone is the M-line, a network of accessory proteins that holds the thick filaments in place. This highly ordered, repeating pattern ensures that the force generated by each sarcomere is efficiently summed up across the entire muscle fiber.
The Role of Contractile and Regulatory Proteins
The myofibril is built from two categories of proteins: contractile proteins that generate force, and regulatory proteins that control contraction timing. The thick filaments are primarily composed of the motor protein myosin, which has a long tail and two globular heads. Each myosin head possesses a site capable of binding to the thin filament and another site that can break down adenosine triphosphate (ATP), the cell’s energy currency.
The thin filaments are mainly built from the protein actin, which polymerizes into a twisted double strand. Each actin molecule has a specific binding site where the myosin head can attach to begin the contraction cycle. In a resting muscle, these myosin binding sites are physically covered and blocked by the regulatory protein tropomyosin.
Tropomyosin is a rod-shaped protein that coils around the actin filament, preventing interaction between actin and myosin. Attached to the tropomyosin strand is troponin, a complex of three proteins that acts as the control switch for muscle contraction. Troponin directly interacts with calcium ions, the necessary signal to move the tropomyosin blockage. Therefore, the presence or absence of calcium determines whether the thin and thick filaments can physically interact.
The Sliding Filament Mechanism of Contraction
Muscle contraction is explained by the sliding filament theory, which posits that muscle shortening occurs because the thick and thin filaments slide past one another. The filaments themselves do not shorten; instead, the overall length of the sarcomere decreases as the Z-discs are pulled closer together. This sliding motion is generated by the cyclic interaction of the myosin heads with the actin filaments, a process called the cross-bridge cycle.
The process is initiated by an increase in calcium ions (Ca2+) within the muscle cell cytoplasm, released upon nerve stimulation. These Ca2+ ions immediately bind to the troponin complex on the thin filament. This binding causes a conformational change in troponin, which then pulls the attached tropomyosin molecule away from the myosin binding sites on the actin.
With the binding sites exposed, the cross-bridge cycle begins. The energy that energized the myosin head came from the prior breakdown of ATP into adenosine diphosphate (ADP) and inorganic phosphate (Pi), which remain temporarily bound to the myosin.
Attachment
The energized myosin head binds strongly to the actin filament, forming the cross-bridge. The release of the Pi molecule triggers the next step.
Power Stroke
The myosin head pivots and bends, pulling the thin actin filament toward the center of the sarcomere. This mechanical movement shortens the sarcomere and generates the force of the muscle contraction. Following the power stroke, the ADP molecule is released from the myosin head, leaving the myosin firmly attached to the actin in a stiff state.
Detachment
This step requires a new molecule of ATP to bind directly to the myosin head. The binding of this fresh ATP causes the myosin head to immediately release from the actin filament, breaking the cross-bridge. ATP is necessary for both contraction and relaxation.
Resetting (Reactivation)
The newly bound ATP is hydrolyzed—broken down into ADP and Pi—by the myosin’s ATPase activity. The energy released from this breakdown cocks the myosin head back into its high-energy, ready-to-bind position, preparing it for the next cycle. This cycle will continue to repeat as long as Ca2+ remains bound to troponin and a sufficient supply of ATP is available.