Nearly every observable movement in the animal kingdom, from the blink of an eye to the beat of a heart, is powered by two proteins: actin and myosin. This pair functions as a microscopic engine, converting chemical energy into mechanical force. Their interaction is the basis for muscle contraction and is also a central process in the life of most eukaryotic cells, enabling both large-scale movement and essential internal activities.
Structural Components of Muscle Fibers
Skeletal muscle is composed of long cells called muscle fibers, which contain cylindrical bundles known as myofibrils. Each myofibril is a chain of repeating contractile units called sarcomeres. The sarcomere’s highly organized structure, formed by two main protein filaments, gives skeletal and cardiac muscle their striated, or striped, appearance.
The primary components are thick filaments, composed of myosin molecules, and thin filaments, made mostly of actin. The thick myosin filaments create the dark “A bands” seen under a microscope, while the light “I bands” contain only thin actin filaments. Actin filaments are anchored at their ends to a structure called the Z-disc, which marks the boundary of each sarcomere, and extend inward toward the center.
Myosin filaments are situated in the center of the sarcomere, anchored at the M-line. These filaments feature globular heads that protrude outward to interact with the surrounding actin filaments. This creates an overlapping region where both filament types are present, which is necessary for contraction.
The Mechanism of Muscle Contraction
Muscle contraction is explained by the sliding filament model. In this process, the filaments do not change in length; instead, thin actin filaments slide past thick myosin filaments, pulling the Z-discs closer together and shortening the sarcomere. This process is driven by a sequence of molecular events known as the cross-bridge cycle.
The cycle begins when the muscle is stimulated, and myosin heads attach to binding sites on the actin filaments, forming a cross-bridge. Once bound, the myosin head pivots in a “power stroke,” pulling the actin filament toward the sarcomere’s center. During this step, adenosine diphosphate (ADP) and inorganic phosphate (Pi) are released.
For the myosin head to release the actin, a new molecule of adenosine triphosphate (ATP) must bind to it, breaking the cross-bridge. The ATP is then hydrolyzed back into ADP and Pi, which releases energy. This energy re-cocks the myosin head into its original position, ready to bind to another site on the actin. This cycle repeats as long as signals are present, causing continuous filament sliding.
Regulation of the Interaction
The interaction between actin and myosin is tightly controlled to ensure muscles contract only when needed. This regulation is managed by two accessory proteins on the thin actin filaments: troponin and tropomyosin. In a resting muscle, tropomyosin physically blocks the myosin-binding sites on actin, preventing cross-bridge formation and keeping the muscle relaxed.
The signal for contraction is the release of calcium ions (Ca2+). When a muscle fiber receives a nerve signal, calcium is released from an internal storage organelle called the sarcoplasmic reticulum. These calcium ions then bind to the troponin complex, inducing a change in its shape.
As troponin shifts, it pulls the attached tropomyosin strand away from the myosin-binding sites on the actin filament. With these sites exposed, myosin heads can bind to actin and begin the power stroke cycle. Contraction continues as long as calcium levels remain high. When the nerve signal ceases, calcium is pumped back into the sarcoplasmic reticulum, troponin returns to its original shape, and tropomyosin again blocks the binding sites, relaxing the muscle.
Functions Beyond Muscle Contraction
While known for their role in muscle, the actin-myosin partnership is a motor system used by nearly all eukaryotic cells for movement and internal organization. These non-muscle functions are important for cellular life.
A prominent example is during cell division, or cytokinesis. After a cell’s genetic material is separated, a contractile ring of actin and non-muscle myosin forms at the cell’s equator. This ring tightens like a purse string, pinching the cell membrane inward until the parent cell is cleaved into two daughter cells.
The actin-myosin system also serves as an internal transport network. Actin filaments form tracks, and myosin motors “walk” along them, carrying cargo like vesicles and organelles. These interactions also drive cell motility, allowing cells like amoebas or immune cells to crawl and migrate through dynamic changes in cell shape.