Muscle contraction is a process where muscle fibers shorten. The “sliding filament theory” explains how muscles shorten, describing the interactions of protein filaments within muscle cells as they slide past one another, rather than shortening themselves.
The Muscle’s Basic Unit
The fundamental unit of muscle contraction is the sarcomere, an organized structure found along a muscle fiber. Each sarcomere is defined by Z-lines, which serve as anchoring points for the thin filaments. Within this unit, two protein filaments are arranged in an overlapping pattern.
These two filament types are actin, which forms the thinner filaments, and myosin, which constitutes the thicker filaments. Actin filaments are anchored to the Z-lines and extend inward, while myosin filaments are centrally located within the sarcomere. Their overlap is crucial for the sliding mechanism during contraction.
The Molecular Switch
Muscle contraction is regulated by proteins and ions that act as a molecular switch, controlling when actin and myosin filaments interact. Associated with the actin filaments are two regulatory proteins: troponin and tropomyosin. Tropomyosin is a long, fibrous protein that, in a resting muscle, wraps around the actin filament, blocking the sites where myosin would normally bind.
Troponin is a smaller protein complex attached to tropomyosin. The presence of calcium ions triggers muscle contraction. When calcium ions are released into the muscle cell, they bind to the troponin complex. This binding causes a conformational change in troponin, which in turn pulls tropomyosin away from the myosin-binding sites on the actin filament.
With the binding sites on actin now exposed, the myosin heads are free to attach, allowing muscle contraction. This ensures contraction only occurs when signaled, preventing uncontrolled shortening.
How Muscles Contract
The cross-bridge cycle, which shortens the muscle, involves repetitive interactions between myosin heads and actin filaments. This cycle begins with the myosin head in an energized, “cocked” position, ready to bind to actin. Once the binding sites on actin are exposed by calcium, the myosin head readily attaches, forming a cross-bridge.
Following attachment, the myosin head undergoes a conformational change, called the “power stroke.” During this stroke, the myosin head pivots, pulling the attached actin filament toward the center of the sarcomere. This movement causes the filaments to slide past each other, shortening the sarcomere. The force generated by many myosin heads performing power strokes simultaneously leads to macroscopic muscle contraction.
For the cycle to continue, a new molecule of adenosine triphosphate (ATP) must bind to the myosin head. The binding of ATP causes the myosin head to detach from the actin filament, breaking the cross-bridge. Without new ATP, the myosin head remains firmly attached to actin, leading to a state of rigor, as seen in rigor mortis.
Once detached, the ATP molecule is hydrolyzed into adenosine diphosphate (ADP) and an inorganic phosphate (Pi) by an enzyme on the myosin head. This hydrolysis reaction releases energy, which re-energizes the myosin head, returning it to its “cocked” position. The myosin head is now ready to bind to a new site further along the actin filament, initiating another power stroke.
The Energy Source
Adenosine triphosphate, or ATP, serves as the energy currency that powers muscle contraction and relaxation. ATP’s role is central to the cross-bridge cycle. ATP binding to the myosin head is necessary for the detachment of the myosin from actin, preventing the muscle from remaining locked in a contracted state.
The hydrolysis of ATP into ADP and inorganic phosphate provides the energy required to “re-cock” the myosin head, preparing it for its next interaction with actin. This energy ensures the myosin head returns to its high-energy state, initiating another power stroke. ATP is also consumed by calcium pumps, which transport calcium ions back into storage within the muscle cell after contraction. This removal of calcium from the cytoplasm is what allows tropomyosin to re-cover the actin binding sites, leading to muscle relaxation.