What makes your muscle move?

Muscle contraction is the process by which muscles generate force and movement. It occurs when the muscle fibers shorten, pulling on the tendons that connect them to bones. The sliding filament theory explains how muscle contraction works at the molecular level.

According to the sliding filament theory, muscle contraction occurs when thin filaments (actin) slide past thick filaments (myosin) within the muscle fibers. This process is driven by the hydrolysis of ATP, the cell's energy currency. Here's a step-by-step explanation:

1. Resting State: When a muscle is at rest, the thin and thick filaments are partially overlapped, but they are not actively interacting.

2. Action Potential: When a muscle receives a signal from the nervous system, an action potential is generated. This electrical impulse travels along the muscle cell membrane and into the muscle fibers.

3. Calcium Release: The action potential causes the release of calcium ions from the sarcoplasmic reticulum (SR), the muscle's internal calcium store. Calcium binds to receptors on the thin filaments, exposing myosin-binding sites.

4. Myosin Heads Bind to Actin: The exposed myosin-binding sites on the thin filaments allow the myosin heads (projections from the thick filaments) to bind to them, forming cross-bridges.

5. Power Stroke: Each myosin head contains an ATPase enzyme that hydrolyzes ATP into ADP and inorganic phosphate (Pi). The energy released from ATP hydrolysis causes a conformational change in the myosin head, generating a power stroke. This power stroke pulls the thin filaments toward the center of the sarcomere, the basic unit of muscle contraction.

6. Thin Filaments Slide: As the myosin heads undergo power strokes, the thin filaments slide past the thick filaments, causing the muscle fiber to shorten. This sliding motion continues as long as there is ATP available and calcium ions are present.

7. Muscle Contraction: The shortening of individual muscle fibers leads to the overall contraction of the muscle. The force generated by the muscle depends on the number of cross-bridges formed and the frequency of the power strokes.

8. Relaxation: When the action potential ends, calcium is actively pumped back into the SR, and the myosin heads detach from the actin filaments. This causes the muscle fibers to relax and return to their resting length.

The sliding filament theory provides a detailed understanding of the molecular mechanisms underlying muscle contraction. It explains how the interaction between actin and myosin filaments, fueled by ATP hydrolysis, generates the force necessary for muscle movement and contraction.