Final answer:
The sliding filament theory describes how muscles contract by the sliding of myosin over actin filaments within muscle fibers, which shortens sarcomeres and leads to muscle contraction without changing the length of the filaments.
Step-by-step explanation:
The sliding filament theory is a widely accepted model that explains the process of muscle contraction. It posits that the thick myosin filaments use the energy from ATP to 'walk' along the thin actin filaments, creating cross bridges, which cause the actin filaments to slide over the myosin filaments. This action pulls the Z lines closer together, thereby shortening the sarcomere and leading to muscle contraction. During this process, although the sarcomeres shorten, the actual length of the thin and thick filaments does not change.
The process begins when a motor neuron stimulates a muscle fiber, causing Ca²² to enter the sarcoplasm. This triggers the myosin heads to bind with the actin filaments, initiating their sliding motion. The Z discs, located at the ends of each sarcomere, are pulled closer together as the filaments slide past one another, which results in shortening of the muscle fiber and ultimately muscle contraction. Electron microscopy supports this model, showing the changes in the sarcomere bands during muscle contraction and relaxation.