Final answer:
The sliding filament theory describes how a muscle contracts by myosin filaments sliding actin filaments toward a sarcomere's center after being stimulated by a motor neuron, resulting in muscle contraction.
Step-by-step explanation:
When a motor neuron stimulates a muscle fiber, a chain of events unfolds leading to muscle contraction. This is explained by the sliding filament theory. Myosin and actin are two types of protein filaments integral to this process. Myosin, the thick filament, uses energy from ATP to attach to and pull the thin actin filaments, which are connected to the Z discs at the ends of a sarcomere. As the myosin filaments 'walk' along the actin, they effectively slide it towards the sarcomere's center, shortening the sarcomere and causing the muscle to contract.
The sequence of molecular events occurring during contraction is known as crossbridge cycling. This cycle is a series of attachments and detachments between the myosin heads and actin filaments, powered by the hydrolysis of ATP. The resulting action pulls the Z discs towards each other, shortening the sarcomere. When all sarcomeres in a muscle fiber contract in this manner, the entire muscle fiber contracts.