Final answer:
Muscle contraction occurs when muscle fibers get shorter. The sliding filament model of muscle contraction explains that when a motor neuron signals a muscle fiber, the actin and myosin filaments slide past each other, causing the muscle to contract.
Step-by-step explanation:
Muscle contraction occurs when muscle fibers get shorter. Each muscle fiber contains myofibrils, which are made up of actin and myosin filaments. The sliding filament model of muscle contraction explains that when a motor neuron signals a muscle fiber, the thin actin filaments slide past the thick myosin filaments, resulting in the contraction of the muscle fiber. This sliding can only occur when myosin-binding sites on the actin filaments are exposed by the presence of calcium ions and ATP.