How the Muscular System Works
Motion is a result of muscle fibers contracting and relaxing. The actions of your muscular system are controlled by your nervous system, which sends a never-ending stream of messages along your nerves. When a skeletal muscle fiber receives a signal, it will either relax or contract. If told to contract, the resulting motion will pull on a bone and create body movement.
For a muscle to exert force, it must be able to shorten or contract. This happens when thick and thin filaments in the muscle cells slide past each other. Think of a centipede walking on a thin branch: the centipede represents the red myosin filament, while blue actin illustrates the branch. The centipede’s feet are the myosin heads, which act as little motors to pull the centipede along the thin filament branch. The myosin heads can attach to and detach from the thin filament, moving the muscle back and forth.
In the muscle, movement looks like this: initially, the thick filaments’ myosin heads are detached from the thin filaments. Due to certain intracellular chemical reactions, the myosin heads become “cocked” or tense, which readies them for the next stroke. An incoming nerve cell signal orders the thin filaments to attach to the myosin heads and uncock, which makes the myosin heads and thin filaments pull past each other.
The thick filament's myosin heads and their movement along the thin filaments are vital for contraction. With millions of myosin heads in your muscles continuously cycling back and forth, substantial force is produced.