So. Take your arm for instance.
.
Everything... everything, even how you think of the Question of the Ultimate Purpose of Life, begins with the brain. The brain sends a signal through the body through nerves (NERVOUS SYSTEM) whose signal is connected to the muscle (MUSCULAR SYSTEM). Then, since the muscle is attached to the bone, (SKELETON), the muscle contracts. The arm then pivots on the elbow joint.
Hope I helped!