The system that enables humans to react to their environment is called the nervous system. It is a complex network of specialized cells that carry electrical impulses rapidly between different parts of the body. The nervous system detects changes, or stimuli, from the environment through receptors, processes this information in the central nervous system (brain and spinal cord), and coordinates responses by sending signals to muscles or glands (effectors) to react accordingly. This system allows humans to sense and respond quickly to changes both outside and inside the body.