A stimuli is an environmental cue from either the internal environment or the external environment The stimuli is detected by receptors, which pass a signal along to the brain or spinal column through sensory neurones. The brain and spinal column make up the CNS, and they coordinate the body's response to the stimuli. The CNS sends a signal to an effector via motor neurones. An effector is a hormone or a muscle in the body that brings about a response.