The nervous system allows the body to respond to changes in the environment in a process usually coordinated by the brain. Reflex actions are extra-rapid responses to stimuli; this process also involves the nervous system but bypasses the brain.
Receptors are groups of specialised cells that can detect changes in the environment called stimuli. Receptors are often located in the sense organs such as the ear, eye and skin. Each organ has receptors sensitive to particular kinds of stimulus.
Eyes: sensitive to light
Ears: sensitive to sound and position of head
Tongue: sensitive to chemicals in food
Nose: sensitive to chemicals in the air
Skin: sensitive to touch, pressure, pain and temperature
The human central nervous system (CNS) consists of the brain and spinal cord. When a receptor is stimulated it sends a signal along the nerve cells, also called neurones, to the brain. The brain then co-ordinates the response.