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The nervous system allows the body to respond to changes in the environment. This is a process usually coordinated by the brain. Reflex actions are extra-rapid responses to stimuli [stimuli: things that set off a reaction in the nervous system - for example, light, heat, sound, gravity, smell, taste, or temperature. The singular is stimulus ], and this process also involves the nervous system, but bypasses the brain.
Receptors and effectors
Receptors
Receptors are groups of specialised cells. They can detect changes in the environment, which are called stimuli, and turn them into electrical impulses. 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.
Receptors
| sense organs | receptors sensitive to |
|---|---|
 Skin | touch, pressure, pain and temperature |
 Tongue | chemicals in food |
 Nose | chemicals in the air |
 Eyes | light |
 Ears | sound and position of the head |
The central nervous system - CNS - in humans consists of the brain and spinal cord. When a receptor is stimulated, it sends a signal along the nerve cells - neurones - to the brain. The brain then co-ordinates the response.
Effectors
An effector is any part of the body that produces the response. Here are some examples of effectors:
- a muscle contracting to move the arm
- a muscle squeezing saliva from the salivary gland
- a gland releasing a hormone [hormone: chemical messengers produced in glands and carried by the blood to specific organs in the body ] into the blood
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