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.
Skin is sensitive too- Touch, pressure, pain and temperature
Tongue is sensitive too- Chemicals in food
Nose is sensitive too- Chemicals in the air
Eyes are sensitive too- Light
Ears are sensitive too- sound and position of the head
CNS Central Nervous System
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.
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 into the blood.
Neurones, nerve cells and Synapses
Neurones are nerve cells. They carry information as tiny electrical signals. There are three different types of neurones, each with a slightly different function.
- Sensory neurones carry signals from receptors to the spinal cord and brain.
- Relay neurones carry messages from one part of the CNS to another.
- Motor neurones carry signals from the CNS to effectors.
Where two neurones meet, there is a tiny gap called a synapse. Signals cross this gap using chemicals. One neurone releases the chemical into the gap. The chemical diffuses across the gap and makes the next neurone transmit an electrical signal.
When a receptor is stimulated, it sends a signal to the central nervous system, where the brain co-ordinates the response. But sometimes a very quick response is needed, one that does not need the involvement of the brain. This response is known as a reflex action.
Reflex actions are rapid and happen without us thinking. For example, you would pull your hand away from a hot flame without thinking about it. The animation below allows you to step through each stage of the reflex arc.
This is what happens:
- Receptor detects a stimulus (change in the environment).
- Sensory neurone sends signal to relay neurone.
- Motor neurone sends signal to effector.
- Effector produces a response.