2.79 Understand that a coordinated response requires a stimulus, a receptor and an effector

To carry out a response, 3 main things are needed:

• A stimulus: A change in environment
• A receptor: Something that detects the change 
• An effector: To carry out a response

Example:

• Stimulus: you touch a hot object
• Receptor: nerves in your finger
• Effector: muscles that pull your hand away from the potentially dangerous hot object