AQA GCSE Biology Paper 2 What is homeostasis? ANS:The maintenance of a constant internal environment. What is a stimulus? ANS:A change in your environment than requires a response. e.g Light, sound, touch, pressure, pain, chemical or temperature. What do the receptors do? ANS:Detect the stimulus or change in environment. What happens after the receptors? ANS:Receptors send messages to the CNS via the sensory neurone. What makes up the central nervous system? ANS:The brain or spinal cord Wh...