A stimulus is a detectable change in internal or external conditions that produces a response in an organism
There is a considerable advantage for organisms that can correctly perceive stimuli and respond to them. For example, a plant moving towards light and a mouse running away from danger both are examples of useful responses that aid survival.
Organisms that respond to their environment are more likely to survive, producing a selection pressure which favours these organisms.
A stimuli is detected by a receptor, which converts energy of the stimulus into a form that the organism can interpret and respond to.
A response is then carried out by an effector, which is a specialised cell, tissue or organ.
Because receptors and effectors can be physically seperate, a from of communication is needed. In plants, hormones mediate the receptor-effector complex, while animals developed a nervous system to respond
How a nervous response works
- A stimulus is always the first step in a response. It is usually a physical variable, such as light intensity, temperature or blood pH.