Fast responses
Animal nerve signals can activate muscles in a fraction of a second. The hand withdrawal and pupil examples show protective reflexes.
Slow responses
Plants often change growth or water pressure in cells. These responses are slower but still help the plant obtain resources or avoid harm.
Moving toward or away
Movement toward a useful stimulus is a positive response. Movement away from danger is a negative response.
Threshold and adaptation
A threshold prevents responses to signals that are too weak. Adaptation reduces the response to a stimulus that stays unchanged.