Sensitivity: responding to stimuli

All living organisms must be able to detect changes in their environment and respond appropriately. Changes in the environment are called stimuli(singular: stimulus). A stimulus may be in either the external environment (outside the organism) or the internal environment (inside the organism). Sensitivity,the ability to respond appropriately to stimuli, is one of the characteristic features of life. Each organism has its own specific type of sensitivity that improves its chances of survival. A single-celled amoeba, for example, can move away from a harmful stimulus such as very bright light, and move towards a favourable stimulus such as food molecules, but it can only distinguish between a limited number of different stimuli.

In an amoeba, the detection of the stimulus and the response to the stimulus must both take place in a single cell. However, in large multicellular animals such as mammals, stimuli are detected in sense organs,and organs that respond are called effectors. The sense organs and effectors may be in quite different parts of the body. In addition, responses usually involve the coordinated actions of many different parts of the body. To achieve this coordination, one part of the body must be able to pass information to another part. In mammals, there are two major systems that convey information: the nervous system and the endocrine (hormonal) system.