In order to protect itself the body has developed ways react to changes in its environment. For example, if you get too hot you will sweat to cool down. If light is too bright, your pupils will constrict so that your eyes are not damaged. In order to do this, the body needs to be able to detect internal and external changes (called stimuli) and make the appropriate response.

To function effectively, there needs to be good communication inside the body. Hormones are just one of the tools used to send messages to the various parts of the body and nerve impulses are used to send messages within the body. Hence both the systems coordinate to perform the stimulatory reactions in our body.