The brain is the organ that signals the body to react to changes. Your senses help you react to your environment by detecting the change. When you see danger coming, your brain is triggered to respond accordingly.
Sensory receptors in the special senses bring in information about the environment and get processed in an area of the brain called the limbic system. The first component is the thalamus and it acts like a switchboard that directs traffic into the brain, and then a part called the amygdala is more like a doorman or bouncer by identifying if the approaching signals are coming from friend or foe, good or bad, pleasant or painful sources.