Both the nervous system and the endocrine system are organ systems that facilitate information and regulate body function. Both are essential for informing the automatic processes of the body; the nervous system is primarily responsible for transmitting information about external stimuli, while the endocrine system translates some of that stimulus information into hormones that affect different biological reactions. The nervous system reacts very fast, for example activating muscles to respond to a pain stimulus. The endocrine system can act fast or slow depending on the function, and since hormones travel via the bloodstream a reaction from the endocrine system can take hours.
The most important connection between the two systems is the hypothalamus, which connects them together via the pituitary gland. This allows the nervous system to send and receive information from the endocrine system, which then monitors and regulates biological reactions around the body in direct response. In many cases, an external stimulus from the nervous system activates a hormone response in the endocrine system, which then travels to the appropriate part of the body for a biological change.