The Endocrine System
The endocrine system works very closely with the nervous system, although it is not classified as part of it. Endocrine glands secrete a variety of hormones, which are biochemicals that travel through the bloodstream. The endocrine system is connected to the nervous system by a gland known as the hypothalamus. This tiny structure is responsible for regulating an enormous chunk of behavior, including sleep, stress responses, eating, thirst, and hunger.
The hypothalamus also controls the pituitary gland, sometimes referred to as the master gland, which secretes some hormones that directly affect emotionality and arousal, and some that stimulate other glands to secrete theirs. The onset of puberty and menopause are examples of complex processes that are governed by hormone levels.
The effects of hormones take a while to wear off. Thus, if you get whiplash in a traffic accident, you initially don't feel much pain, but it becomes excruciating later when the endorphins subside. If you have a terrifying near miss, your “adrenaline rush” may produce so much arousal that you have to pull over for a while to get yourself under control.
During the fight-or-flight reaction, the pituitary releases endorphins, which are natural painkillers that work at the synaptic level and reduce the sensation of pain — a highly adaptive reaction if someone or some thing is assaulting you. The pituitary also stimulates the adrenal glands to secrete epinephrine (also called adrenaline), a hormone that is the agent by which the sympathetic nervous system increases heart rate, blood sugar level and red blood cells, in each case providing you with more energy — highly adaptive if you decide to run or fight back.