Just think, the next time you are about to have sex, do you feel your heart beating faster?, yourself feeling a little hotter, even though the temperature in the room is the same and even though certain parts of your are not being overly stimulated by touch, you are suddenly very aware of them!
This can also happen just by sitting in a chair doing nothing, suddenly you have a thought to do with feeling angry or sad and suddenly your pancreas secretes some hormone. Your pancreas? How did you manage to do that with your pancreas? I bet you don't even know where your pancreas is! LOL
Your liver is making an enzyme that wasn't there before, your spleen is text-messaging something to your thymus gland, blood flow in little capillaries in your ankles has just changed, all from a thinking thought!
I think we all kind of know the brain can regulate functions throughout the body, but it is still mind blowing to be reminded of how far-reaching the effects can be.
Hormones and stress response
As the master gland, the brain can experience or think of something stressful and activate components of the stress response hormonally.
Two hormones vital to the stress response are epinephrine (Adrenaline) and norepinephrine (noradrenaline), released by the sympathetic nervous system. Another important class of hormones in the response to stress are called glucocorticoids. (Glucocorticoids are steroid hormones), steroid is used to describe the general chemical structure of 5 classes of hormones, Androgens the famed "anabolic" steroids like testosterone - estrogens, progestins, mineralocorticoids, and glucocorticoids, they are secreted by the adrenal gland and they often act in similar ways to to adrenaline.
Adrenaline acts within seconds; glucocorticoids back this activity up over the course of minutes or hours.
When something stressful happens or you think a stressful thought, your brain secrets an array of hormones into the circulatory system that gets the ball rolling.
In addition, in times of stress your pancreas is stimulated to release a hormone called glucagon. Glucocorticoids, glucagon, and the sympathetic nervous system raise circulating levels of the sugar glucose. These hormones are essential for providing energy during stress.
Other hormones are activated as well. The pituitary secretes prolactin, which, among other effects, plays a role in suppressing reproduction during stress, both the pituitary and the brain also secrete a class of endogenous morphine-like substances called endorphins and enkephalins, which help blunt the pain perception among other things. (Just think back to a time where you were highly stressed, you might have hurt yourself, but you don't feel nothing until later on).
Finally, the pituitary also secretes vasopressin also known as antidiuretic hormone, which plays a role in the cardiovascular stress-response.
Just as some glands are activated during stress, various others are inhibited during stress, the secretion of reproductive hormones such as estrogen, progesterone, and testosterone are inhibited. (think of your menstrual cycle being affected by this or if you male, think of your sex drive being quite low). Hormones related to growth, such as growth hormone, are also inhibited, as is the secretion of insulin, a pancreatic hormone that normally tells your body to store energy for later use.
This, then, is an outline of our current understanding of the neural and hormonal messengers that carry the brain's news that something bad is happening.
A lot of this information was taken from the book Why Zebras Don't Have Ulcers by Robert M Sapolsky