When was the last time your hair stood up on the back of your neck? Ever wonder why?

Goosebumps and your neck hair standing on end are sibling responses, so to speak, caused by fear, low temperatures, stress, or other extreme emotions or conditions. They are both caused by contractions of miniature muscles (attached to each hair) that create a shallow depression on the skin surface. In turn, the surrounding area protrudes causing both bumps on your skin and the hair on your arms and neck to stand on end. Each of these physiological phenomena is inherited from our animal ancestors, according to George A. Bubenik, a physiologist and professor of zoology at Ontario’s University of Guelph, writing in Scientific American.

In animals with a thick hair coat, this response serves any number of purposes. It often occurs as a response to cold and so helps warm the body. Elevated hair expands the layer of air that naturally insulates the body. The response to cold, then, helps an animal’s body more effectively retain heat. In other situations, say, a cat being attacked by a bigger feline, the raised hair, together with an arched back, makes the cat appear bigger and scarier to a predator. Threatened equals threatening, courtesy of a little biophysiology.

However, we humans lack a hair coat, so this physiological response is useless, most people would say. Still, it could be argued that this unusual physical response raises our awareness of possible danger — helps us feel the fear, so to speak.

Underneath the mechanical muscle contraction, after all, is the release of a stress hormone. Adrenaline, also called epinephrine, is produced in two beanlike glands that sit atop the kidneys and released into the bloodstream. Not only does this hormone cause the contractions of skin muscles, it also causes the cascade of physiological responses we fondly refer to as flight-or-fight. The suddenly racing heart, the feeling of "butterflies" in the stomach, trembling hands, sweaty palms — all of these signs tell us that adrenaline is coursing through our veins. In practical terms, the sudden flood of this hormone increases our muscle strength. (No one is joking about the fight part of the equation.)

What is strangest about this hormone, though, is it also may be released during our happiest moments. Sometimes we tremble with joy as we do with fear. "There is a thin line between pleasurable and unpleasurable stress," Bubenik said. Since the brain clearly does not distinguish between danger and delight, we must assume that is a job for the soul.