Last night I woke up to banging and realized that a man was outside my bedroom window trying to break the lock to steal my bicycle that was parked in my patio. I jumped up and started screaming, running around in circles for what seemed like very long minutes looking for my clothes and keys, confused but full of energy. Moments later, when I realized I had scared off the thief with my screams and he had left my bike as it was, I noticed that my whole body was shaking, trembling from every cell. What chemical and physiological processes were responsible for that animal instinct reaction that came to me when I jumped up ready to fight, bypassing any mental decision whatsoever? And what was happening in my body when that energy drained and left me scared and confused?
The hypothalamus is what is responsible for producing this fight or flight response in your body. The hypothalamus can activate two systems: the sympathetic nervous system and the adrenal-cortical system. The sympathetic nervous system uses nerve pathways to initiate reactions in the body, and the adrenal-cortical system uses the bloodstream.
When the hypothalamus tells the sympathetic nervous system to kick into gear, the overall effect is that the body speeds up, tenses up and becomes generally very alert. When I heard that banging, I immediately jumped into action without a thought. The sympathetic nervous system sends out impulses to glands and smooth muscles and tells the adrenal medulla to release epinephrine (adrenaline) and norepinephrine (noradrenaline) into the bloodstream. These "stress hormones" cause several changes in the body, including an increase in heart rate and blood pressure, which is what I felt as my heart raced and I started to tremble.
At the same time that I was experiencing this, my hypothalamus was releasing corticotropin-releasing factor (CRF) into my pituitary gland, activating the adrenal-cortical system. The pituitary gland (a major endocrine gland) secretes the hormone ACTH (adrenocorticotropic hormone). ACTH moved through my bloodstream and ultimately arrived at the adrenal cortex, where it activated the release of approximately 30 different hormones that prepare me for a threat. Some of the symptoms that I experienced are common of the fight or flight response, such as an increase in heart rate and blood pressure, a tensing of muscles energized by adrenaline and glucose (responsible for goose bumps -- when tiny muscles attached to each hair on surface of skin tense up, the hairs are forced upright, pulling skin with them), a flow of oxygen into the lungs (allowing me to scream so strongly perhaps), as well as trouble focusing on small tasks (as the brain is directed to focus only on big picture in order to determine where threat is coming from). The physical experience for me was unlike anything I have experienced, and though at the time it was not a good feeling, thinking about it now I understand how it has allowed us to survive millennia of evolution.