When you experience a sudden fright, it triggers your body's fight-or-flight response (pardon the rhyme). This is an instinctive, immediate reaction to danger that prepares your body to help defend itself by fighting or escaping. Your body pumps adrenaline to your muscles, boosting them to respond powerfully, and your breathing and heart rates increase to pump more blood throughout your body.
The brain's hypothalamus initiates the body's fight-or-flight response by simultaneously activating both the sympathetic nervous system (which triggers the nerves) and the adrenal-cortical system (which dumps hormones into the bloodstream). The action of the sympathetic nervous system causes the body to become tense and very alert. Meanwhile, the hypothalamus alerts the pituitary gland to activate the adrenal-cortical system, which releases about 30 different hormones to prepare the body to handle the threat.
The hormones released into the body during a fear response cause the following physical reactions:
- Increase in heart rate and blood pressure
- Dilated pupils
- Constriction of veins in the skin, which causes the chilly sensation often associated with fear
- Increased blood glucose
- Tensing of muscles and goose bumps
- Relaxation of smooth muscles
- Shutting down of nonessential systems such as digestion and the immune system
- Difficulty concentrating on small tasks
Once the threat diminishes, the body releases the hormone cortisol to calm itself back down to normal. The entire fight-or-flight cycle is part of a defense mechanism that has developed over thousands of years. Without it, one of your direct ancestors may not have been able to defend themselves or escape from predators or enemies.
While it can save your life, there is a downside to the fight-or-flight mechanism. Sufferers of posttraumatic stress disorder (PTSD), which was formerly known as shell shock, will often have elevated levels of the hormones associated with fight or flight, they may experience some of the symptoms when the situation does not warrant it, or their bodies may react as though the threat is real when they are simply reminded of a traumatic event [source: WebMD]. PTSD can be caused from a single traumatic event, or it may be caused by prolonged periods of elevated fight-or-flight hormones -- the body loses its ability to shut the process off when it's not needed. That is why PTSD is so common among soldiers. The Pentagon estimates that one in six soldiers returning home from war suffer PTSD [source: CNN]. Researchers are looking into ways of helping them bring the disorder back under control, and the solution may exist in learning to regulate the sympathetic nervous system [source: Wired]
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