“Flight or Fright” Response


To produce the fight-or-flight response, the hypothalamus activates 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. The combined effects of these two systems are the fight-or-flight response.

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. If there’s a burglar at the door, you’re going to have to take action — and fast. 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.

At the same time, the hypothalamus releases corticotropin-releasing factor (CRF) into the pituitary gland, activating the adrenal-cortical system. The pituitary gland (a major endocrine gland) secretes the hormone ACTH (adrenocorticotropic hormone). ACTH moves through the bloodstream and ultimately arrives at the adrenal cortex, where it activates the release of approximately 30 different hormones that get the body prepared to deal with a threat.

The sudden flood of epinephrine, norepinephrine and dozens of other hormones causes changes in the body that include:

  • heart rate and blood pressure increase
  • pupils dilate to take in as much light as possible
  • veins in skin constrict to send more blood to major muscle groups (responsible for the “chill” sometimes associated with fear — less blood in the skin to keep it warm)
  • blood-glucose level increases
  • muscles tense up, 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)
  • smooth muscle relaxes in order to allow more oxygen into the lungs
  • nonessential systems (like digestion and immune system) shut down to allow more energy for emergency functions
  • trouble focusing on small tasks (brain is directed to focus only on big picture in order to determine where threat is coming from)

­All of these physical responses are intended to help you survive a dangerous situation by preparing you to either run for your life or fight for your life (thus the term “fight or flight”). Fear — and the fight-or-flight response in particular — is an instinct that every animal possesses.

Consider this stressful situation: At a meeting for which you have thoroughly prepared, the chair criticizes you and accuses you of failing to attend to tasks that were, in reality, someone else’s responsibility. As all eyes turn on you, you feel your face getting hot, your jaw tightening, and your fist clenching. You would not shout or hit anyone—doing so would only make things worse. But you feel like shouting or striking out.

Now consider another stressful situation: You walk into class a few moments late, only to find everyone putting books and notes away—apparently preparing for a test you did not realize had been scheduled for today. Your heart seems to stop, your mouth is dry, your knees feel weak and you momentarily consider hurrying back out the door. Your life is not really in danger, and running away will not solve your problem—so why should you feel a physical urge to escape?

These two scenarios illustrate the two poles of the fight-or-flight response, a sequence of internal processes that prepares the aroused organism for struggle or escape. It is triggered when we interpret a situation as threatening. The resulting response depends on how the organism has learned to deal with threat, as well as on an innate fight-or-flight “program” built into the brain.

The learned fight response

Evidence that the fight response can be learned is seen, for example, in studies showing that reactions to a perceived insult are strongly dependent on culture. In the United States the learned fight response has been nurtured in the “culture of honor” that developed in the South—which some experts believe may account for the southern states’ much higher murder rate in comparison to the northern states.  Learning can also affect our internal responses to stress. For example, in a study of patients with high blood pressure (which can be a stress response), those who took placebos along with their medication for high blood pressure maintained a healthy blood pressure after the medication was removed, as long as they continued taking the placebo.
This suggests that their expectation that the placebos would control their blood pressure was enough to reduce the emergency response of the blood vessels.

While the fight or flight response clearly can be learned, it also involves an innate reaction that operates largely outside consciousness. This was first recognized in the 1920s by physiologist Walter Canon, whose research showed that a threat stimulates a sequence of activities in an organism’s nerves and glands. We now know that the hypothalamus controls this response by initiating a cascade of events in the autonomic nervous system (ANS), in the endocrine system and in the immune system.

As you will recall, the autonomic nervous system regulates the activities of our internal organs. When we perceive a situation as threatening, this judgment causes the hypothalamus to send an emergency message to the ANS, which sets in motion several bodily reactions to stress. This response is helpful when you need to escape a hungry bear or confront a hostile rival.

It served our ancestors well, but it has a cost. Staying physiologically on guard against a threat eventually wears down the body’s natural defenses. In this way, suffering from frequent stress —or frequently interpreting experiences as stressful —can create a serious health risk: an essentially healthy stress response can become distress.

Article Sourced from:  Adapted from Psychology, Third Edition, by Philip G. Zimbardo,
Ann L. Weber and Robert Lee Johnson & http://science.howstuffworks.com/life/fear2.htm

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