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events associated with fear is often called the "fight or flight" reaction: increased heart rate and force of each beat ("pounding heart"); increased muscle tension that can even cause tremors; sweaty but cold palms; and even nausea and diarrhea. Another aspect of fear is a physical "conditioning", so that even a minor stimulus can bring on the whole fear reaction.

The brain structure which appears to be at the very center of most of the brain events associated with fear is the "amygdala" (Greek for "almond", its shape).

The amygdala seems to respond to severe traumas with an un-erasable fear response

It seems to be genetically different and "wired" for a higher level of fear in some individuals, such as those with panic disorder.

There appears to be a remarkable similarity between (a) the physiological and behavioral consequences of response to a conditioned fear stimulus and (b) a panic attack.

In animals, these responses are mediated by a "fear network" in the brain that is centered in the **amygdala** and involves its interaction with the hippocampus and medial prefrontal cortex

It is speculated that a similar network is involved in panic disorder. A convergence of evidence suggests that both in heritable factors and stressful life events, particularly in early childhood, are responsible for the onset of panic disorder.

Medications, particularly those that influence the serotonin system, are hypothesized to desensitize the fear network from the level of the amygdala through its projects to the hypothalamus and the brainstem. Effective psychosocial treatments (i.e. cognitive-behavioral therapy, CBT) may also reduce contextual fear and cognitive misattributions at the level of the prefrontal cortex and hippocampus. Neuroimaging studies should help clarify whether these hypotheses are correct.