The finding set Feinstein on a decade-long search for the truth about the amygdala’s role in fear. It turns out that there are actually two different fear pathways in the brain, depending on whether the threat is external or internal.
Getty ImagesWhen it comes to external threats, the amygdala acts like an orchestra conductor, directing the other parts of the brain and body to produce a response. First it receives information from the brain areas that process vision, smell, taste and hearing. If the amygdala detects a threat, such as an approaching burglar, snake or bear, it then sends messages to the hypothalamus, a region just above the back of the neck. The hypothalamus then communicates with the pituitary gland, which in turns gets the adrenal glands to release cortisol and adrenaline into the bloodstream.
“This will cause your heart rate to go up, blood pressure to rise, and all the classic sort of fight-or-flight symptoms of a typical fear response will be evoked,” says Feinstein.
However, when it comes to internal threats, such as detecting raised levels of CO2 in the blood, the brain manages things in a different way. The body interprets the high CO2 as a sign of impending suffocation, as there aren’t any oxygen sensors in the bloodstream. Feinstein’s research has shown that it is the brainstem, a region that regulates unconscious bodily functions such as breathing, that senses the rise in CO2 and initiates a sense of panic. The amygdala puts the brakes on this response, preventing fear; hence why patients like SM, who are missing their amygdala, have such an exaggerated response. (Scientists still don’t know why, however, the amygdala behaves this way.)
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