Amygdala
The brain's smoke detector. Fast, crude, and it can't tell a bear from an email.
Two small almond-shaped clusters deep in the brain. One of their main jobs is to spot threat and get the body ready before you've thought about it: heart up, muscles tense, attention narrowed. Evidence: solid That's why you can be angry before you know what the message said. Evidence: decent
It's fast because it's crude. It reacts to tone, faces, and anything that once went badly. It doesn't do context; that's the slower front of the brain, which gets there a second later. Evidence: decent
Why it matters
Most of the advice on this site about breathing, cold water, naming the feeling and calling a break is about giving the slow part of the brain time to catch up with the fast part. Evidence: decent