When you are afraid, you are likely to have:
worried thoughts; and
physical sensations like a faster heart rate, sweating, increased breathing; and
behaviors, like trying to escape the situation that made you afraid in the first place!
There is a complicated series of events in the nervous system that leads to the physical sensations and behaviors of fear (the thoughts, we can't localize so well). Below you'll find details of the anatomy of this process -- but for a simple look at the "fear circuitry" of the brain, have a look at Joseph LeDoux's lab site. Inside his site you can look at a brief essay on fear, based on their research. After that, come back here if you want a more detailed look at the amygdala (you'll know what that is by the time you come back!)
One of these physical 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 ("post-traumatic stress disorder", or PTSD; click for a superb site by the Madison Institute of Medicine on PTSD). It seems to be genetically different and "wired" for a higher level of fear in some individuals, such as those with panic disorder. And it recently has been shown to be larger in some people with bipolar disorder, though what that means is still a mystery.
You can see the amygdala clearly in this drawing: it's the small (almond-shaped, almost) bulge at the lower tip of the gold loop (yellow arrow). If you've had the "R Complex" Tour, you'll know the gold loop as the "caudate", so the amygdala is a small bulge at the lower tip of the caudate, if that helps. Here's another way to see it:
There's the caudate again, making that big loop, and at it's lower end there's this thing we're calling the amygdala. This drawing shows the "almond" shape of the bulge. You can see how the amygdala is located within the temporal lobe (see the temporal lobe outline there?) If you've already had the Tour of the hippocampus, you've seen this territory before: the amygdala lies just in front of the hippocampus, in about the same spot on the inside of the temporal lobe.
What does this thing really look like in a real brain? Here are two images: in the first, the amygdala has been circled in yellow; the second is the same picture without the highlight (see #17) :
Not much to look at, is it. When some clear images of changes in the amygdala due to a mental health problem become available, I will try to post those here. If you've had enough, you can go back to the Brain Tours options, or to the home page of this website.
If you would like a little more yet, these MRI images may help you see how the amgydala sits in the temporal lobe:
This hopefully will look like something you understand, if you've had the Tour of the hippocampus. If not you can go there or review. Notice that just ahead of the hippocampus (HIP) is the amygdala (AMG). You can see through the outline and the letters that there's really nothing which identifies "amygdala" as separate from the rest of the tissue on this MRI. Rather, we know from examining real brains that the amygdala can be found in the location circled above (so if you can't see a separate structure here, that's not your problem, that's how these scans look. This is why it takes a skilled radiologist to be able to say "that's the amygdala, right there" looking at one of these scans).
Finally, if you're really sharp you'll have noticed that you know roughly where the amygdala is, but only relative to the eyeballs and back of the head; and relative to the neck and the top of the head (roughly halfway in each case). But you don't know, at least from these MRI scans, where it is relative to your two ears. Is it halfway? That would be straight behind your nose, right? Or maybe one third of the way toward the nose from the ear?
In this scan the amygdala is circled on both sides, labeled on the left. Again you see that it is located on the inner fold of the temporal lobe, circled in yellow.
Now you know where it is. To learn more about what it does, in mental health terms, you can come back soon to read the planned section on panic disorder (see "What's New?"). Until then, if you'd like to see how the amygdala is now regarded as in the center of the fear process in panic, try the abstract I've copied in below. A little technical but not too tough for you?
(PsychEducation.org (home) Brain Tours)