Brain Tours: Fear

(updated 12/2014)

The anatomy of fear

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).

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 “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 a difficult-to-erase fear response (post-traumatic stress disorder, PTSD).  It seems to be genetically different and “wired” for a higher level of fear in some individuals, such as those with panic disorder . (More on that genetic basis).

Simplified brain anatomy: where is the amygdala?

amygarrow

 

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:

bgtrans3gif

 

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 imagine the temporal lobe on your side of these central structures, the one that was removed so that you could see almost to the middle of the brain in the line drawing above. The amygdala is part of the temporal lobe, way on the toward-the-middle side of it, and rather near the front.  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) :

amygdahelp

amygdalabook

If you’re really following all this, you’ll have figured out that the amygdala circled in yellow two pictures up is the Right Amygdala.  Think about it: all the rest of the brain has been removed, on “your side” of the amygdala you’re seeing here. We’re looking at the toward-the-middle-of-the-brain side of this brain’s right temporal lobe.  The leafy-vegetable-looking thing off the back, on the right, is the cerebellum. The broad white area is nerve axons (“white matter”, get it?) heading up to the thin bit of cortex around the outer upper edges.

Head back for more Brain Tours. What follows is just more “where am I?” training on amygdala location.  Unless you wanted more?

More detail on amygdala location: MRI images

If you would like a little more yet, these MRI images may help you see how the amgydala sits in the temporal lobe:

hippolat

 

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?

coronamyg

 

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.  You’ll see it how its involved in mental problems on many other pages on this site, as — unfortunately — the amygdala is very closely tied to mood disorders generally and bipolar disorder variations in particular.

More Brain Tours