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Memory, Learning, and Emotion: the Hippocampus
Emotion and memory are very closely related. You know this from your experience. Go to a party, meet a bunch of new people. Which faces are you going to remember? The woman who made you laugh, the man who made you feel embarrassed, and your new boss -- the ones who had an emotional impact.
So perhaps you would not be surprised to learn that the a portion of the emotion system of the brain (the "limbic system") is in charge of transferring information into memory. From years of experiments and surgical experience, we now know that the main location for this transfer is a portion of the temporal lobe called the hippocampus.
You won't particularly impressed by its appearance. The hippocampus is included here because it is always cited as part of the "limbic system", and because there is striking new "news" about:
- how it works;
- its vulnerability to stress; and
- its rich supply of estrogen and progesterone receptors.
We'll consider each of those in turn, after we look at just what and where it is.
Here's Harry. Harry's looking to your left. His left ear would be right in the middle of this picture. Notice the green portion of the brain: this is called the temporal lobe. It lies right under your temple. Memories of names live in this left temporal lobe: particularly people's names, names of animals, and names of tools. If you would like two paragraphs on "why these names?", click here.
The portion of the brain that helps those names get into memory in the first place, this hippocampus thing we are talking about, is also part of this temporal lobe. But you can't see it here, because it's an inside fold, not these outside folds you see above. To see the hippocampus, we'll have to use x-ray vision. Imagine you could just squint and see right through the temporal lobe to what's underneath. Using some beautiful images from a great medical illustrator, here's what you'd see:
Can you see the temporal lobe, there on the outside, just as before? We're here to look at the purple part at the bottom: that's the hippocampus. For the moment you can ignore all the other limbic system elements shown here; we'll be back to learn more of them in other Tours.
So, there's that purple thing, part of the innermost fold of the temporal lobe. How "inner" is that? To see that, imagine you could look straight down from the top of Harry's head, and use your x-ray vision to see the hippocampus, outlined in purple -- you could see how far toward the middle it sits:
Ignore the lines, they're for the poor medical students who have to memorize all the latin names of the structures shown. You can see it on the left side, outlined; the right one is also shown -- see it over there on the right, in shadow a bit? A few more structures have been deleted on the right so you can really see the entire "body" of the hippocampus there, and the "head" with its bulging folds.
Why should you care where this thing is? Here are three reasons. First, this part of the brain appears to be absolutely necessary for making new memories. If you didn't have it, you couldn't live in the present: you'd be stuck in the past of old memories. And this is common: Alzheimer's disease affects the hippocampus first and severely, before other parts of the cortex (later, the frontal lobes too). So memory is usually the first thing to start to falter in Alzheimer's -- the ability to make new ones, that is. Who visited yesterday? Where did I put the car keys? Why isn't there any mail today (when you brought it in 3 hours ago)?
Secondly, the hippocampus seems to be involved in severe mental illnesses. In both schizophrenia and some severe depressions, the hippocampus appears to shrink. However, there is recent evidence that this shrinkage can be reversed and perhaps prevented in people with depression and bipolar disorder, with effective treatment. You'll find a summary of this research in Part Two of the essay on the Brain Chemistry of Depression. One of the mini-chapters there includes this story about neurogenesis, the birth of brand new brain cells, which occurs in the hippocampus and frontal lobes.
Third, the hippocampus is known to be directly affected by estrogen. Look at these slides from the important work of Catherine Woolley and colleagues at Northwestern University:
The slide on the left (a) shows the arms and branches of a hippocampus nerve cell. The bumps are places where connections to other cells occur ("synapses" -- for a diagram of what synapses look like and how they work, read the section on how antidepressants work inside cells). The slide on the right (b) shows the same thing -- except this cell was exposed to estrogen. If you look closely you'll notice that there are more bumps, and the bumps are bigger. The whole thing is "spinier" (which one would you rather brush against if they were rose bushes, for example?) So, estrogen increases "synaptic density" -- the number of connections to other nerve cells -- in the hippocampus especially.
There is research on the role of estrogen in preventing Alzheimer's; on its role in causing seizures, perhaps through this increase in synaptic connection; and on the relationship to "trophic" molecules that are now thought to potentially be the basis of depression and protection against depression (for more on that subject, read the "how antidepressants work inside cells" section). Since some of the work on estrogen focuses on the hippocampus, I thought you might want to know where it is.
For those of you who are really interested in the hippocampus, read on for more pictures and details. The rest of you can Tour elsewhere if you wish.
More Details of Hippocampus Shape and Location
Here we'll see how it looks:
On MRI Scans
Ok, so Harry is fortunate to have a fairy godmother and she's going to take him to dinner now -- well, 2/3 of him anyway. The last third will stay behind for us to look at, and it's Harry's right "third". So where Cindy got split down the middle, Harry will divide along a line back from his right eye. His nose and left eye and left ear and everything all went for dinner. You can see the other half of his right eye and his (maroon) right temporal lobe, below. If you've got the geometry of this picture, you'll understand that you're looking at the inside of his temporal lobe, instead of the outside of the left one you saw at the start of this Tour. If you're lost, check out the next photo below.
Here's a look at Cindy's brain again: her entire right half.
If you've read the mood Tour , you'll recognize the red and blue areas. Now notice the yellow outline: that's her right temporal lobe, poking out from behind the brainstem. You're seeing the inner surface of the temporal lobe there.
But to find the hippocampus, we have to look farther up the temporal lobe, and Cindy's brainstem is in the way of our view. The yellow outline traces the temporal lobe, if we could see it behind the brainstem, and the green X marks where we should be looking.
To see it, we'll look at Harry's temporal lobe again, outlined in maroon:
TP marks the "temporal pole", the front of the lobe. But within the lobe, right there on its inner surface, is the structure we're after: the hippocampus, outlined in white, and marked HIP. (AMG is the amygdala). Try comparing this view to the artist's view from the same perspective, above.
Cindy's pictures are sections of real brain; Harry's are called MRI's
-- in case you wondered.
Diseases of the Hippocampus
The hippocampus can be injured by too little blood flow, Alzheimer's Disease -- and epilepsy, which has some striking similarities to bipolar disorder. Now that you know what it looks like, you may be able to follow a discussion of these disease processes for medical students. The pictures are fantastic, and will increase your understanding of the anatomy even further. Rather than cut up the presentation, I will refer you to the first year medical student course at a major medical school. When you get there, scroll down to section E right away (it's easier, and it has the pictures). If you're really fascinated and not too intimidated, then go back and look at the rest of their page. Y'all come back now, y'hear?
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