So let's talk about
the limbic system. What is the limbic system? Well, it's a set of
structures in the brain. And many of those structures
play an important role in regulating emotion. Now, something that
gets kind of confusing when you talk about
the limbic system is that experts
can't actually agree on what structures make up
the entire limbic system. So for our purposes,
I'm going to address some of the most important
structures and ones that everyone pretty much agrees
are part of the limbic system. Now before I get going into
the nitty-gritty so to speak, I want to give you a quick
overview of what structures we're going to talk about. And the way I remember
these structures is through this
little cartoon here. This is a hippopotamus
and he's wearing a hat. Now, why this is
this hippopotamus wearing this stylish hat? Well, this is my way of
remembering in the four most important components
of the limbic system when it comes to emotion. So we see a hippopotamus here. I'll write "hippo." And we see him wearing a hat. I'll write "hat." Now for this to be a mnemonic,
it has to be something useful. And the reason I
think of this is these are the four main
structures of the limbic system when it comes to emotion. So "HAT" stands for
Hypothalamus, "A" for Amygdala, "T" for Thalamus, and "hippo,"
short for hippocampus. And these happen to
be the four structures that I'd like to talk about. So let's get to a little
more complicated diagram. And what you see here
is my best attempt at drawing the limbic system. Now, limbic system structures
sit on top of the brain stem. And this is the brain stem. And you can imagine this as
the very bottom of your brain. And here's the spinal
cord coming out of it. And the spinal cord
goes all the way down your back to
about your tailbone. Now, the limbic system are
these structures up here, that are drawn in bright colors. Now to orient you
to this diagram, this is what you would
see if you pulled off like the top part of your brain,
which is called the cortex. And it's facing
in this direction. In other words, while this
isn't anatomically correct, let's say your eyes are
here, your nose is here, and your mouth is here. Again, this is not
anatomically correct. But this you can see is the
front, and this is the back. So I kind of drew it at an angle
so you kind of get a 3D idea. So let's remove this
and go back to talking about the anatomical structures. So this blue thing here,
this is called a thalamus. And you actually have
two of these, one here and one on the other side. So your thalamus functions as
like a sensory relay station, meaning the things
that you see, hear, taste, touch, all
these senses you have come through your
nerves and ultimately end up in your thalamus. And the thalamus
directs these senses into the appropriate
areas in the cortex, as well as other
areas of the brain. And I mentioned this
in terms of an emotion lecture because emotions are
very contingent on the things that you see, the things
that you touch and hear. And you may have
noticed there's one sense that I didn't mention. And that's a sense of smell. And the sense of smell
actually is the only sense that you have that actually
bypasses this thalamus. And instead, it has its
own private relay station that, when it comes
from the nose, it goes to a certain
area in the brain. And that area of
the brain actually happens to be very close
to other areas that regulate emotion, which explains
why sometimes certain scents can evoke very powerful
memories and bring you back to a certain moment in time. But in terms of emotion,
I mentioned thalamus because of how the senses
play an important role in your emotions. Now, you see here there's
these two purple structures. And this is known
as an amygdala. Now, the amygdala is sometimes
called the aggression center. And experiments
have actually shown that if you stimulate
the amygdala, you can produce feelings
of anger and violence, as well as fear and anxiety. I'm going to put
"stimulate" and represent it as dark green plus sign. So you stimulate the amygdala. It evokes feelings of anger,
violence, fear, and anxiety. On the other hand, if you've
destroyed your amygdala-- and I'll represent destruction
as a negative sign-- if you destroy the
amygdala, it can cause a very mellowing effect. I'll write "mellow." And this mellowing effect in the
context of a destroyed amygdala was actually noted by
a psychologist named Dr. Kluver and a neurosurgeon
by the name of Dr. Bucy. And I mention Kluver and Bucy
because in medicine there's actually a syndrome known
as Kluver-Bucy syndrome. And that's when there's
a bilateral destruction of your amygdala. And "bilateral" means both. And if you have bilateral
destruction of the amygdalas, that can result in certain
symptoms that are often seen, like hyperorality,
which means you put things in their mouth a lot;
also hypersexuality; as well as
disinhibited behavior. And disinhibited
behavior is when you ignore social conventions. You can act very impulsively. You don't consider the
risks of your behavior. So you do dangerous,
reckless things. So that's Kluver-Bucy syndrome. And that's again
when you destroy both sides of your amygdalas. And the way I remember
this is I think if you stimulate
the amygdalas, that can cause fear and anxiety. And people who have anxiety
disorders or experiencing an anxiety attack
sometimes are given a medication known
as a benzodiazepine. Sometimes they're
called "benzos." And these benzodiazepines
medications function pharmacologically
very similar to alcohol. And think of what
happens when people consume too much alcohol. Sometimes you see these
types of behaviors. You see hyperorality. You might be eating a lot. You might have hypersexuality. And, of course, you get
disinhibited behavior. Think of the person with a
lamp shade on their head. They're ignoring certain
social conventions because of the
effect of alcohol. So that's how I remember
the effect of stimulating versus destroying the amygdala. And this green structure
here that you curving around the thalamus is known
as the hippocampus. And the hippocampus plays a key
role in forming new memories. What it does is it helps
to convert your short-term memory-- I'll
abbreviate it as "STM"-- it helps convert that short-term
memory into your long-term memory. And I mention that
in this conversation because when you think
back on your memories, whether it's short-term
memory or long-term memory, these memories can
evoke emotions as well. So the hippocampus is
an important structure in forming long-term memories. And people with
damage to this area, they have difficulty
forming new memories. So everything that they
experience just basically fades away. Now what's interesting about
this is if your hippocampus is destroyed, while you
can't form new memories, you still have your
old memories intact. So your long-term memory
functions just fine. So that's the hippocampus. Now lastly, this
orange structure here, this orange structure
is the hypothalamus. And "hypo" means below. So hypothalamus is
below the thalamus. And here's the thalamus. And it's below it. So that's where it
gets its name from. And the hypothalamus is
actually a very tiny structure. And this diagram here
really exaggerates the size of the hypothalamus. The hypothalamus is so small
that it actually makes up less than 1% of the total
volume of your brain. It's about the size
of kidney bean. And the hypothalamus
plays an incredible role in regulating so many
functions in your body. But for our purposes, we're
talking about the limbic system structures in terms of emotion. So when it comes to
emotion, the hypothalamus you can think of as regulating
the autonomic nervous system. I'll abbreviate it as "ANS." And the autonomic
nervous system you can think of as fight or
flight versus rest and digest. Now, I'm going to discuss this
further in a different video. But right now, just
think of it as regulating the autonomic nervous system. And it does this by controlling
the endocrine system, by triggering the release of
hormones into your bloodstream. And some of these hormones
that are triggered to release are things like epinephrine
or norepinephrine. And epinephrine is actually very
commonly known as adrenaline. So if you ever
think of the phrase like "a lot of adrenaline
pumping through your veins," that's actually being
regulated by the hypothalamus. Your hypothalamus
is also involved in regulating other basic
drives, like hunger, thirst, sleep, sex. But in terms of
emotion, I think it's most important to
note that it regulates the autonomic nervous system,
that fight or flight or rest and digest response. So that's the limbic system. And these are the four basic
structures, the thalamus, the amygdala, the hippocampus,
and the hypothalamus. So these are the
basic structures of the limbic system.
