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Limbic system is complex both structurally and functionally. It is located on either side of the thalamus, immediately below the cerebrum and consists of both It is located on either side of the thalamus, immediately below the cerebrum and consists of both the grey mater and white mater.
So let's talk aboutthe limbic system. What is the limbic system? Well, it's a set ofstructures in the brain. And many of those structuresplay an important role in regulating emotion. Now, something thatgets kind of confusing when you talk aboutthe limbic system is that expertscan't actually agree on what structures make upthe entire limbic system. So for our purposes,I'm going to address some of the most importantstructures and ones that everyone pretty much agreesare part of the limbic system. Now before I get going intothe nitty-gritty so to speak, I want to give you a quickoverview of what structures we're going to talk about. And the way I rememberthese structures is through thislittle cartoon here. This is a hippopotamusand he's wearing a hat. Now, why this isthis hippopotamus wearing this stylish hat? Well, this is my way ofremembering in the four most important componentsof 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 Ithink of this is these are the four mainstructures of the limbic system when it comes to emotion. So 'HAT' stands forHypothalamus, 'A' for Amygdala, 'T' for Thalamus, and 'hippo,'short for hippocampus. And these happen tobe the four structures that I'd like to talk about. So let's get to a littlemore complicated diagram. And what you see hereis my best attempt at drawing the limbic system. Now, limbic system structuressit on top of the brain stem. And this is the brain stem. And you can imagine this asthe very bottom of your brain. And here's the spinalcord coming out of it. And the spinal cordgoes all the way down your back toabout your tailbone. Now, the limbic system arethese structures up here, that are drawn in bright colors. Now to orient youto this diagram, this is what you wouldsee if you pulled off like the top part of your brain,which is called the cortex. And it's facingin this direction. In other words, while thisisn't anatomically correct, let's say your eyes arehere, your nose is here, and your mouth is here. Again, this is notanatomically correct. But this you can see is thefront, and this is the back. So I kind of drew it at an angleso you kind of get a 3D idea. So let's remove thisand go back to talking about the anatomical structures. So this blue thing here,this is called a thalamus. And you actually havetwo of these, one here and one on the other side. So your thalamus functions aslike a sensory relay station, meaning the thingsthat you see, hear, taste, touch, allthese senses you have come through yournerves and ultimately end up in your thalamus. And the thalamusdirects these senses into the appropriateareas in the cortex, as well as otherareas of the brain. And I mentioned thisin terms of an emotion lecture because emotions arevery contingent on the things that you see, the thingsthat you touch and hear. And you may havenoticed there's one sense that I didn't mention. And that's a sense of smell. And the sense of smellactually is the only sense that you have that actuallybypasses this thalamus. And instead, it has itsown private relay station that, when it comesfrom the nose, it goes to a certainarea in the brain. And that area ofthe brain actually happens to be very closeto other areas that regulate emotion, which explainswhy sometimes certain scents can evoke very powerfulmemories and bring you back to a certain moment in time. But in terms of emotion,I mentioned thalamus because of how the sensesplay an important role in your emotions. Now, you see here there'sthese two purple structures. And this is knownas an amygdala. Now, the amygdala is sometimescalled the aggression center. And experimentshave actually shown that if you stimulatethe amygdala, you can produce feelingsof 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'vedestroyed your amygdala-- and I'll represent destructionas a negative sign-- if you destroy theamygdala, it can cause a very mellowing effect. I'll write 'mellow.' And this mellowing effect in thecontext of a destroyed amygdala was actually noted bya psychologist named Dr. Kluver and a neurosurgeonby the name of Dr. Bucy. And I mention Kluver and Bucybecause in medicine there's actually a syndrome knownas Kluver-Bucy syndrome. And that's when there'sa bilateral destruction of your amygdala. And 'bilateral' means both. And if you have bilateraldestruction of the amygdalas, that can result in certainsymptoms that are often seen, like hyperorality,which means you put things in their mouth a lot;also hypersexuality; as well asdisinhibited behavior. And disinhibitedbehavior is when you ignore social conventions. You can act very impulsively. You don't consider therisks of your behavior. So you do dangerous,reckless things. So that's Kluver-Bucy syndrome. And that's againwhen you destroy both sides of your amygdalas. And the way I rememberthis is I think if you stimulatethe amygdalas, that can cause fear and anxiety. And people who have anxietydisorders or experiencing an anxiety attacksometimes are given a medication knownas a benzodiazepine. Sometimes they'recalled 'benzos.' And these benzodiazepinesmedications function pharmacologicallyvery similar to alcohol. And think of whathappens when people consume too much alcohol. Sometimes you see thesetypes of behaviors. You see hyperorality. You might be eating a lot. You might have hypersexuality. And, of course, you getdisinhibited behavior. Think of the person with alamp shade on their head. They're ignoring certainsocial conventions because of theeffect of alcohol. So that's how I rememberthe effect of stimulating versus destroying the amygdala. And this green structurehere that you curving around the thalamus is knownas the hippocampus. And the hippocampus plays a keyrole in forming new memories. What it does is it helpsto convert your short-term memory-- I'llabbreviate it as 'STM'-- it helps convert that short-termmemory into your long-term memory. And I mention thatin this conversation because when you thinkback on your memories, whether it's short-termmemory or long-term memory, these memories canevoke emotions as well. So the hippocampus isan important structure in forming long-term memories. And people withdamage to this area, they have difficultyforming new memories. So everything that theyexperience just basically fades away. Now what's interesting aboutthis is if your hippocampus is destroyed, while youcan't form new memories, you still have yourold memories intact. So your long-term memoryfunctions just fine. So that's the hippocampus. Now lastly, thisorange structure here, this orange structureis the hypothalamus. And 'hypo' means below. So hypothalamus isbelow the thalamus. And here's the thalamus. And it's below it. So that's where itgets its name from. And the hypothalamus isactually a very tiny structure. And this diagram herereally exaggerates the size of the hypothalamus. The hypothalamus is so smallthat it actually makes up less than 1% of the totalvolume of your brain. It's about the sizeof kidney bean. And the hypothalamusplays an incredible role in regulating so manyfunctions in your body. But for our purposes, we'retalking about the limbic system structures in terms of emotion. So when it comes toemotion, the hypothalamus you can think of as regulatingthe autonomic nervous system. I'll abbreviate it as 'ANS.' And the autonomicnervous system you can think of as fight orflight versus rest and digest. Now, I'm going to discuss thisfurther in a different video. But right now, justthink of it as regulating the autonomic nervous system. And it does this by controllingthe endocrine system, by triggering the release ofhormones into your bloodstream. And some of these hormonesthat are triggered to release are things like epinephrineor norepinephrine. And epinephrine is actually verycommonly known as adrenaline. So if you everthink of the phrase like 'a lot of adrenalinepumping through your veins,' that's actually beingregulated by the hypothalamus. Your hypothalamusis also involved in regulating other basicdrives, like hunger, thirst, sleep, sex. But in terms ofemotion, I think it's most important tonote 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 basicstructures, the thalamus, the amygdala, the hippocampus,and the hypothalamus. So these are thebasic structures of the limbic system.