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- [Voiceover] Think about a time when you felt happy or particularly rewarded and this could be maybe somebody gave you a hug or you received some verbal praise or maybe you just ate a particularly excellent piece of cake. In any of these situations, your brain is responding in a similar way. Even though you had different stimuli, they all indicated that you were feeling rewarded. So what we're gonna talk about is the reward pathway in the brain. So this is your brain. Pretend you've sliced a brain in half and you're looking at the right hemisphere here. So here's your brain stem, prefrontal cortex, and the rest of it. So what I'm gonna focus on is a few specific parts of the brain and when you first experience pleasure, your brain releases a neurotransmitter called dopamine. So I'm just gonna write dopamine off to the side. So the dopamine is primarily produced in this area which is called the ventral tegmental area or VTA. The vental tegmental area is in the mid-brain and when it releases dopamine in the reward circuit it actually goes to a lot of different parts of the brain. So one of the places the VTA sends dopamine is to the amygdala so that's kind of in this area. The amygdala deals with emotions among other things. It also sends dopamine up to the nucleus accumbens which is around here and the nucleus accumbens controls your body's motor functions. So then we also send dopamine up to the prefrontal cortex which helps focus attention and planning. And the last area that we're gonna talk about where the VTA sends dopamine is the hippocampus which is kind of right around here. Just to note, that hippocampus dot should probably be a little closer to the amygdala, a little more to the left. The hippocampus is in the temporal lobe not the brain stem. We're just drawing it here so it's a little easier to separate out from the other parts that we're talking about. And the hippocampus is responsible for the formation of memories. So now we've set up all the parts of this pathway and what happens is that when you experience a stimulus and the dopamine in the VTA is released and travels along these pathways, it basically tells your body that this was good, let's do it again so this is your natural response to some pleasurable stimuli such as food, sex, social interactions, also certain drugs, particularly stimulants such as cocaine or amphetamines can initiate this response. And of course, different stimuli activate the circuit to different degrees and when we're talking about drugs, that's one reason that some drugs are easier to become dependent on than others. They activate the reward circuit to a greater degree than others. The nucleus accumbens, the amygdala, and the hippocampus are all part of something called the mesolimbic pathway and as a side note, try not to get too caught up in the terms. These terms like mesolimbic pathway, mesostriatal pathway, sometimes they're used in slightly different ways by different people and sometimes they're broken down into even more detail. Our purpose here is to hit the highlights, the really important parts of the reward pathway so I'm just gonna use some of the more common terms. The mesolimbic pathway is a big part of the reward circuit in the brain. So what happens is the VTA releases dopamine and it goes to all these different parts of the brain which have dopamine receptors so they uptake with dopamine and the result is a feeling of happiness or euphoria which is the reward you get. So for example, the amygdala, which helps process emotions and is connected to the hippocampus, will say, "This was a pleasurable sensation, "I enjoyed it," and then your hippocampus will say, "Well, let me remember everything "about this environment so we can do this again." For example, let's go back to that excellent piece of cake you might be eating. Your amygdala says, "This is delicious, "I love this, I'm feeling so happy right now," and your hippocampus says, "Well, let me "remember what restaurant I'm at, "what exact piece of cake I ordered, "who I'm with, let's remember things "about this experience." Then your nucleus accumbens, which helps control motor functions, says, "Well, let's take another bite. "Let me use my hand to use the fork "to get another piece and eat it." And your prefrontal cortex helps focus on that cake and divert some of your attention to it. And then you take another bite and it's delicious and the reward circuit goes crazy again and the dopamine goes out, and that's why you experience the sort of continued pleasure. And one interesting thing to note is that with the continued activation of this reward circuit, we talked about how dopamine goes up and at the same time a neurotransmitter called serotonin goes down and serotonin is partially responsible for feelings of satiation. So this is why drugs can be problematic when you continually activate this dopaminenergic circuit, this reward circuit. Your dopamine goes up so you have this increasing sense of euphoria but also serotonin levels can go down which means you're less likely to be satiated or content. What you might notice about this cycle is that it's a very biologically driven process. A long time ago, people used to think that drug addiction was completely driven by a failure or morals or willpower. And while people's choices are definitely strongly involved, we know now that addiction has physiological components as well. It's similar to looking at your family history to see if you might have a genetic predisposition for high blood pressure or something. If someone in your family has high blood pressure or has suffered from a drug addiction, then you may have some increased risk to develop the same condition. However, environment and your choices matter too so don't worry, nothing's set in stone. Some evidence for the biological basis of drug dependence actually comes from animal models. Scientists could hook rats up to IVs that give them cocaine if they push a lever and when they do this, rats learn very quickly to push that lever and they'll even engage in drug-seeking behaviors and will increase their dosage if they're allowed to. What's also really interesting is that negative consequences don't affect an addicted brain in the same way that they do a normal brain. For example, when you give a rat regular food that it likes paired with a substance that makes it sick, it learns to avoid that food. It doesn't like it anymore. But when you give an addicted rat its favorite drug paired with a substance that makes it sick, it still wants that drug. So whereas with the regular food it learns that "Oh, something bad happened when I ate this food, "I'm not going to do it anymore," with the drug it says, "Oh, something bad "happened when I took this drug, "but I don't really care, I really need that reward." And what those kinds of studies show us is that addiction seems to take over a rational mind. So what we'll talk about in the next video is tolerance or how you get accustomed to certain levels of reward and withdrawal so how you react when those pleasurable sensations are taken away from you.