If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Video transcript

- So, in the previous video, we talked about the reward circuit in the brain at sort of a broad level. And now, what we're gonna talk about is how that works at a lower level, at the neuron level, so that we can discuss issues like tolerance, addiction, and withdrawal. Now, behaviorally, tolerance just means that you get used to a drug so that you need more of it in order to achieve the same effect. So, let's look at how that works inside your brain. So let's take a look at what's happening with the neurons here. So, imagine that this is the axon of one neuron, and the dendrite of another, and then in here is the synapse. All right. So then, we have these neurons. They're coming from the VTA and they're sending dopamine. So imagine that you've just taken some cocaine, so all of this dopamine is flowing, pleasure signals going crazy, you're pretty happy. Okay, so tons of dopamine. And what normally happens here is that the postsynaptic neuron has receptors for certain neuro-transmitters, such as dopamine. It has these little spots for the dopamine to come, and it gets taken up, and it sends the signal on, and that's how you experience the euphoria. It's that stimulation of the neurons along that pathway. Now, in a normal situation, if your stimulus had been, say, a hug or something, then you would still experience this feeling of pleasure, but then your brain chemistry would go back to normal after a second. It would balance itself out. With drugs, long-term stimulation can actually alter your brain chemistry. So what happens when your brain is just constantly over-stimulated with dopamine, it's too much for it, and it tries to balance it out. Believe it or not, you don't always wanna be super-duper happy. At least your brain doesn't. So what happens after a while is it says, "Okay, I need to calm down. "I'm gonna shut down some of these receptors, so that way, "the same amount of drugs won't cause me "to be so over-stimulated. "I won't get as much of a high from the same amount of drugs." And so, when that happens, that is called tolerance, because you have built up a tolerance to the same amount of drugs, and it doesn't have the same effect anymore. Now, with drugs like cocaine in particular, this can cause a problem, because you've started to develop a dependence on it. We usually talk about a combination of emotional dependence-- a feeling like you need the drug-- as well as a physical dependence-- you actually experience negative physical symptoms without it. So once you've built up this tolerance and you still wanna feel that high, you end up taking more cocaine to get the same feeling. And so then, you just have to keep kinda increasing your dosage over time. Okay, so that's what happens if you have just free and steady access to drugs: you just keep increasing your dosage up to a point. On the other hand, you might not always have access to the drug, and if you go through a period of not having it, then that is when you experience withdrawal symptoms. So, remember, now your body has gotten accustomed to this very high level of dopamine, and it's gotten accustomed maybe to not producing it on its own, but relying on the drug. Once you start taking cocaine, I mean, things like chocolate and hugs won't make you quite as happy as the cocaine does. You end up seeking out this cocaine and the pleasurable sensations it can produce in the place of other types of stimulation. So then, without it, you don't have the same level of dopamine, your body's not producing it on its own, so you tend to feel depressed, you feel highly anxious, and the specific symptoms will vary by type of drug. Sometimes you'll sweat, have headaches. Generally, anxiety and depression are pretty common, and when those are extreme enough, you'll really do whatever it takes to make yourself feel happy again. The thing is, though, you're not even going for the euphoria anymore, you're going for normal. Once you've built up this tolerance, you need the drug to feel normal again, not even euphoric. And this is usually a sign that you are addicted to the drug, which means that you feel a need to keep taking it. The good news is that even though withdrawal is miserable, just like your brain can get used to the presence of drugs, it can used to the absence of drugs again. So, with some time and effort, even if the drugs have caused some irreparable damage to other parts of your brain, you can get your reward system back to functioning at a more normal level.