– 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.