- [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.