I'm Andrew Huberman, and I'm a professor of neurobiology and ophthalmology at Stanford School of Medicine.

For today's podcast, we're going to talk about the parts list of the nervous system.

Now that might sound boring, but these are the bits and pieces that together make up everything about your experience of life, from what you think about to what you feel, what you imagine, and what you accomplish from the day you're born until the day you die.

By the end of this podcast, I promise you're going to understand a lot more about how you work and how to apply that knowledge.

So let's talk about the nervous system.

The reason I say your nervous system and not your brain is because your brain is actually just one piece of this larger, more important thing, frankly, that we call the nervous system.

The nervous system includes your brain and your spinal cord, but also all the connections between your brain and your spinal cord and the organs of your body.

It also includes, very importantly, all the connections between your organs back to your spinal cord and brain.

So the way to think about how you function at every level from the moment you're born until the day you die, everything you think and remember and feel and imagine is that your nervous system is this continuous loop of communication between the brain, spinal cord, and body, and body, spinal cord, and brain.

In fact, we really can't even separate them.

It's one continuous loop.

The way to think about how the nervous system works is that our experiences, our memories, everything is sort of like the keys on a piano being played in a particular order, right?

If I play the keys on a piano in a particular order and with a particular intensity, that's a given song.

We can make that analogous to a given experience.

Our brain is really a map of our experience.

We come into the world and our brain has a kind of bias towards learning particular kinds of things.

It's ready to receive information and learn that information, but the brain is really a map of experience.

So let's talk about what experience really is.

What does it mean for your brain to work?

Well, I think it's fair to say that the nervous system really does five things, maybe six.

The first one is sensation.

Sensation is a non-negotiable element of your nervous system.

You have neurons in your eye that perceive certain colors of light and certain directions of movement.

You have neurons in your skin that perceive particular kinds of touch, like light touch or firm touch or painful touch.

You have neurons in your ears that perceive certain sounds.

Your entire experience of life is sort of filtered by these, what we call sensory receptors, if you want to know what the name is.

Perception is our ability to take what we're sensing and focus on it and make sense of it, to explore it, to remember it.

So really perceptions are just whichever sensations we happen to be paying attention to at any moment.

Perception is under the control of your attention.

And the way to think about attention is it's like a spotlight, except it's not one spotlight.

You actually have two attentional spotlights.

Anyone that tells you you can't multitask, tell them they're wrong.

And if they disagree with you, tell them to contact me.

Because in old world primates, of which humans are, we are able to do what's called covert attention.

We can place a spotlight of attention on something, for instance, something we're reading or looking at or someone that we're listening to.

And we can place a second spotlight of attention on something we're eating and how it tastes or our child running around in the room or my dog.

You can split your attention into two locations, but of course you can also bring your attention, that is your perception, to one particular location.

You can dilate your attention, kind of like making a spotlight more diffuse, or you can make it more concentrated.

This is very important to understand if you're going to think about tools to improve your nervous system.

Attention is something that is absolutely under your control.

The nervous system can be reflexive in its action or it can be deliberate.

Deliberate thoughts are top down.

They require some effort and some focus, but that's the point.

You can decide to focus your behavior in any way you want, but it will always feel like it requires some effort and some strain.

Whereas when you're in reflexive mode, just walking and talking and eating and doing your thing, it's going to feel very easy.

And that's because your nervous system basically wired up to be able to do most things easily without much metabolic demand, without consuming much energy.

But the moment you try and do something very specific, you're going to feel a sort of mental friction.

It's going to be challenging.

So we've got sensations, perceptions, and then we've got things that we call feelings slash emotions.

And these get a little complicated because almost all of us, I would hope all of us, are familiar with things like happiness and sadness, or boredom or frustration.

Certainly emotions and feelings are the product of the nervous system.

They involve the activity of neurons.

But as I mentioned earlier, neurons are electrically active, but they also release chemicals.

And there's a certain category of chemicals that has a very profound influence on our emotional states.

They're called neuromodulators.

And those neuromodulators have names that probably you've heard of before, things like dopamine and serotonin and acetylcholine, epinephrine.

Neuromodulators are really interesting because they bias which neurons are likely to be active and which ones are likely to be inactive.

A simple way to think about neuromodulators is they are sort of like playlists that you would have on any kind of device where you're going to play particular categories of music.

So for instance, dopamine, which is often discussed as the molecule of reward or joy, is involved in reward.

And it does tend to create a sort of a upbeat mood when released in appropriate amounts in the brain.

But the reason it does that is because it makes certain neurons and neural circuits, as we call them, more active and others less active, okay?

So serotonin, for instance, is a molecule that when released tends to make us feel really good with what we have, our sort of internal landscape and the resources that we have.

Whereas dopamine, more than being a molecule of reward, is really more a molecule of motivation toward things that are outside us and that we want to pursue.

And we can look at healthy conditions or situations like being in pursuit of a goal where every time we accomplish something in route to that goal, a little bit of dopamine is released and we feel more motivation, that happens.

We can also look at the extreme example of something like mania, where somebody is so relentlessly in pursuit of external things like money and relationships that they're sort of in this delusional state of thinking that they have the resources that they need in order to pursue all these things when in fact they don't.

I want to emphasize also that emotions are something that we generally feel are not under our control.

We feel like they kind of geyser up within us and they just kind of happen to us.

And that's because they are somewhat reflexive.

We don't really set out with a deliberate thought to be happy or deliberate thought to be sad.

We tend to experience them in kind of a passive reflexive way.

And that brings us to the next thing, which are thoughts.

Thoughts are really interesting because in many ways they're like perceptions, except that they draw on not just what's happening in the present, but also things we remember from the past and things that we anticipate about the future.

The other thing about thoughts that's really interesting is that thoughts can be both reflexive, they can just be occurring all the time, sort of like pop-up windows on a poorly filtered web browser, or they can be deliberate.

We can decide to have a thought.

And a lot of people don't understand or at least appreciate that the thought patterns and the neural circuits that underlie thoughts can actually be controlled in this deliberate way.

And then finally, there are actions.

Actions or behaviors are perhaps the most important aspect to our nervous system.

Because first of all, our behaviors are actually the only thing that are going to create any fossil record of our existence.

After we die, the nervous system deteriorates, our skeleton will remain, but in the moment of experiencing something very joyful or something very sad, it can feel so all-encompassing that we actually think that it has some meaning beyond that moment.

But actually for humans, and I think for all species, the sensations, the perceptions, and the thoughts and the feelings that we have in our lifespan, none of that is actually carried forward except the ones that we take and we convert into actions such as writing, actions such as words, actions such as engineering new things.

And so the fossil record of our species and of each one of us is really through action.

And that in part is why so much of our nervous system is devoted to converting sensation, perceptions, feelings, and thoughts into actions.

The other way to think about it is that one of the reasons that our central nervous system, our brain and spinal cord include this stuff in our skull, but also connect so heavily to the body is because most everything that we experience, including our thoughts and feelings, was really designed to either impact our behavior or not.

And the fact that thoughts allow us to reach into the past and anticipate the future and not just experience what's happening in the moment gave rise to an incredible capacity for us to engage in behaviors that are not just for the moment.

They're based on things that we know from the past and that we would like to see in the future.

And this aspect to our nervous system of creating movement occurs through some very simple pathways that reflexive pathway basically includes areas of the brainstem we call central pattern generators.

When you walk, provided you already know how to walk, you are basically walking because you have these central pattern generators, groups of neurons that generate right foot, left foot, right foot, left foot kind of movement.

However, when you decide to move in a particular deliberate way that requires a little more attention, you start to engage areas of your brain for top-down processing where your forebrain works from the top down to control those central pattern generators so that maybe it's right foot, right foot, left foot, right foot, right foot, left foot if maybe you're hiking along some rocks or something and you have to engage in that kind of movement.

So movement is just like thoughts, can be either reflexive or deliberate.

And when we talk about deliberate, I want to be very specific about how your brain works in the deliberate way because it gives rise to a very important feature of the nervous system that we're going to talk about next, which is your ability to change your nervous system.

And what I'd like to center on for a second is this notion of what does it mean for the nervous system to do something deliberately?

Well, when you do something deliberately, you pay attention, you are bringing your perception to an analysis of three things, duration, how long something is going to take or should be done, path, what you should be doing, and outcome, if you do something for a given length of time, what's going to happen.

Now, when you're walking down the street or you're eating or you're just talking reflexively, you're not doing this, what I call DPO, duration, path, outcome, type of deliberate function in your brain and nervous system.

Let's give an example where perhaps somebody says something that's triggering to you, you don't like it, and you know you shouldn't respond.

You feel like, oh, I shouldn't respond, I shouldn't respond, I shouldn't respond.

You're actively suppressing your behavior through top-down processing.

Your forebrain is actually preventing you from saying the thing that you know you shouldn't say or that maybe you should wait to say or say in a different form.

This feels like agitation and stress because you're actually suppressing a circuit.

We actually can see examples of what happens when you're not doing this well.

Some of the examples come from children.

If you look at young children, they don't have the forebrain circuitry to engage in this top-down processing until they reach age 22, even 25.

But in young children, you see this in a really robust way.

A kid sees a piece of candy that it wants and will just reach out and grab it, whereas an adult probably would ask if they could have a piece or wait until they were offered a piece in most cases.

People that have damage to the certain areas of the frontal lobes don't have this kind of restriction.

They'll just blurt things out.

They'll just say things.

Impulsivity is a lack of top-down control, a lack of top-down processing.

So a lot of the motor system is designed to just work in a reflexive way.

And then when we decide we want to learn something or do something or not do something, we have to engage in this top-down restriction.

And it feels like agitation because it's accompanied by the release of a neuromodulator called norepinephrine, which in the body we call adrenaline, and it actually makes us feel agitated.

So for those of you that are trying to learn something new or to learn to suppress your responses or be more deliberate and careful in your responses, that is going to feel challenging for a particular reason.

It's going to feel challenging because the chemicals in your body that are released in association with that effort are designed to make you feel kind of agitated.

And so this is really important to understand because if you want to understand neuroplasticity, you want to understand how to shape your behavior, how to shape your thinking, how to change how you're able to perform in any context.

The most important thing to understand is that it requires top-down processing.

It requires this feeling of agitation.

In fact, I would say that agitation and strain is the entry point to neuroplasticity.

So let's take a look at what neuroplasticity is.

Neuroplasticity is the ability for these connections in the brain and body to change in response to experience.

And what's so incredible about the human nervous system in particular is that we can direct our own neural changes.

We can decide that we want to change our brain.

In other words, our brain can change itself and our nervous system can change itself.

For a long time, it was thought that neuroplasticity was the unique gift of young animals and humans, that it could only occur when we're young.

And in fact, the young brain is incredibly plastic.

Children can learn three languages without an accent, reflexively, whereas adults, it's very challenging.

It takes a lot more effort and strain, a lot more of that duration path outcome kind of thinking in order to achieve those plastic changes.

We now know, however, that the adult brain can change in response to experience.

In order to understand that process, we really have to understand something that might at first seem totally divorced from neuroplasticity, but actually lies at the center of neuroplasticity.

And for any of you that are interested in changing your nervous system so that something that you want can go from being very hard or seem almost impossible and out of reach to being very reflexive, this is especially important to pay attention to.

Plasticity in the adult human nervous system is gated, meaning it is controlled by neuromodulators.

These things that we talked about earlier, dopamine, serotonin, and one in particular called acetylcholine are what open up plasticity.

They literally unveil plasticity and allow brief periods of time in which whatever information, whatever thing we're sensing or perceiving or thinking, or whatever emotions we feel can literally be mapped in the brain such that later it will become much easier for us to experience and feel that thing.