That Neuroscience Guy - The Neuroscience of the Growing Brain

Episode Date: May 29, 2022

You've probably noticed that children and teenagers tend to behave much differently than adults. A lot of this can actually be attributed to how the brain grows and changes as we develop. In today's episode of That Neuroscience Guy, we discuss how the growing brain develops over time and affects behaviour. 

Hi, my name is Olof Krogolsen, and I'm a neuroscientist at the University of Victoria. And in my spare time, I'm that neuroscience guy. Welcome to the podcast. You have probably noticed by now that children and teenagers do not behave in quite the same manner that adults do. Indeed, while we can forgive infants and young children for some of the decisions that they make and some of the things that they do,

my 14-year-old son leaves my head scratching at times. I'm sitting there going, what's going on, buddy? Like, why did you do that? So why is this? You know, what's going on in the growing brain? On today's podcast, the growing brain. So let's start with the neuroanatomy of brain growth. Basically, the brain begins to form at week three. So when a baby is conceived and it's still in the first trimester and in the womb,

by week three, we see the growth of something called the neural tube. Effectively, it's the structure that's going to turn into the brain. So only three weeks in from the start of growth, there's already the beginning of a brain. By week 10, we see a brain region, something that actually looks like a precursor to the human brain, and the brain region formation begins. By week 13, neurons are beginning to migrate. They're beginning to spread out across the brain-like structure. By week 15, now remember this is all while the baby's still in the womb, we see structural formation within the brain. You actually begin to recognize structures that we've talked about, like the prefrontal cortex, the hippocampus, you know, the parietal cortex,

the temporal lobes, the brainstem, these structures begin shaking up. Now, by week 25, synaptic pruning begins. Now, we're going to talk more about pruning in a bit, but essentially when the brain begins to grow, it makes a whole bunch of connections. So, all of the regions are interconnected, but as the brain begins to start doing its work, connections are pruned or they're broken. Basically, think of it as a connection that's not used. So maybe a road that you don't take no longer is needed. So pruning begins at week 25. Until birth from week 25, basically, there's a massive sprout in brain growth and myelinization begins. So myelinization

is when the axons begin to get the fatty sheath that allows them to conduct electrical impulses. And this is all while the baby's still inside the womb. Now, you might have heard of fetal alcohol syndrome. This is why it's so bad. So if you're not familiar, fetal alcohol syndrome is when a pregnant mother consumes alcohol and it basically results in brain damage. It's really that straightforward. And this is why doctors warn pregnant women to not drink too much and or possibly not even drink at all. There's still a bit of debate about the right amount of alcohol to consume during pregnancy. You know, some doctors say a glass of wine is okay, some say zero,

but the reality is more than that's a really bad idea because it will impact the growing brain. Now at birth, a human baby's brain is only about a quarter of its adult size, but by the time the baby is age two, so it's a child now, the brain is three quarters of the size of the adult brain. And this change in size is totally related to growth of neurons and the testing and pruning of connections. So in terms of the growth of neurons, during this rapid growth period, when the young child's brain is just sort of massively expanding in size, the neurons are making tons and tons of connections. They're sprouting and growing and reaching out across

all the different regions of the brain. So brain growth makes sense. You want the brain to get bigger and you want to form new connections. But remember, all of the information we process, all of the decisions we make, all of our motor movements are based on having a bunch of neural connections. But at the same time, pruning occurs. And pruning is a little bit more tricky to understand. Basically, when the brain is growing, like I said earlier, all of these new connections are forming. So the brain is totally interconnected. But as we experience things, there's connections that simply aren't used. They're redundant connections or useless connections. And these get pruned, or basically

they break effectively. So neural pruning is as big a part of the growth process. And by the time the brain is fully mature, there's actually less neural connections than the peak of brain growth. And that's due to pruning. Now for girls, the brain reaches its biggest size around 11 years old on average, of course. And for boys, the brain reaches its biggest size around age 14. This is due to differences in growth weight between boys and girls. But this difference does not mean that boys or girls are smarter than one another. It's just a difference in rate of development. Now, although the brain's growing in size pretty early on, it doesn't finish development and maturation until the mid to late 20s.

People used to think it was earlier than that. But now it's sort of accepted that brain growth, at least in terms of development and maturation, can occur right into the late 20s. Although for some people it's done earlier. And the last part of the brain to develop is the prefrontal cortex. Now we're going to talk about that a little bit more when we get to teenagers. Now, this is an area that's responsible for skills and planning. But like I said, more on that in a bit. Now, as we develop and as the brain develops, what's happening?

Well, the most obvious one is physiological development. Okay, muscle growth and the ability to control muscles. So the development of the motor system when you think of a young infant they're uncoordinated and the reason they're uncoordinated is they're still trying to figure out the neural circuitry that they need to move and this is coordination in the cerebellum the cerebellum is a structure at the back of the brain and it plays a massive role in coordinating our movements and it just hasn't learned to do that yet which is why infants stumble and fall.

They're literally trying out movements and figuring out what works. And if you look at young infants, when they reach out to grab something, especially the first couple of times, they're just guessing. Like they're firing random neurons in the limb. They're flinging their limbs out into space. And they're literally observing that process and going, huh, that's what that did. Let's try something else. And when they're successful, well, then they try to repeat that action. That's why once a baby or an infant gets a movement down, they tend to learn fairly quickly.

And parents that have got newborns or young infants in their lives, parents that have young infants in their lives who've seen this, it can take quite a while for a baby to walk, but once they get the handle of walking, they're off and they're gone. And this is true for reaching or grasping or other motor movements. And if you think back to season one, we talked about grabbing an apple. This is tied to the development of what we call inverse and forward models. Inverse models

are basically a computer program, if you will, within the brain that allows you to plan a movement. And it does it in a predictive fashion. Basically, it says, if I want to grab that apple, what do I need to do to achieve that? And forward models allow us to control our movements. Again, I'd refer you back to that episode. But in brief, a forward model is basically evaluating the motor command that's generated. So the inverse model says, here's the things I need to do to reach this apple. The forward model evaluates what we call the motor plan or the computer program for the movement, if you will, and says, is this going to work or not? And if it isn't, it tweaks it. So these things are all developing, which is why our movements start as janky and uncoordinated and they move to becoming coordinated and smooth.

Now, our perceptual systems are developing as well as we work through this brain growth process. Basically, we're linking what we see and hear and smell and taste and feel to constructs. And I remember, you know, with my son Owen, I used to carry him around on one of those slings that you, you know, you see parents wearing where he was attached to my front. And he would point at things and say truck. It was his first real word other than mum. And he had pointed a tree and go truck. And he would point at a fence and go truck. And then eventually he'd pointed a truck and go truck. And I'd go, yes, Owen, that's 100% right. And that is how infants learn. Their perceptual system

can identify things and it comes online and there's obviously growth and a young infant can't see as clearly as we can as adults. But as neurons grow and they're pruned, our visual systems and other sensory systems come online, if you will. But it's this linking to constructs, which is a key state of brain development. And last, there are cognitive systems. So we develop our cognitive systems. Quite frankly, our learning systems are crucial here because there's so much to learn. You know, basically you're born with what we call a tabula rasa or a blank slate. Very little is innate to us. And all of these things need to be learned. And if you

think back to the discussions we've had about learning, this is where prediction errors come in. Basically, the growing brain doesn't have a lot of expectations. When you make a movement, or you make a choice, or you experience something, you don't know what to expect. And when you expect it, it's unexpected. And we have these massive prediction errors. And if you remember, these really drive learning. Early in learning, these massive prediction errors really shape our changes in behavior or our improvements in the motor system. And our memory systems are growing as well. Interestingly enough, they're maximal at around 18. Our best memory is when we're about 18. But these systems are coming online as the brain grows and those neurons continue to grow and to sprout and also to prune.

Now, I'm going to spend a little bit of extra time on the teenage brain because, well, it's near and dear to my heart being the parent of a teenager. One mantra that I always use when I talk about brain growth is you have to think about the brain growing from inside to out. So that neural tube that starts at three weeks, that's effectively the start of the brainstem, if you will. And so it grows from this basic structure on the inside of the brain, it grows outwards. And it also grows from back to front. So like I said earlier, the last thing to develop is the prefrontal cortex. Now, what does that mean? Well, basically, this is why teenagers do dumb things. If you think back to the many discussions we've had about how decision-making works, you know that our analytical or rational decision-making system is the prefrontal cortex. So when this thing is the

last thing to develop, well, guess what? That's why teenagers make poor choices. Their emotional system is running wild. Everyone knows that teenagers experience a lot of hormones. Testosterone and estrogen are being released to promote growth, along with all of the other hormones that are in the human body. And so teenagers are just this emotional mess. The amygdala is firing like crazy saying, hey, let's do this. What about this? And sadly, that prefrontal cortex is just literally turned off. It's not doing its job because it just hasn't grown yet. And this is why teenage females generally tend to be a little bit more mature, if you will, earlier than teenage boys.

And that relates just to the differences in rate of brain development. The prefrontal cortex comes on a little bit earlier in terms of females, which is why they start making more rational choices a little bit earlier. Now remember, on average, than teenage boys. Now, because the teenage brain is still developing, teens can respond to stress differently than adults. And this is why stress is so bad for mental health in teenagers with issues such as anxiety and depression. And if you take this in light of the COVID pandemic, one of the things that we're going to be learning is just how badly the children have been impacted, I believe, in the coming years. And there's also

a reason why teenagers need more sleep than even young children than adults. You might think that young children need the most sleep, but actually peak sleep is needed for teenagers. And that's because research shows that melatonin, which is basically your sleep hormone, the levels in the blood are naturally higher at night and drop later in the morning in teens than relative to children and adults. And that explains why teens stay up late and struggle with getting up in the morning. It's because their melatonin cycle is off relative to the rest of us and more

melatonin is being released. So the average teenager needs about nine to 10 hours of sleep a night. But those of us that parent teens know that most of the teenagers don't get that much sleep. But in terms of optimal brain growth, the lesson would be try to sleep nine to 10 hours a night. of optimal brain growth, the lesson would be try to sleep nine to 10 hours a night. And if teenagers don't get that sleep, basically that leads to difficulties in paying attention. Surprise, surprise, classic thing you see with teenagers. It increases impulsivity. And if you think back to the prefrontal cortex, bam, that explains a lot. And of course it increases the risk for irritability or depression. And again, if you've had a teenager, you know that they can go through mood swings or they can be irritable. So take it easy on teenagers, everyone.

These brains are still growing. Now, before I wrap up this bit on brain growth and development, I'm just going to give you some cool brain trivia related to stage of life. Peak memory, like I said earlier, 18 years old. Ability to learn unfamiliar names, around about 22 years old. Facial recognition ability, though, peaks at about 32 years old. Concentration abilities peak at 43 years old. Basic arithmetic peaks at about 50 years old. Understanding new information, again, around 50. and vocabulary skills peak at around 67. So those of us that are, let's say, past 30, don't worry. You've got better facial recognition, better concentration, better basic arithmetic, better ability to understand new information, and better vocabulary skills. And that sets us up nicely for next week, where I'm going to talk about the aging brain. So what happens to the brain when you get beyond 50 to 60?

So I hope you enjoyed that bit on the teenage brain and the growing brain. Remember, if you can, try to support us on Patreon. If you're not familiar with Patreon, I've walked you through it a few times. Basically, you sign up, you create an account, you can pledge money to the podcast on a weekly or monthly basis, and all of that money is going to the graduate students in my lab to help them get through grad school. Remember, we got some t-shirts on our Etsy store. The website is there, thatneuroscienceguy.com. It's got links to everything. Remember, if you've got ideas for podcasts, follow me on Twitter at thatneuroscienceguy. I drop information about my lab and the science we're doing

and other stuff that I'm doing that I think is cool and awesome. And of course, please subscribe to the podcast. Thank you so much for listening. My name is Olof Kregolsen, and I'm that Neuroscience Guy. I'll see you on Wednesday for another Neuroscience Bite.