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Today Hank talks about your central nervous system. In this episode, we'll explore how your brain develops and how important location is for each of your brain's many functions.

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Introduction: Broca's Aphasia 00:00
Central Nervous System Structure & Function 1:48
Brain Development: Neural Tube 3:27
Brain Development: 3 Primary Vesicles 3:47
Brain Development: 5 Secondary Vesicles 4:04
Brain Development: Major Adult Brain Regions 4:18
Brain Development: Cerebellum & Brain Stem 4:42
Brain Development: Reptilian Brain 5:45
Brain Development: Cerebral Hemispheres 6:35
The Brain's Lobes 7:35
Review 9:14
Credits 9:48

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 Introduction (0:00)

James was a healthy professional, a father of two. He had lots of friends, loved telling jokes, and played softball on Sundays. Then one day, at the age of 45, he suffered a stroke. He bounced back fairly quickly, with one major exception: He was no longer able to speak.

The stroke damaged a specific area in the left hemisphere of his brain, called Broca’s area, and left him with what’s known as Broca’s aphasia. Broca’s area is partly responsible for the ability to produce and process language, and Broca’s aphasia often leaves its sufferers with some ability to understand speech, but an inability to produce intelligible words.

James could understand his wife when she asked if he wanted cereal for breakfast, but he could only respond by repeating the word “too” - although he could still intonate as though he were speaking a whole sentence. Then, after some time and therapy, something rather unexpected happened - James regained some ability to communicate through singing.

Broca’s aphasia can sometimes be treated by teaching patients to sing, because singing uses a different region of the brain - one that’s on the right side and that’s analogous to Broca’s area on the left. So after some practice, James could sing words, and he eventually relearned how to talk by teaching the right side of his brain to speak rather than sing.

Whether it’s a stroke affecting your speech, a tumor destroying your memory, a concussion affecting your aggression, or that fateful iron rod that shot straight through Phineas Gage’s skull - a lot of what we know about how the brain works has come through studying injuries to it.

And what we’ve learned so far is that, even though it looks like a 1.4-kilogram lump of gray, congealed oatmeal, the brain is made up of super-specific areas that have super-specific functions.

You might actually say the same thing about your brain that’s sometimes said about politics: Everything is local.

[Opening music]

 The Layout of the Central Nervous System (1:49)

You’ll remember that our nervous system is divided into two main networks that work in harmony - the central nervous system, consisting of your amazing brain and spinal cord, and the peripheral nervous system, made up of the nerves coming out of that central nervous system.

The central nervous system’s main game is integrating the sensory information that the peripheral system collects from all over the body, and responding to it by coordinating both conscious and unconscious activity. The sun is bright, so I’ll shade my eyes; I’m hungry, so I’m calling the pizza man; the phone is ringing, maybe I’ll answer it.

All these sensations, thoughts, and directions process through this two-part system. It’s the brain, of course, that sorts out all that sensory information and gives orders. It also carries out your most complex functions, like thinking, and feeling, and remembering.

Meanwhile, your spinal cord conducts two-way signals between your brain and the rest of your body, while also governing basic muscle reflexes and patterns that don’t need your brain’s blessing to work - this is how a chicken can still run around even if the poor thing has been decapitated.

Both your spinal cord and brain are made of fragile, jelly-like nervous tissue that is extremely susceptible to injury. So all that goo is well-protected by the bones of your vertebrae and cranium, as well as membrane layers, or meninges, before being bathed in a cushy waterbed of clear cerebrospinal fluid.

This fluid actually allows your brain to float somewhat in your skull, reducing its weight and letting it slosh around while you and your head are free to move. But even with all that extra protection, your brain is still vulnerable. And one thing James’s story taught us is that its vulnerabilities can be incredibly specific, because your brain is divided into specialized regions that may, or may not, interact with each other to produce a given action.

  How the Brain Develops (3:21)

We can better understand this division of labor by looking at how the brain first develops into its main component parts.

Inside a developing embryo, the central nervous system starts off as a humble little neural tube. Soon the caudal, or lower, end of the tube stretches out, forming the spinal cord, while the cranial end begins to expand, divide, and enlarge into three primary brain vesicles, or interconnected chambers.

This is kind of your proto-brain. We call these chambers the prosencephalon, the mesencephalon, and the rhombencephalon - or fore-brain, mid-brain, and hindbrain.

By an embryo’s fifth week of development, these main three chambers start morphing into five secondary vesicles that essentially form the roots of what will become your grown-up brain structures. The prosencephalon divides into two sections - the telencephalon and the diencephalon.

The rhombencephalon forms into another pair, called the metencephalon and the myelencephalon. And in between, the mesencephalon, thanks to evolution, remains undivided.

The real action starts as these five secondary vesicles start developing into the major adult brain regions that you might be more familiar with - the brainstem, the cerebellum, the diencephalon, also known as the interbrain, and finally the cerebral hemispheres.

But, in order to go from a simple tube into that classic, wrinkly icon we think of as the “brain,” each of these five vesicles grows in different ways. Basically, some develop a lot more than others.

The least dramatic changes occur in the three most caudal or lower sections: the mesencephalon, the metencephalon, and the myelencephalon. They go on to form the cerebellum, which mostly helps coordinate muscular activity, and the brainstem, which plays a vital role in relaying information between the body and the higher regions of the brain.

The brainstem actually has three main components - and I know this is getting to be a lot of vocabulary here - you have the mid-brain, the pons, and the medulla oblongata. Together they regulate many of the most basic, vital involuntary functions, like keeping your heart on pace, keeping your lungs working, and controlling things like sleep, and appetite, and pain sensitivity, and awareness.

But of the three brainstem parts, it’s your mid-brain that carries out the higher-level functions. Like, when your eyes track a fast moving object, or when you look behind you after hearing some sudden loud sound, it’s the mid-brain that receives and processes that sensory information and sends out the reflexive motor signals, so you react without thinking. The mid-brain also passes that data to regions like the cerebral cortex, which do the actual conscious thinking about the stimuli, like “What is that thing whizzing across the sky?” or “WHAT JUST EXPLODED BEHIND ME?!”

So with the brainstem and cerebellum covering your basic life and motor functions, you start to see somewhat more complex tasks being carried out in the next major brain structure, the diencephalon. This is where you find the thalamus, hypothalamus, epithalamus, and the mammillary bodies, which regulate things like homeostasis, alertness, and reproductive activity.

Here we also find part of the limbic system, which is a center for strong emotions, like fear. This area is sometimes called the “reptilian brain” because we share it with some of our less philosophical animal brethren like lizards and fish. I’m not putting these guys down, but by our standards, they don’t think so much as focus on the more instinctual pursuits that are ruled by the caudal regions of the brain - eat, drink, sleep, mate, stay safe.

All those things are awesome. But it wasn’t until the appearance of birds and mammals that some animals’ brains came to be dominated by the last of the five vesicles, the telencephalon. During your brain’s growth, the telencephalon undergoes the biggest changes of all, as it develops into the most brainy part of your brain - the two classic, walnut-looking hemispheres we collectively call the cerebrum, that cover the rest of your brain like a mushroom cap on its stalk.

That’s the cerebrum - not to be confused with Cerebro, which is Professor X’s telepathy-enhancing device - and it is the largest region of the brain and performs the highest functions. It’s made up of the wrinkled, outer layer of “gray matter” called the cerebral cortex, and the inner squishy layer of “white matter” beneath it.

And it’s the cerebrum that rules our voluntary movements and our most advanced tricks, like thinking, and learning, and regulating and recognizing emotions, and experiencing consciousness in general. You’ll remember that higher processing requires lots of synapses, which require lots of nervous tissue. So as the cerebrum grew through evolutionary time, it got more massive but our skull didn’t exactly keep up.

So in order to squeeze all that material into your skull, the brain forms little creases, called gyri, and larger grooves, or sulci, giving it more folds than than an origami pineapple. And although a big fissure separates the left and right hemispheres, the two halves communicate, through a series of myelinated axon fibers called the corpus callosum.

And each hemisphere has other, smaller fissures that divide it into lobes - each with a different set of major functions. The frontal lobe, for example, governs muscle control and cognitive functions like planning for the future, concentration, and preventing socially unacceptable behaviors.

In most people, this area doesn’t finish developing until after the teenage years, which tells you a lot about the teenage years. Since Broca’s area lives in this lobe in the left hemisphere, it also is important in language comprehension and speech.

If you’re enjoying a beautiful sunset, you can thank your occipital lobe at the back of your head for processing those bright visual cues. And the next time you step on a Lego, you can curse your parietal lobe, which processes the sensations of touch, pain, and pressure.

Meanwhile the temporal lobe helps sort out auditory information, including language. It contains Wernicke’s area - another important region of the brain associated with the production of written and spoken language. This part of the limbic system includes your short-term memory keeper, the hippocampus, and the emotional amygdala, which controls sexual and social behavior.

So, if you damage the wrong part of your temporal lobe, you may never again be able to remember what you ate for lunch or you might suddenly become a total jerk who kicks kittens and cuts in line.

We could do a whole course on the finer-grained functions and consequences of malfunction in every bit of brain in your gourd, but, well, we can’t do that today. And you got to remember that, when it comes to your body, no organ or system is an island. Your brain would be pretty useless if it weren’t hooked up to the outside world. That’s where the peripheral nervous system comes in, which we’ll be spending the next few lessons exploring.

  Review and Credits (9:13)

Meanwhile, you learned today about the central nervous system and how important location is to brain function. We looked at how the brain develops from an unassuming neural tube into three primary vesicles, and then five secondary vesicles, and finally into our complex set of four adult structures and their basic functions.

Crash Course is now on Patreon! Thank you so much to all of our supporters on Patreon who help make Crash Course possible for themselves and for everyone else in the world through their monthly contributions. If you like Crash Course and you want to help us keep making great new videos like this one, you can check out

This episode was written by Kathleen Yale. The script was edited by Blake de Pastino, and our consultant, is Dr. Brandon Jackson. It was directed by Nicholas Jenkins and Michael Aranda, and our graphics team is Thought Café.