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  • James was 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 wordtoo” -- 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 weve 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.

  • Youll 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.

  • 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 forebrain, midbrain, 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

  • thebrain,” 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 midbrain, 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 midbrain 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 midbrain that receives and processes that sensory information

  • and sends out the reflexive motor signals, so you react without thinking.

  • The midbrain also passes that data to regions like the cerebral cortex, which do the actual

  • conscious thinking about the stimuli, likeWhat is that thing whizzing across the sky?”

  • orWHAT 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 thereptilian brainbecause 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 animalsbrains 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 ofgray mattercalled the cerebral cortex,

  • and the inner squishy layer ofwhite matterbeneath 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.

  • Youll 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 youre 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 lunchor 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 well be spending the next few lessons exploring.

  • 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 Patreon.com/CrashCourse

  • 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.

James was healthy professional, a father of two. He had lots of friends, loved telling

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