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Unser Gehirn - Aufbau und Funktion einfach erklärt | Neurologie mit Dr. Janis | YouTubeToText
YouTube Transcript: Unser Gehirn - Aufbau und Funktion einfach erklärt
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The brain is one of the most complex constructs in the known universe and yet,
or perhaps because of this, it is (still) the only one
that can think about itself and at least try to understand itself.
This video is all about neuroanatomy - the parts
that make up the brain and their functions.
Hey, I'm Janis and on this channel we always talk about our brain, so it's
not hard to guess that I'm absolutely fascinated by our brain, so much so
that I even studied medicine, worked in neurology for the last two years,
and even wrote my doctoral thesis on neuroanatomy. And yet it would be absolutely
untrue to say that I really understand the brain. But the basics of the brain
don't have to be complicated and they are still exciting and that's what we're going to cover now.
So, if we zoom out, our whole body is completely permeated
by our nervous system. And we can distinguish between the central nervous system
with the brain and spinal cord and the peripheral nervous system with the spinal nerves and
cranial nerves. These connect the central nervous system to the muscles
and other peripheral organs. But if we remove all of these and then follow
the spinal cord from bottom to top, we arrive at the brain. This - by definition - starts
at the point directly above the top spinal nerve where it exits. Below that
is the spinal cord, above it is the brain. And it also makes sense to look at the brain
from bottom to top step by step. This is how it more or less developed evolutionarily
and it makes it easier to assign sections and functions.
And the first brain region is the brainstem. It is aptly named
because it is like a small trunk that has its crown at the top, which is the cerebrum,
and a small lateral crown at the back, which is the cerebellum. And it connects them
with each other and also downwards with the spinal cord and indirectly
with the spinal nerves, just like a tree trunk connects the branches, the crown,
and the roots. And even though the exceptional and exciting achievements
of the human brain are mainly achieved elsewhere, the brainstem
is still essential because without it, survival would not be possible. Firstly, because it
transmits all the pathways, but above all because most of our unconscious but
vital functions are located there. So it is ultimately the control center
for many important vital functions and many vital reflexes,
such as our circulatory function, controlling blood pressure and heart rate,
and breathing. And for reflexes such as coughing, swallowing, or vomiting. This means
that any really major damage to the brainstem is absolutely life-threatening,
while it is still possible to survive with major damage to the cerebrum. Even
if practically the entire cerebrum is broken, it can sometimes still be the case in a coma
that survival is still maintained through the brainstem. But then
no consciousness is possible. So to really lead life as we want to,
we definitely need both - brainstem and cerebrum. And what is really impressive
about the brainstem is that it is actually only about the size of our thumb. And because
it is the oldest part of the brain in evolutionary terms, the differences between animals
and humans are relatively small here. The brainstem can be divided into three different
sections and we will look at them individually now. So, as we said,
it starts above the level of the first spinal nerve. Above that is the medulla oblongata, which literally
translates to “continued spinal cord”, so this is a sensible name because it kind of is
an upward extension of the spinal cord. Apart from this definition of where it starts, a identifying a border
between the two is not so easy. Next comes the pons.
In German, you can impress experts if you use the correct male pronoun - "der" Pons - even though
the translation is "die" Brücke, because the noun is male in latin.
Even though I never had Latin classes but that's what I’ve been told and it’s also what everyone says wrong, so it’s an insider tip...
And the bridge is easy to recognize because it is simply this transverse swelling.
The name of the pons makes sense because it looks like a bridge
that spans the rest of the brainstem. If we look at the brainstem from the back,
we can see that the bridge is heavily connected to the cerebellum, which
essentially lies on top of the brainstem from the back. These connections are called the cerebellar peduncles
and they give us the main function of the bridge: it serves as
a relay station for information that needs to go from the brain or spinal cord to
he cerebellum, and then back and on to other targets.
These connections are quite thick, so what kind of information
needs to go into the cerebellum? Ultimately, it's more or less anything
related to our motor and movement systems. The cerebellum is primarily responsible for
coordinating and fine-tuning our movements. Even though
a lot of movement planning occurs in our cerebral cortex, it doesn't work properly
without the cerebellum. You may have already tested the function
of your own cerebellum voluntarily or involuntarily - namely, when you're really
drunk. The coordination disorders that occur are largely due
to the cerebellum not functioning properly. It's quite susceptible
to alcohol. And that's ultimately why the examinations done by police officers
during a DUI check or neurologists in the emergency room - like walking in a straight line
or touching your nose with your finger - are quite similar. Both
are actually trying to figure out if the cerebellum is functioning properly. But the neurologist does this
to check for neurological disorders, while the police officer is probably
more interested in detecting signs of drunkenness. But let's hope that your cerebellum
has passed the police check - then there is one more fascinating fact about the cerebellum.
But first we have to take a step back, or more literally, actually look much closer
at it all. Basically, the entire nervous system is made up of many, many small
nerve cells. It's estimated that the human brain has about 86 billion
nerve cells, which is over 10 times as many people as we have on Earth. And what
do you think is the percentage of these nerve cells that are actually located in the cerebellum? Actually,
over 50%. That seems a bit strange - because it's relatively small compared to the cerebral cortex. But ultimately,
this works through two tricks: first, the population of all these
nerve cells is quite diverse. There are many different sizes, ranging from
very large to tiny. The cerebellum contains tons of granule cells, which are
quite small compared to other cells. That means that a lot of them fit into the cerebellum.
But there is another trick to make this work. And for that,
we need to take a closer look at the general structure of a neuron. Ultimately, it consists of three essential parts:
the cell body and two processes. On one side is the axon, which is
ultimately the process where information is transmitted electrically. It then branches off
and connects to other neurons and sends the information to these chemically
via synapses. And on the other end, the neuron has this other process,
the dendritic tree. This is exactly where the synapses of other neurons
arrive. And the evolution has apparently found out with these neurons
hat it doesn't work very well if you just lump them together chaotically
and just cram them into the brain all mixed up. Instead, they are actually neatly sorted. And that
is why all these nuclei of the neurons always gather in one place. And then the axons,
which sometimes have to run very far until they arrive at their target cell, run relatively
ordered and separated from them. And these places where these nerve cell bodies gather,
they actually look different. They are a bit darker, so these are really your gray
cells, the gray matter. Even if it actually looks mostly pink
in living humans, much more than gray. And the areas that only axons run through,
that is the so called white matter. If you want to gather a lot of these neuron cell bodies
in one place, you can of course either do this in a cluster,
and this option does actually exist, for example the cranial nerve nuclei in the brainstem, or
the basal ganglia in the cerebral cortex, and there are also a few small ones in the cerebellum. But another possibility
is to arrange all the neuron cell nuclei in a layer. And this is
crucial both in the cerebellum and in the cerebral cortex. So if we take a closer look
at this cerebellum, it becomes apparent that it has lots of very fine grooves and fine elevations. And if
we cut it diagonally once, we can see that this grayish band is really at the top
of the surface. That is the gray matter, that is the cerebral cortex of the cerebellum. And that is exactly
where all these countless granule cells gather. And that is the second trick,
that the cerebellum is simply so incredibly folded, because it likes to organize
all these cell bodies in this band, in this cortex. And if the
cerebellum were just a ball, there’d be very little surface area, but due to
this strong folding, it has much more surface area and that's how all these neurons
can fit in there orderly and beautifully sorted. But enough about the cerebellum. Now let's go back
to the brainstem, of which we are still missing the third and final part. That is the midbrain,
or the mesencephalon. And that actually looks quite interesting too. So, from the front we see
these cerebral peduncles, that's what they are actually called. And when we look at it from behind,
with the cerebellum hidden, we can see the tectum with these
four elevations. In birds and reptiles, the midbrain is super important. Many pathways
from sensory organs end there: the visual pathway, the auditory pathway, and also the one for sensitivity
and then they are really finally processed there. For this, the space in the midbrain
was no longer sufficient for us humans, so it was largely outsourced to the cortex of the cerebral cortex.
But even if the poor midbrain had to give up all these functions in our human brains,
it still remains important, like everything in the brain, no space is wasted.
What is left over now, for example, is the control of our eye muscles. And
if you cut across here, you can see an area on both sides that is
a bit blacker, that is because it is a collection of nerve cells that
have more iron than the other cells, plus the pigment melanin. That's why it looks so dark. And this is
the substantia nigra, which can already be counted as part of the basal ganglia, which also are important
for movement. And if the substantia nigra no longer functions properly, it leads
to Parkinson's disease. Above the midbrain, the brainstem ends, so that's ticked off now.
But it's not directly the cerebrum that we see here, there is yet another
area in between. This is aptly called the diencephalon, which is greek
for "intermediate brain". And that's where it starts to really get exciting. So, the diencephalon consists of
several levels, all of which have Thalamus in their name in some way. There is the Thalamus itself,
then, for example, the hypothalamus and the epithalamus. And some others, but
these are the most exciting ones. Let's start with the hypothalamus. It sits here in front of
the pituitary gland and is actually the most important interface between our brain
and our body. So it is the control center of the autonomic nervous system
and it also controls the pituitary gland, where many hormones are secreted. And so
the hypothalamus controls very important behaviors, such as hunger, thirst, sleep and wakefulness,
and also our sexual behavior. But if we look from the pituitary gland that is attached here right below,
the anterior gland, back to the diencephalon, there is also a gland there
and that is the pineal gland. It is called the pineal gland
because it apparently looks similar to a cone from the pine tree. And the pineal gland
is probably the region of the brain around which most myths revolve.
This is probably because it is the one structure of the brain that lies pretty much in the middle
and is one of the very few structures in our brain that is not paired. Most structures
in the brain are actually mirrored on the left and right sides. But because
the pineal gland is right in the middle, there is only one of it. And that led, among other things,
the French philosopher René Descartes to suspect that this was
the seat of the soul. There is not much, if any, scientific truth to any of those myths, but they seem to still
be quite popular even today. But what the pineal gland really does and what is also important is
that it secretes melatonin and thus controls our sleep-wake cycle. And
the last major element of the diencephalon, of the intermediate brain, is the thalamus. It is located
more here in the middle, but on both sides - so unlike the pineal gland,
it is not just one, but there are two of them. And the thalamus is pretty much
the bouncer of our brain. Everything we somehow see, hear, taste or feel,
wants to get to our cerebral cortex somehow so that we can become aware of it
and so that we can consciously deal with it. But as this is incredibly much information,
they all have to go through the thalamus first and it then sorts out what is allowed to continue
into the cerebrum and what is not. And without the thalamus, nothing gets through. But if
the thalamus is in a good mood and lets the information pass, then it gets into the cerebrum. And the cerebrum is
what really makes us humans really special. So, unlike for the brainstem,
the difference to the mouse is rather clear here, just from the size alone. And in humans,
the cerebrum actually makes up about 80 to 85% of the total mass of our brain. It also is
the part that is most differentiated, meaning it is the part that is most recently and furthest developed,
of all our brain. And the most exciting thing about it is the outermost layer, the cerebral cortex. The fact
that you can see me and hear me right now and hopefully even understand what I'm saying
- and maybe even remember something about it - for all of that, the cortex
is essential. And it's huge. It really covers the entire cerebrum here.
But different areas have different tasks. And thanks to its ridges
and furrows - of which at least the really big ones actually are pretty similar in all people -
we can quite easily distinguished these different areas, even macroscopically.
The technical terms may also be quite useful for this. Such a furrow
is called a sulcus in the cerebral cortex, and such a ridge is called a gyrus. Not gyros,
but gyrus with a “u”. Looking at the main gyri and sulci, the cerebral cortex can be divided into four
major lobes. They are: the frontal lobe, then the parietal lobe, the occipital lobe, and the
temporal lobe. And of course we’ve got two of all of those, as is the case with most of our brain structures. That means
there is a left hemisphere and a right hemisphere. And, amazingly,
they are largely separated from each other. But there is one big connection, and that is the corpus callosum
in the middle. About 200 million nerve fibers run through it. And in people who had severe
epilepsy, a treatment that was sometimes attempted, was to sever this corpus callosum surgically.
The crazy thing about it is that these people showed hardly any abnormalities in everyday life.
They were actually able to live pretty well without this connection, without this corpus callosum.
But with more detailed investigations, it was found that the connection
of course does have its relevance and that it’s not just there for no reason. There are some crazy things
because the function of the two cerebral hemispheres is partially distributed differently. Well, many tasks
are also executed identically by both hemispheres - and, surprisingly, usually for the other half of the body.
But one thing that no longer worked, for example, was when these people
saw something in their left visual field, for example an apple, then this information was passed on
so that it ultimately arrived in the occipital lobe on the right side of the brain, regardless
of this cutting of the beam - there the apple was recognized as a picture.
But naming objects, in general, occurs in the left hemisphere. And the relaying of the visual information
about the apple, over to the language center on the left side, did not work any longer.
This meant that people could no longer name the apple they saw in their
left visual field. This was already a bit of a preview of
which functions are located where in the brain. Let's take a closer look at the
most exciting aspects. Let's start at the back with the occipital lobe, which comes from latin and quite literally
means “the lobe of the back of the head”. The task of that is quite simple, quite clear,
it is responsible for seeing. It's a bit weirdly constructed - the eyes
are in front and the brain region that is responsible for seeing is at the very back. Why?
That’s hard to say. But it works. The visual information we receive with our eyes
is first checked by the thalamus, by the bouncer, of course,
and then arrives at the back of the occipital lobe. And here, we already see an exciting general principle.
Because, in order for us to actually recognize something and make sense of what we see,
it is a multi-stage process that must take place. First, this information arrives in the
primary visual context where it is further processed. But that's not enough yet.
There are other cortical areas, also initially still in the occipital lobe, which are then called secondary
and further association cortices. And only through this multi-stage processing here
- and then even further steps with the information forwarded to the parietal and temporal lobes -
can we ultimately really do something with what we see. Let's move on straight
to the parietal lobe. It lies in front of the occipital lobe and extends forwards up to the central fissure,
the central sulcus. The parietal lobe is responsible for all of our somatosensory perception.
And we already know the logic. Again, there is an area where
the information arrives first. So the somatosensory information (for example from our skin)
from the whole body arrives here at the very front, in the postcentral gyrus. For further
processing of this perceptual information, he also needs support.
And the posterior area of the parietal lobe brings together almost all of our perceptions - not only
from our skin, but also visual and auditory information from our ears,
and builds a 3D image for us, a complete 3D-perception of ourselves and our environment.
If this region stops working, for example after a stroke,
then a crazy phenomenon can occur, especially if the right side is affected,
namely a so-called neglect. And this means, that things that one perceives - as most everything crosses sides
sides in the brain, that’s things from the opposite side that one sees, hears, or feels -
even though one could still perceive them, are no longer properly
incorporated into this 3D image and are therefore no longer properly recognized. That means,
for example, if someone is talking to an affected person and standing on the left side,
it can happen that they do not perceive them at all - do not hear or see them -
even though the perception would in fact work as far as the rest of the brain is concerned. It can even go so far
that one's own body parts, for example one's arm, even if one can feel it,
are no longer felt to belong to oneself. So that's sensitivity and perception. Let's go
one step further to the front, then we get to the frontal lobe,
which begins behind this central fissure. This lobe is actually responsible for two different things.
First of all, above all, for motor function, as we still lack that.
Well, the cerebellum already does a lot of that, but the actual planning of motor skills
actually takes place in the cerebral cortex in the frontal lobe. And then fine tuning and coordination in the cerebellum.
The primary motor cortex has an exciting commonality with the primary sensory cortex
which we saw just before. Namely, both are somatotopically arranged. Somatotopically, what does that mean?
Let's take another look at the example of the primary sensory cortex. As with the motor cortex,
you can draw a homunculus over it, a kind of small human being.
And if you draw this little man on it here, it shows us
which part of the body is perceived by surface sensitivity at which point on the cortex.
So that means, here again, that things are not just random and messy,
but everything is finely ordered. And for example, the knee is here on the edge,
while the perception of the hand is further out and the perception of the face
is even further down. And you can see that the dimensions don't match
with how big our body parts really are. So if you 3-dimensionally transform this little man,
it would look like this. And here you can basically see how fine (or not) the perception is
in the different body regions. Well, at the back, for example, we perceive very little, while in our hands, which are
much more sophisticated, as we use them very precisely, or our face, there are many more sensory cells
and accordingly they have much larger representations here in the brain. The same principle
also applies to the primary motor cortex. And if there is now a very small damage,
for example a small stroke, in one place, then the symptoms are according.
So if there is a mini-stroke here, for example, where the hand is controlled,
then only the hand (on the opposite side of course) can be paralyzed.
But as I said, the frontal lobe has another task besides motor skills. And
- well, I've said so several times now... - but here it really gets exciting! So, the foremost area,
the prefrontal cortex, is needed for attention, for thinking, for planning,
or making decisions, and it is also said that this is where the seat of personality is.
There is a really famous story about how it was first thought that these functions
could be localized there. This is the story of Phineas Gage. He was
a worker at a railway company and in an accident during a blast,
an iron bar shot through his skull,
in under his left eye and then out of his head again, damaging
or at least partially destroying his prefrontal cortex. And the crazy thing was that Phineas Gage
survived this accident and was even conscious throughout and could report on it.
And afterwards, even crazier, he didn't appear to have any recognizable damage, at first.
Well, we now know that motor skills, sensory skills and also vision are located further back.
And it didn't damage these areas. That means he could feel and
move everything normally. But what his friends, relatives,
and coworkers quickly noticed was that he had simply changed in personality,
that he was no longer the same Phineas Gage they knew.
He used to be quiet and reliable and then became much more impulsive and unreliable.
So, really a totally crazy story. But it also illustrates very well how things about the brain
used to be mainly discovered, namely simply by chance through accidents
and then being able to determine where the brain was damaged and what kinds
of neurological deficits resulted from that. This is still partly how it works today, for example,
While working in a stroke unit, I had many opportunities to precisely see
the location of damage in a patients brain on a CT or MRI scan
and then correlate this directly to the results of the patient’s physical examination findings
and thus recognize which region of their body they can no longer move or feel, for example, as a result of this brain damage.
But fortunately, there are other possibilities in brain research today, namely functional MRI (fMRI)
where you can basically follow along live and without causing any damage
which regions of the brain consume more energy, i.e. work more strongly, while having the examined person perform certain tasks.
Here, for example, is a fMRI-image of a person who was instructed to move their fingers of the left hand.
And now we can test our knowledge again.
The area that lights up in red-yellow here is mainly in the gyrus praecentralis,
where motor skills are controlled. And if we place the homunculus over it again, it fits quite precisely
that the spot that lights up in red and yellow is where the hand is controlled.
So, this is a tremendous advancement in research. And it's working really well in this example
- the crazy thing about it is that it's rarely as simple as this,
as in most cases you rather find out that things are more complicated than expected
and that for many functions multiple brain regions are involved
But these basic distinctions that we learned so far, fit pretty well.
But we are still lacking one last lobe, namely the temporal lobe,
which is located here on the side.
It does not have a clearly visible border to the occipital and parietal lobes,
but it is separated from the frontal lobe by this huge fissure here, the so-called lateral sulcus.
And the temporal lobe, its function is a bit less clear. We had seen
vision, touch, and motor skills before, and then personality and all the complex things.
The temporal lobe is rather multimodal. So it does many things, including smelling, hearing,
many functions for understanding and for visually recognizing objects.
And then, crucially, memory or memory formation. For that,
let's take a look at the brain from below again. And then we see this little hook,
which is called the uncus. Superficially, the olfactory cortex is located in this uncus, and underneath it,
it gets even more exciting, there is the amygdala. It is responsible for the emotional coloring
of our perceptions and also of our memories. However, it is somewhat inclined
to color things negatively. It is also part of the limbic system,
which also contains several other brain regions. One very important thing that belongs to it
is the hippocampus. It sits right next to the Amygdala in the gyrus parahippocampalis
- quite a tongue twister... I secretly recorded that ten times, but I'm editing all of that out -
but this ist where the hippocampus is hidden, and it's super exciting too. It is crucial
for forming and retrieving memories. Long-term memories are
not stored in the hippocampus itself, but rather in the cortex.
But without the hippocampus, we cannot store things and cannot retrieve them again.
The transfer of memories from short-term to long-term memory
largely takes place during sleep. So, I really wish you
good sleep today so that you can remember as many of these things about the brain as possible.
But if you still have some time before going to sleep, I have a created a playlist for you
with more videos about the individual brain regions, which you can find here.
Thank you very much for watching, I hope to see you in one of the next videos! Bye.
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