So hindbrain, midbrain, diencephalon, and
then Telencephalic hemisphere right, and Telencephalic hemisphere left,
this join right here is a very small join, and it's right here.
That's the only place where it joins.
So how come this is so huge?
Well, what happens is that the telencephalon expands
in every single direction, and it expands down,
it expands in the front, it expands a lot towards the back.
And so if you look at it from the side
you'd see here is the diencephalon,
here is one Telencephalic Hemisphere.
So let's make that the right Telencephalic Hemisphere,
this comes back, it goes forward, it goes down.
But it really goes back, and so
what you end up with is, here's the diencephalon,
midbrain and hindbrain is going back that way.
Here's the telencephalon, actually we'll put in the midbrain,
we'll put in the hindbrain, and what happened is that
this telencephalon goes back and arches over and expands.
And it also comes out this way, so it envelops the rest of the brain.
This enormous expansion is what fuels the behavioral
flexibility of humans and other mammals.
This telencephalon has- Several parts.
One is the cerebral cortex, and, in the cerebral cortex,
the bulk of the cerebral cortex is what we call neocortex.
Neocortex is, it's got six layers and
it's only present in mammals, only mammals have neocortex.
And the bulk of this is Neocortex.
That's why we have these enormous brains, is to house all these neocortex,
to allow mammals to have a lot of behavioural flexibility.
So you have this comb-over of cerebral cortex,
and why is the comb-over so important?
Because cerebral cortex, cortex means bark,
it's actually just the outer,
the very outer rind of this is cerebral cortex.
So in order to get a lot of surface area, we need to make this this vesicle,
we need to make this part of the brain very large.
And it gets so hungry for territory that it even comes around and
forms a temporal lobe.
And so, if we come back to the slide,
I think you can see it here,
the diencephalon is sitting deep in here and
the telencephalon grew out.
It grew out, it grew back, and then it still didn't have enough room, so,
off it goes out to form the temporal lobe and that whole expansion is a ram's horn.
It's in the shape of a ram's horn.
So, we have a temporal lobe, there are several other mammals that have
a temporal lobe, but for instance, a rodent doesn't have a temporal lobe.
There is one more thing that we've done to expand
the territory that cerebral cortex can occupy.
And that's to make, what you see here,
are these invaginations, this is in and
out, this are called sulci and gyri.
Sulcus is the singular and gyrus is the singular,
gyrus is the outpouching, and sulcus is the valley between the hills.
And sulci saying gyri are the plurals.
So by doing this, you've expanded the territory
that the cerebral cortex can occupy.
What we learned was that the neural tube forms at, is complete by day 28.
Over the next eight months of a human gestation
all of these cerebral cortex expands.
Lots of neurones are born and they grow into this.
They expand over, they do the come over, they get and
they populate the temporal lobe and
they also create these invaginations, the sulci and gyri..
What happens if that gets interrupted?
And if that gets interrupted, you don’t get a full blown brain, and
that is essentially what has happened with in the case of microcephaly.
Microcephaly, as you may know, we are recording
this now in 2016, has recently been in the news,
because unfortunately, there's a virus,
the Zika virus, which is carried by mosquitoes,
which is infecting pregnant women and causing a high proportion of their
babies to be born with some degree of microcephaly.
And the press has been calling it as a small head,
it's not about a small head, it's about a small brain, and
it's not about a small brain, it's about a small telencephalon.
This is a reduction in the size of the cerebral cortex,
and the degree to which it's reduced is probably
related to how early in gestation the infection occurs.
So, the earlier it occurs, the more devastating affects are going to be,
the later occurs, you might get a milder form of microcephaly.
The head is simply going to then be as large as it needs to
be to contain the brain.
The head is, it's a cranium, it's a volt for the brain.
So this is a really unfortunate problem, and
the degree of dysfunction, or the limitations,
of these individuals born with microcephaly will
face will depend on the severity, some will talk some will not talk.
There'll be a wide range of clinical presentations.
I'm just sad about this.
I hope that some vaccine is developed,
I hope that somehow we can combat this modern problem.
Okay, great.
[MUSIC]
Peggy MasonProfessor
