Introduction to the Central Nervous System !!

 

the adult human brain weighs

approximately one and a half kilograms

making up less than 2% of the person's

body weight and yet it defines our

humanity and makes us the individuals

that we are the brain is responsible for

the generation of language and thought

attention consciousness memory and

imagination in order to fit into the

skull and accommodate the massive number

of neurons and connections needed the

brain is highly folded this results in

the creation of gyri or ridges and sulci

or furrows if we were to unfold the

entire human brain it would take up

approximately one square metre perhaps

the most impressive feature of the brain

is the amount of connections formed

between neurons there are an estimated

86 billion neurons in the brain each of

which forms an average of 7,000

connections with other neurons resulting

in between 100 and 500 trillion synapses

within the brain in an attempt to

conceive of the enormity of this system

the number of neurons in the human brain

has been equated to the number of stars

in the Milky Way now that we have begun

to appreciate the complexity of the

human brain let's begin to examine its

structure

the brain can be divided into functional

and anatomical regions we will now start

with an anatomical overview to establish

a common terminology and to describe the

areas of the brain the brain has

multiple surfaces here we have the

superior surface and this here is the

inferior surface there are also anterior

and posterior aspects to the brain

here's a section through the brain

within the skull and we can appreciate

the axis within the CNS connections can

travel either towards the anterior pole

here or towards the inferior end of the

spinal cord fibers can move either

rostra Li towards the rostral pole or

caudally towards the caudal pole now

let's look at these component parts of

the brain this is the forebrain which is

composed of the cerebral hemispheres and

deep structures the right and left

hemispheres are roughly symmetrical and

are connected through corpus callosum

which you can see here here we have cut

the brain in half separating the two

hemispheres from each other this is the

cut surface of corpus callosum the

largest white matter tract in the brain

impressively it consists of an estimated

200 to 250 million projections this

structure here is the thalamus and

together with the hypothalamus here and

the sub thalamus which you cannot see it

makes up the diencephalon the cerebral

hemispheres and the diencephalon

together comprise the forebrain we now

move into the brain stem which is caudal

to the diencephalon

the brainstem can

into three parts the midbrain located

here just caudal to the thalamus

contains the large fiber bundles with

cerebral peduncles that connect the

forebrain with all caudal structures the

pons is located caudal to the midbrain

it is connected to the cerebellum

through the cerebellar peduncles the

most caudal part of the brainstem is the

medulla it is continuous with the spinal

cord as it exits through foramen magnum

of the skull the cerebellum is

embryologically part of the pons but its

functions are so distinct that it is now

considered its own entity separate from

the brainstem the cerebral hemispheres

can be divided into lobes along some key

surface landmarks the central sulcus

separates the frontal lobe from the

parietal lobe the lateral fissure

separates the temporal lobe from both

the frontal and parietal lobes the

occipital lobe is separated from the

parietal lobe via the parietal occipital

sulcus which you can see on this medial

aspect of the brain you can draw a line

onto the lateral surface right here to

differentiate between the occipital lobe

and the parietal and temporal lobes in

this medial view of the brain you can

identify a continuous strip of cortex

which swings around the surface of the

brain this lobe has been dubbed the

limbic lobe due to its intimate

relationship with the limbic system it

spans the frontal parietal and temporal

lobes

deep within the brain our space is

filled with cerebrospinal fluid or CSF

this is a 3d reconstruction of the

ventricular system you can see the two C

shaped lateral ventricles in the

cerebral hemispheres they have an

anterior horn deep within the frontal

and parietal lobes of the forebrain a

posterior horn which extends into the

occipital lobe and an inferior horn

which extends into the temporal lobe

these lateral ventricles are connected

to the third ventricle in the midline

the third ventricle is connected to the

fourth ventricle through the cerebral

aqueduct on this mid sagittal section of

the brain we can identify different

components of the ventricular system

this here is the lateral ventricle the

anterior horn will extend anteriorly

here into the frontal lobe and the

posterior horn will extend into the

occipital lobe this here is the third

ventricle the thalamus is on either side

of the third ventricle the third

ventricle connects to the fourth

ventricle via the cerebral aqueduct

right here this here is the fourth

ventricle at the level of the pons and

medulla the fourth ventricle is going to

close off into the central canal let's

look at the various planes a brain can

be sectioned in this is a very important

concept because imaging of the brain

uses these planes I'm going to cut this

brain in the coronal plane

so you can now see corpus callosum here

and we've started to look into the

ventricular system so this is the

anterior tip of the anterior horn here

so this next slice will take us into the

anterior pull of the temporal lobe right

here those pieces might fall off because

they're not connected to the plane that

I'm cutting in there we go let's have a

look at that alright so what you can see

on this slide is really the lateral

ventricle again you've got the head of

the caudate nucleus right there and the

putamen here this is the anterior limb

of the internal capsule

alright so in this section now you can

see the two lateral ventricles as well

as the third ventricle here on either

side of the thalamus

all right so here in this section now

you can again see the two lateral

ventricles you are going into the

cerebral aqueduct on our way to the

fourth ventricle this here is the

inferior horn of the lateral ventricle

okay so in this section here again here

are the two lateral ventricles here

again is the inferior horn the

hippocampus right there in the floor of

the inferior horn all of this here is

the thalamus where quite posterior now

as you can tell in the brain so all of

this is the Loomis here we're actually

getting into the posterior parts of the

thalamus there right here we are in the

midbrain and the pawn so it's a bit of

an oblique section here through the

brainstem but you can see the cerebral

aqueduct here and of course this is the

pons

alright so in this section here you can

see that we're now getting into the

posterior horn of the lateral ventricle

this is the very very tail end of corpus

callosum there you can see the cut

surface here of the cerebellum it's the

middle cerebellar peduncle moving in

there and here you can see the central

canal who's going to open up into the

fourth ventricle which will light just

on the other side of that opening there

in this coronal section we can see both

gray and white matter white matter is

the sum of all myelinated axons or

tracts as they travel through the CNS

gray matter is the sum of all nerve cell

bodies it can be seen here along this

cortical band on the surface of the

brain which is why an increased surface

area is so important to accommodate the

large number of neurons in the cortex

gray matter can also be found in these

deep nuclei of the forebrain including

basal ganglia and limbic structures

we're going to cut this brain in a

horizontal orientation

they were now going to cut through the

brainstem here in an axial orientation

so here's the midbrain here we've cut

through the pons in the superior

cerebellar peduncle as it projects to

the cerebellum so this is the last

section here through the pons and you

can see some of the deep cerebellar

nuclei here this cut here you can see

the inferior part of the cerebellum and

then part of the open medulla and here

you can see the closed medulla as the

ventricle has closed over the central

canal here finally the brain can be cut

along the sagittal plane here we have

cut the brain along the mid sagittal

plane to separate the two hemispheres

with this overview we now have a common

terminology as we begin our journey

through the brain