2. Main parts of the brain
The brain is composed of 4 main parts, as shown in the figure;
the two cerebral hemispheres, In fact 90% of the brain if formed by the cerebral hemispheres.
two diencephalons:
Each diencephalon is subdivided into 4 parts;
the thalamus,
the hypothalamus (which connects to and
controls the pituitary gland),
the epithalamus (which is formed mainly from
the pineal gland),
and the fourth part is the subthalamus, and as its
name indicates, it is located below the thalamus
but at the same time it is lateral to the
hypothalamus, so we can't see the subthalamus
unless we make a transverse section below the
thalamus, and we expect to see the subthalamus
lateral to the hypothalamus.
one brain stem (the picture represents half
brain stem), that is composed of three parts
arranged from superior to inferior as the brain
stem, the pons, and lastly the medulla oblongata
which in turn will continue downward as the
spinal cord.
and finally the cerebellum, it represents the main balance center in our nervous system, the part of
the brain that maintains the coordination between muscles and in this way it will maintain our
balance.
The cerebral hemispheres
The nerve cells in the cerebral hemisphere are organized into superficially located gray matter called the
cerebral cortex and centrally located white matter. The cerebral
cortex is made mainly of cell bodies and dendrites while the
cerebral white matter contains the axons. Anyway, in addition to
the white matter that is formed by the cell axons, the central
part contains groups of nuclei that are called basal ganglia.
The surface of the cerebral hemispheres is very irregular, the
elevated parts are known as gyri (single; Gyrus), and the grooves
between them are called sulci (single; sulcus).
3. LANDMARKS
In each side we divide the cerebral hemispheres into 4 lobes; frontal lobe, parietal lobe, temporal lobe, and
occipital lobe, in the figure below we can see a lateral aspect to the left and a medial aspect to the right.
Of course, we use landmarks to divide these cerebral hemispheres on the lateral surface into these lobes,
two of these landmarks are true, while the other two are imaginary lines:
The two true lines are:
1- The lateral fissure : located on the lateral surface as seen in the figure, it has three branches; the
anterior ramus which is directed
forward, the ascending ramus which
ascends upward, in addition to the
posterior ramus that is directed
backward
2- the central sulcus : that is located in
the lateral surface of the brain, in the
center, that is separating the frontal
lobe from the parietal lobe. It
ascends upward and slightly
backward from the lateral fissure to
the superior margin of the brain and
usually continues for a short distance
on the medial surface as you can see.
The two imaginary lines are:
1- the first imaginary line : passes
upward vertically from the
preoccipital notch, which is a small notch or groove that is located anterior to the occipital lobe, to
the superior margin of the brain.
2- the second imaginary line : that extends from the middle of the first one to the lateral fissure.
The first imaginary line is parallel to a sulcus that is located on the medial surface, this sulcus is called
parieto-occipital sulcus, which –as the name indicates- is located between the parietal lobe and the
occipital lobe.
THE LOBES :
you should know the location of each lobe as the following:
The frontal lobe is located anterior to the central sulcus and above the lateral fissure.
The parietal lobe, is located posterior to the central sulcus, anterior to the first imaginary line , and above
the posterior ramus of the lateral lobe and second imaginary line.
The occipital lobe is located posterior to the first imaginary line.
The temporal lobe is located below the second imaginary line and the lateral fissure, anterior to the first
imaginary line.
4. Gyri and Sulci of the cortex
Before we start we should know that the numbers that are present in the following pictures are called
Brodmann areas, and Brodmann is a scientist who classified the different cortical areas based on their
microscopic anatomy (such as the thickness of layers and other criteria) into 52 areas. You should know the
numbers of the areas that we will talk about them.
THE GYRI OF THE FRONTAL LOBE
The gyrus that is located anterior and parallel to the
central sulcus is known anatomically as the precentral
gyrus (corresponding to the Brodmann area n. 4).
Anyway, this gyrus is known functionally as the motor
gyrus (or motor cortex, motor area), because the nerve
cells of this gyrus generate the motor orders that enable
the muscle to contract, so damage to the motor area will
lead to paralysis.
like the central sulcus, the precentral gyrus starts at the
lateral fissure, ascending upward and slightly backward
to the top of the brain, then it will descend for short
distance in the medial surface.
The muscles of each side of our body are presented in a special way in this gyrus; this presentation
of the muscles within the motor gyrus is known as the motor homunculus (homunculus is a Latin
word that means the very small body, this area is named like this because it represents the motor
function of the whole body).
5. This presentation –motor homunculus- has three features;
first, it is contralateral, that’s why if a damage occurrs in the left motor gyrus, for example,
the paralysis will be in the right side, because the axons that leave the motor cortex while
descending in the brain stem, will cross the midline to the contralateral side in the medulla
oblongata.
Second, it is inverted as you can see in the figure, the body is represented in an inverted
manner with the head is represented in the most inferior part of the motor area on the
lateral surface, and the legs are presented in the medial side.
Third, it is disproportional to the muscles size, on the other hand, it is proportional to the
significance of the muscles. You can see that the hand, fingers and the lips take a wide area,
while the muscles of the abdomen are large in size, but the complexity of their movements
is not that much, so they will occupy a small area.
Anterior to the motor gyrus we have the
premotor area (corresponding to the Brodmann
area number 6), this area will store the motor
programs of previous motor experience, so
damage to the motor programs will not lead to
paralysis, actually he can move his muscles but
the patient can't use his muscles in the proper
way to produce the normal motor activity
The premotor area continues on the medial
surface what we call supplementary motor
cortex.
The rest of the frontal lobe is divided by two sulci into three gyri. The two sulci are the superior and
the inferior frontal sulci, and the three gyri are the superior frontal gyrus, the middle frontal gyrus
and the inferior frontal gyrus.
The superior frontal gyrus continues on the
medial surface, the middle frontal gyrus
contains an area called the frontal eye field
(which corresponds to the Brodmann area
n. 8). The frontal eye field in the right side
moves the two eyes to the left (the
contralateral side) and the frontal eye field
in the left side moves the eyes two the right.
Now, if you have an unconscious patient,
after car accident, for example, suppose
that you saw his eyes are shifted to the left
side, this will indicate that the lesion is in the left side and because the right side is normal,
it shifted the eyes to the left.
The middle frontal gyrus, the doctor said nothing
about this area. (=Brodmann 46, according to wiki)
6.
The inferior frontal gyrus contains two important gyri as part of it, the triangular and the
1
opercular gyri. The triangular gyri is located between the anterior ramus and the ascending
2
ramus of the lateral fissure. Behind the triangular gyrus we have the other gyrus; the
opercular gyrus. Functionally; these two gyri together are known as the motor speech area
(Broca's area, corresponding to the Brodmann areas 44 and 45), which store the speech
programs. In fact when we talk we don’t need to think of each letter and each word because
we have programs stored here in this region. Damage to the motor speech area will result in
motor aphasia, aphasia means disturbance of language. In this case, we don’t have speech
muscle paralysis, but the patient can't use his muscles in the proper way to produce the
normal speech, so the patient will either stammer or he will be unable to produce
understandable language at all , depending on the extent of damage.
The remaining part of the frontal lobe is called the prefrontal gyrus, its function is related to the
higher functions of our brain, including thinking, learning, decision making and social behavior.
Damage to the prefrontal gyrus on one side will lead to nothing, while damage to the prefrontal
gyri will lead to loss of these functions. For example, the social behavior will change for someone
from being very gentle and very kind to a rude person if the damage occurs in both sides. ( I really
wonder if the opposite may occur !!)
7. THE GYRI OF PARIETAL LOBE
The gyrus that is located immediately posterior to the central sulcus is called the postcentral gyrus,
and it corresponds to the Brodmann areas 1,2 and 3. This gyrus is parallel to the central sulcus,
starting at the lateral fissure, ascending upward
and slightly backward to the top of the brain,
and then descending for a short distance on
the medial surface.
Functionally, it enables us to feel general
sensations and thus called the primary
somatosensory gyrus. These sensations like
pain, temperature, touch, and proprioception
(which means the sense of position of different
parts of the body and sense of movement,
even when your eyes are closed if someone
moves your finger, you can sense of that, and
you can determine its relative position to other body parts).
The presentation of the body in this area is called the sensory homunculus, which is contralateral,
inverted and disproportional, just like the characteristics of the motor homunculus.
Both the precentral and the postcentral gyri will continue, as you know, for a short distance in the
medial surface, and together they are known as the paracentral lobule, so it forms the motor and
the sensory area for the leg and foot, the lower parts.
The remaining part of the parietal lobe behind parietal lobe is divided into superior and inferior
parietal lobules by a very irregular sulcus called the intraparietal sulcus.
The superior parietal lobule is called
the associated somatosensory area.
While the primary sensory area
enables us to feel the sensation, the
associated sensory area enables us to
appreciate sensation. For example,
we feel the pain in the postcentral
gyrus, but features like the position,
the type, and how much it is severe,
are appreciated in the associated
somatosensory area, which
corresponds here to the Brodmann
areas 5 and 7.
8.
In the inferior parietal lobule contains the supramarginal gyrus and the angular gyrus;
o To identify the supramarginal area in any brain you should follow the lateral fissure,
the posterior ramus until you reach the end of this ramus, this end is called the
margin of the lateral fissure, the gyrus that overlaps the margin is called the
supramarginal gyrus.
The supramarginal gyrus of each side enables us to be aware of the other side; so if
a damage occurs in the left supramarginal gyrus, then the patient will neglect the
right side totally, so if he wants to shave his beard, he will shave the left side but not
the right! and if he will ignore a car that is about to run over him if it is from the
right side. This case is called negligence syndrome.
o The other part of the infraparietal lobule just behind and below the supramarginal,
there is the angular gyrus which enables us to understand what we read, and so if
there is a damage in the angular gyrus this is called visual sensory aphasia (aphasia=
disturbance of language, and in this case we have a problem in understanding the
written language), to be more accurate this is called visual aphasia. If a "Jordanian"
patient has a problem in this region and he is reading the journal, he can see
everything but it is just as he is trying to read a Malaysian journal.
9. OCCIPITAL LOBE
We will consider in the medial side of the occipital
lobe, and in this surface the occipital lobe is
divided by one sulcus into two gyri. The sulcus is
called calcrine sulcus which will divide the
occipital lobe on the medial side into cuneate
gyrus (cuneus) and lingual gyrus.
Functionally, the part of the cuneate gyrus that is
located immediately above the calcrine sulcus and
the part of the lingual gyrus that is located
immediately below the calcrine together they
represent the primary visual area which enables
us to see, so a damage in the primary visual area
results in loss of vision (for example if I put a
pencil in front of the patient; he won't see it)
The remaining part of the cuneate, and the
remaining part of the lingual gyri on the medial
surface along with their continuation on lateral
surface of the occipital lobe, they form
functionally the associated (secondary) visual area and it enables us to understand or appreciate what we
see, so a damage in the associated area results in loss of the ability to recognize what you see (in the same
example you will see the pencil but you won’t recognize it as a pencil).
TEMPORAL LOBE
On the lateral surface; and it is divided by two sulci (superior &middle temporal sulci, while the inferior
temporal sulcus is located on the inferior surface) into three gyri (superior, middle and inferior temporal
gyri which will continue on the inferior surface of the brain).
the superior temporal gyrus:
a part of it will run transversely
toward the lateral fissure and that
is why we call this part transverse
temporal gyrus. Functionally it
enables us to hear so we call it the
primary auditory (hearing) area (or
cortex or gyrus).
10. The remaining part of the superior temporal gyrus is called secondary (associated) auditory
(hearing) area which enables us to understand what we hear.
The most posterior part of the secondary auditory area (corresponding to the Brodmann area n. 22)
enables us to understand the spoken language while the other sounds are understood in the other
parts of the superior temporal gyrus, this area is called wernickes area according to the scientist
wernickes who discoverd it; while its functional name is sensory speech area.
(wernickes= Brodmann #22 = sensory speech area which is the most posterior part of the superior
temporal gyrus that enables us to understand the language that we hear).
So a damage in the primary auditory area results in loss of hearing (the patient can't hear) while a
damage in the sensory speech area results in loss of the ability to understand what you hear (the
patient can hear but can't understand what he hear), and this is one of the sensory aphasia (it is
sensory because the error is in sensation) known as auditory or hearing sensory aphasia.
To sum up "aphasia", we have two main types of aphasia;
1stMotor aphasia caused by damage to the motor speech area (triangular &opercular gyri) and
2ndSensory aphasia which is two types; visual aphasia caused by damage to the angular gyrus where the
patient can't understand what he read &auditory (hearing) aphasia caused by damage to the wernickes
area where the patient can't understand what he hear (so if you ask him the answer won't be related to
the question).
The inferior surface of the cerebral cortex will be covered in the next lecture.
You have to read the hand outs and the lecture, anyone of them is not enough.
Done by:
Ahmad Hdaib
Moath Azmi
إن عجبتكم االمحاضرة ادعولنا، و إن ما عجبتكم االمحاضرة ، بعينكم هللا ، كمان ادعولنا
