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No. 25
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Telencephalon
Ⅳ. The Telencephalon
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The telencephalon consists of right and
left cerebral hemispheres, which together
are referred to as the cerebrum.
The cerebrum has an outer surface of gray
matter and an inner white matter.
The gray matter is composed primarily of
nerve cell bodies and unmyelinated nerve
fibers. This surface layer is called the
cerebral cortex.
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The white matter is called medullary
substance, which is composed of tracts
of myelinated nerve fibers.
Within the hemispheres, there are several
masses of gray matter known as the
basal nuclei.
A fluid-filled cavity called a lateral
ventricle is located within each cerebral
hemisphere.
Ⅰ) External Features and lobes of
telencephalon
 Two hemispheres of cerebrum are almost
completely separated by the cerebral
longitudinal fissure.
 At the bottom of this fissure a large
bundle of transverse fibers, the corpus
callosum, crosses between the two
hemispheres.
 The cerebral transverse fissure
intervenes between the hemispheres and
the cerebellum.
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1. The sulci, fissures, gyri and the lobes of the
cerebrum
1) The sulci, fissures, gyri
The surface of the cerebrum has many rounded
ridges called gyri (singular: gyrus). Separating
the gyri are furrows. The deeper furrows are
called fissures; the shallower ones are sulci
(singular: sulcus). The folding of the cortex that
produces the gyri and sulci makes the surface
area of the cerebral cortex much greater than it
would be if the brain’s surface were smooth. As it
is, a significant percentage of the cerebral cortex
is located in the fissures and sulci and is not
visible from the surface.
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There are three constant sulci (or fissures) for
demarcation on the surface of the hemispheres.
① The lateral sulcus begins as a deep furrow on
the inferior surface of the hemisphere and runs
posteriorly and upward onto the dorsolateral
surface.
② The central sulcus runs from the lateral
sulcus to the longitudinal fissure at right angle.
Each hemisphere is further divided into a frontal
lobe and a parietal lobe by the central sulcus.
③ The parietooccipital sulcus lies on the
medial surface and separates the parietal and
occipital lobes as its name indicates.
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2) The lobes of the cerebrum
There are five lobes of each hemisphere.
The hemisphere can be divided by these
three sulci and an imaginary line into five
lobes.
The imaginary line is drawn from the
upper end of the parietooccipital sulcus to
the preoccipital notch which lies about 4
cm anterior to the occipital pole on the
inferolateral border. They are frontal,
parietal, occipital, temporal lobes and
insula.
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① The frontal lobe
The frontal lobe is the area in front of the
central sulcus and above the lateral sulcus.
② The parietal lobe
The parietal lobe is the area bounded by
central sulcus, the imaginary line and the
lateral sulcus.
③ The occipital lobe
The occipital lobe is the area posterior to
the imaginary line and the parietooccipital
sulcus.
④ The temporal lobe
 The temporal lobe occupies the area
inferior to the lateral sulcus and in front of
the imaginary line.
⑤ The insula
 The insula lies deeply in the floor of the
lateral sulcus, and is overlapped by
portions of the frontal, parietal and
temporal lobes.
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2. The gyri and sulci on the dorsolateral
surface of the hemisphere
1) In the frontal lobe
Precentral sulcus and precentral gyrus
Superior, middle, and inferior frontal
gyri
Superior, middle and inferior frontal
sulci
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2) In the parietal lobe
Postcentral sulcus and postcentral gyrus
Intraparietal (or interparietal) sulcus
Superior and inferior parietal lobules
Supramarginal gyrus and angular gyrus
3) In the temporal lobe
Superior and inferior temporal sulci
Superior, middle and inferior temporal gyri
Transverse temporal gyri
4) In the occipital lobe
Not constant.
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3. The gyri and sulci on the medial and inferior
surfaces of the hemisphere
1) The gyri and sulci on the medial surface
Paracentral lobule,
Corpus callosum,
Callosal sulcus,
Cingulate gyrus and cingulate sulcus,
Paracentral sulcus and Marginal ramus,
Calcarine sulcus,
Cuneus and Lingual gyrus,
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2) The gyri and sulci on the inferior surface
① The inferior surface of frontal lobe
Olfactory bulb, olfactory tract and olfactory
trigone,
Anterior perforated substance.
② The inferior surface of temporal lobe
Lateral and medial occipitotemporal gyri,
Occipitotemporal sulcus,
Collateral sulcus,
Parahippocampal gyrus and uncus,
Hippocampal sulcus,
Dentate gyrus,
Hippocampus,
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Hippocampal formation: It includes
hippocampus and dentate gyrus.
Limbic lobe: On the medial surface of the
cerebral hemisphere, a large arcuate convolution
formed primarily by the cingulate,
parahippocampus gyri and the hippocampus and
dentate gyrus surrounding the upper brain stem,
constitute the limbic lobe.
Ⅱ) Functional Localization of the Cerebral
Cortex
 On the basis of the effects of electrical
stimulation of specific areas of the
cerebral cortex in humans, from observing
the clinical manifestations of brain disease
or damage in humans, and from the
results obtained from detailed
experiments on other mammals, it has
been determined that certain areas of the
cortex are related to specific functions.
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Some of these areas have been precisely
mapped and numbered in a system called
the Brodmann classification, but for our
purposes it is sufficient to consider only
the general locations of the major
functional areas.
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1. First (Primary) somatic motor area, or
motor center
(1) Location
It is located in the precentral gyrus and the
anterior part of the paracentral lobule.
It is equivalent to the 4 and 6 areas of the
Brodmann classification.
(2) Function
The neurons of this area control the conscious
and precise voluntary contractions of skeletal
muscles.
The axons of the neurons of this area form the
pyramidal tract.
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(3) Characteristics
① Inverted image
The projection of all parts of body in this area is
inverted image, but the projection of head and
face is the right image. The cortex of the upper
1/3 part of precentral gyrus and the anterior part
of paracentral lobule control the muscles of
inferior limb. The middle 1/3 part of precentral
gyrus manipulates the muscles of upper limb.
The inferior 1/3 part of precentral gyrus
manipulates the muscles of head and face.
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② Crossed controlling
The first somatic motor area controls the
movement of contralateral limb.
③ Projection area in the motor center
The projection areas of all parts of the
body are related to the functions and
meticulous level of the corresponding
controlled region.
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2. First (Primary) somatic sensory area
(general sensory area)
(1) Location
It occupies the postcentral gyrus and the
posterior part of the paracentral lobule.
It consists of areas 3, 1 and 2 of Brodmann’ map.
(2) Function
Within this area are the terminations of the
sensory pathways that carry general sensory
information concerning temperature, touch,
pressure, pain, and proprioception from the body
to the cortex of the brain.
(3) Characteristics
 The projecting images in the primary
sensory area of all the parts of body are
similar to that in the primary somatic
motor area.
① Inverted image
 The projection of all parts of body in this
area is inverted image, but the projection
of head and face is the right image. The
inverted image of human body. The right
image of head and face.
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② Crossed controlling.
The contralateral half of the body is
represented as inverted.
③ Projection area
The size of the cortical area for a
particular part of the body is determined
by the functional importance of the part
and its need for sensitivity.
Thus the area for the face, especially the
lips, is disproportionately large, and a
large area is signed to the hand,
particularly the thumb and index fingers.
3. The visual area
 It surrounds the calcarine sulcus on the
medial surface of the occipital lobe,
corresponding to area 17 of Brodmann’s
map.
 This area contributes to the interpretation
of visual experience.
 The chief source of afferent fibers to area
17 area is the lateral geniculate nucleus of
thalamus by way of the geniculocalcarine
tract.
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4. The auditory area (acoustic area)
It is located in the transverse temporal
gyri corresponding to areas 41 and 42 of
Brodmann’s map.
This area contributes to the interpretation
of acoustic experience.
The medial geniculate nucleus of the
posterior thalamus is the principal source
of fibers ending in the acoustic cortex;
these fibers constitute the acoustic
radiation in the medullary center.
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5. Vestibular area
It is located in front of superior temporal
gyrus.
6. Olfactory area
In the medial part of uncus of
parahippocampal gyrus.
7. Gustatory area
8. Visceral activation area
In the limbic lobe.
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9. The language areas
Four language cortical areas of special
importance in language have been
demonstrated. They are situated in the
left hemisphere, with few exceptions, and
this hemisphere is therefore the dominant
hemisphere as a rule with respect to
language.
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1) The motor speech area (Broca’s area)
It occupies the posterior 1/3 portion of the
inferior frontal gyrus (to Brodmann’s map 44 and
45 ).
Motor aphasia, caused by a lesion in Broca’s area.
2) The writing area
The posterior portion of the middle frontal gyrus
(Brodmann’s map 8) is the written word area.
Written aphasia is caused by a lesion in the
written word area.
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3) The auditory speech area (sensory
language area)
This area occupies the posterior part of the
superior temporal gyrus (Brodmann’ map 22).
Sensory aphasia, in which comprehension of
language, naming of objects, and repetition of a
sentence spoken by the examiner are all
defective, is caused by a lesion in the sensory
language area, more specifically in sensory
language area.
4) The visual speech area (reading area)
It locates in the angular gyrus (39 area if
Brodmann).
Visual aphasia is caused by a lesion in the visual
speech area.
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Ⅲ) Internal Structures
Each cerebral hemisphere includes a large
volume of white matter constituting the
medullary center, the basal ganglia and
lateral ventricle.
1. The basal nuclei
Located deep within each cerebral
hemisphere are several masses of gray
matter known collectively as the basal
nuclei.
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① Caudate nucleus
It is a long, arching nucleus, remaining in
the wall of the lateral ventricle and grows
around with it in C-shaped course. It
consists of an anterior portion or head,
which tapers into a slender tail extending
backward and then forward into the
temporal lobe and terminates at the
amygdaloid nucleus.
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② Lentiform nucleus
It is subdivided into the putamen and the
globus pallidus;
The caudate nucleus and the lentiform
nucleus are sometimes referred to as the
corpus striatum.
The caudate nucleus and putamen are
new structures phylogenetically, which
have the similar connections, so both
together are called the neostriatum,
while the globus pallidus is referred to as
the paleostriatum.
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③ Claustrum
It is a thin layer of gray matter just deep to the
cortex of the insula; between the insula cortex
and the putamen of the lentiform nucleus, from
which it is separated by the fibers of the external
capsule.
④ Amygdaloid nucleus
It is located at the tip of the tail of the caudate
nucleus.
The band of white matter located between the
basal nuclei and the thalamus is called the
internal capsule.
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2. The medullary center
It composed of three kinds of fibers depending on
the nature of their connections.
1) The association fibers
They are confined to a hemisphere connecting
one cortical area with another.
Superior longitudinal fasciculus,
Inferior longitudinal fasciculus,
Cingulum.
Uncinate fasciculus.
Arcuate fibers.
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2) The commissural fibers
They connect the gyri in one hemisphere to the
gyri in the other hemisphere.
Corpus callosum:
From forward back, it can be divided into 4 parts:
the
rostrum,
genu,
body,
splenium.
Anterior and posterior commissures.
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3) The projection fibers
They are connections between the cortex and the
subcortical structures. The fibers fan out as the
corona radiata in the medullary center. The
projection fibers are concentrated in the internal
capsule.
The internal capsule
It is between the thalamus, caudate nucleus and
lentiform nucleus.
Constitution:
It consists of an anterior limb, a genu and a
posterior limb, which have topographic
relationships with adjacent gray messes.
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① Anterior limb
The anterior limb is bounded by the lenticular
nucleus and the head of the caudate nucleus.
Projection fibers:
Anterior thalamic radiation.
Frontopontine tract.
② Genu
The genu is medial to the apex of the lentiform
nucleus.
Projection fibers:
Corticonuclear tract.
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③ Posterior limb
The posterior limb is bounded by the lenticular
nucleus and the dorsal thalamus.
Projection fibers:
Central thalamic radiation (Thalamocortical
tract).
Corticospinal tract.
Corticorubral tract.
Parieto-occiptto-temporo-pontine tract
(corticopontine tract).
Optic radiation.
Acoustic radiation.
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A lesion on one side of the posterior limb
of the internal capsule may results in:
“three hemiplegia syndorm”.
a. the abnormalities of voluntary motion
of the contralateral trunk and limbs
b. the abnormalities of sensation of the
contralateral half body.
c. the blindness in contralateral halves of
both visual fields.
3. The lateral ventricles
 They are roughly C-shaped cavities lined
by ependymal epithelium one in each
cerebral hemisphere, and filled with
cerebrospinal fluid.
 Each lateral ventricle consists of a body in
the region of the parietal lobe from which
anterior, posterior and inferior horns
extend into the frontal, occipital and
temporal lobes, respectively.
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The lateral ventricles communicate with
the third ventricle through the
interventricular foramen.
Portions of lateral ventricle contain choroid
plexus, formed by an invagination of pia
matter covering layer of ependyma on the
medial surface of the cerebral hemisphere.
This plexus presents in the body and the
inferior horn of the lateral ventricle and is
the main structure for production of
cerebral spinal fluid (CSF).
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Ⅳ) The Limbic System
The limbic system is used to include the limbic
lobe as well as associated subcortical structures,
such as the amygdaloid complex,
hypothalamus, epithalamus, anterior thalamic
nuclei.
Functions:
The limbic system is concerned with:
① emotional and genesis, together with visceral
response to the emotions
② survival of individual and species
③ processes involved in memory
The limbic system is also known as the visceral
brain because of its substantial influence on
visceral functions through the autonomic nervous
system.