CEREBRAL CORTEX

CEREBRAL CORTEX
General appearance of the cerebral hemispheres:
- The cerebral hemisphe res are the largest part of the brain; they are
separated by a deep midline sagittal fissure, the longitudinal cerebral
fissure
- The fissure contains the sickle-shaped fold of dura mater, the falx cerebri,
and the ante rior cerebral arte ries.
- In the depths of the fissure, the great commissure, the corpus callosum,
connects the hemispheres across the midline
- A second horizontal fold of dura mater separates the cerebral
hemispheres from the cerebellum and is called the tentorium ce rebelli.
- To increase the surface area of the cerebral cortex, the surface is thrown
into folds (gyri), which are separated from each other by fissures (sulci)
- For descriptive purposes, each hemisphere is divided into lobes, which are
named according to the cranial bones unde r which they lie

- The central and parieto-occipital sulci and the lateral and calcarine sulci
are boundaries used for the division of the cerebral hemisphe re into
frontal, parietal, temporal, and occipital lobes
Surfaces of the hemispheres:
1-The lateral surface; is convex in adaptation to the concavity of the
corresponding half of the vault of the cranium
2- The medial surface; is flat and vertical, and is separated from that of the
opposite hemisphere by the great longitudinal fissure and the falx cerebri
3- The inferior surface; is of an irregular form, and may be divided into three
areas:
- The anterior area, formed by the orbital surface of the frontal lobe, is
concave, and rests on the roof of the orbit and nose
- The middle area is convex, and consists of the under surface of the
temporal lobe: it is adapted to the corresponding half of the middle
cranial fossa
- The posterior area is concave, directed medialward as well as downward,
and is named the tentorial surface
Main Sulci:
The central sulcus:
- Is of great importance because since it separates the two major cortical
areas (sensory & motor)
- The central sulcus indents the superior medial border of the hemisphe re
about 1 cm behind the midpoint
- It runs downward and forward across the lateral aspect of the
hemisphere, and its lower end is separated from the posterior ramus of
the lateral sulcus by a narrow bridge of cortex.
- The central sulcus is the only sulcus of any length on this surface of the
hemisphere that indents the superomedial border and lies between two
parallel gyri.
The lateral sulcus:
- Is a deep cleft found mainly on the inferior and lateral surfaces of the
cerebral hemisphere.
- It consists of a short stem that divides into three rami.
- The stem arises on the inferior surface, and on reaching the lateral
surface, it divides into the anterior, ascending and continues as the
posterior ramus
- An area of cortex called the insula lies at the bottom of the deep lateral
sulcus and cannot be seen from the surface unless the lips of the sulcus
are separated
The parieto-occipital sulcus:
- Begins on the superior medial margin of the hemisphere about 5 cm
anterior to the occipital pole
- It passes downward and anteriorly on the medial surface to meet the
calcarine sulcus
The calcarine sulcus:
- Is found on the medial surface of the hemisphere
- It commences unde r the posterior end of the corpus callosum and arches
upward and backward to reach the occipital pole, whe re it stops.

- In some brains, however, it continues for a short distance onto the late ral
surface of the hemisphere.
- The calcarine sulcus is joined at an acute angle by the parieto-occipital
sulcus about halfway along its length
Lobes & gyri of the cerebrum:
Superolateral Surface of the Hemisphere:
- The frontal lobe occupies the area anterior to the central sulcus and
superior to the lateral sulcus
- The superolateral surface of the frontal lobe is divided by three sulci into
four gyri.
- The precentral sulcus runs parallel to the central sulcus, and the
precentral gyrus lies between them
- Extending anteriorly from the precentral sulcus are the supe rior and
inferior frontal sulci
- These two sulci divide the remaining part of this lobe into superior,
middle & inferior frontal gyri
- The infe rior frontal gyrus is invaded by the anterior and ascending rami
of the lateral sulcus between them lie the pars triangularis.
- The parietal lobe occupies the area posterior to the central sulcus and
superior to the lateral sulcus
- It extends posteriorly as far as the parieto-occipital sulcus
- The lateral surface of the parietal lobe is divided by two sulci into three
gyri.
- The postcentral sulcus runs parallel to the central sulcus, and the
postcentral gyrus lies between them.
- Running posteriorly from the middle of the postcentral sulcus is the
intraparietal sulcus which divides the reminder of the lobe into superior
& inferiorparietal lobules
- The temporal lobe occupies the area infe rior to the lateral sulcus
- The lateral surface of the temporal lobe is divided into three gyri by two
sulci.
- The superior and middle temporal sulci run parallel to the posterior
ramus of the lateral sulcus and divide the temporal lobe into the superior,
middle, and inferior temporal gyri, the latter is continued onto the
inferior surface of the hemisphere
Medial and inferior surfaces of the hemisphere:
- There are many important areas that should be recognized:
- The corpus callosum, which is the largest commissure of the brain, forms
a striking feature on this surface
- The cingulate gyrus begins beneath the anterior end of the corpus
callosum and continues above it posteriorly
- The gyrus is separated from the corpus callosum by the callosal sulcus &
from the superior frontal gyrus by the cingulate sulcus
- The paracentral lobule is the area of the cerebral cortex that surrounds
the indentation produced by the central sulcus on the superior border
- The anterior part of this lobule is a continuation of the precentral gyrus
on the superior lateral surface, & the posterior part is a continuation of
the postcentral gyrus.
- The precuneus: is an area of cortex bounded anteriorly by the posterior
end of the cingulate & by the parieto-occipital sulci.

- The cuneus: is a triangular area of cortex bounded above by the parietooccipital
sulcus, infe riorly by the calcarine sulcus
- The collateral sulcus is situated on the infe rior surface of the hemisphe re
& runs anteriorly below the calcarine sulcus.
- Between the collate ral sulcus and the calcarine sulcus is the lingual gyrus.
- Anterior to the lingual gyrus is the parahippocampal gyrus which
terminates in front as the hooklike uncus.
- The medial occipitotemporal gyrus extends from the occipital pole to the
temporal pole, it is bounded medially by the collateral and rhinal sulci
and laterally by the occipitotemporal sulcus.
- The occipitotemporal gyrus lies lateral to the sulcus and is continuous
with the inferior temporal gyrus
- On the infe rior surface of the frontal lobe, the olfactory bulb and tract
overlie the olfactory sulcus
- Medial to the olfactory sulcus is the gyrus rectus & lateral to it are a
number of orbital gyri
Structure of the cerebral cortex:
- The cerebral cortex forms a complete covering of the cerebral
hemisphere.
- It is composed of gray matter and has been estimated to contain
approximately 10 billion neurons.
- Some neurones synapse with more than ten thousand neurones, so the
estimated number of synapses in the cerebral cortex is 60 trillion
- The thickness of the cortex varies from 1.5 to 4.5 mm. being thickest over
the crest of a gyrus and thinnest in the depth of a sulcus.
- The cerebral cortex, like gray matter elsewhere in the central nervous
system, consists of a mixture of nerve cells, nerve fibers, neuroglia, and
blood vessels.
- The following types of nerve cells are present in the cerebral cortex:
(1) pyramidal cells, (2) stellate cells, (3) fusiform cells, (4) horizontal cells of
Cajal, and (5) cells of Martinotti
Layers of the cerebral cortex:
1- The molecular (plexiform layer):
- This is the most superficial layer; it consists mainly of a dense network of
tangentially oriented nerve fibers
- These fibers are derived from the apical dendrites of the pyramidal cells
and fusiform cells, the axons of the stellate cells, and the cells of
Martinotti.
- Afferent fibe rs originating in the thalamus and in association with
commissural fibers also are present.
- Scattered among these nerve fibers are occasional horizontal cells of
Cajal. This most superficial layer of the cortex clearly is where large
numbers of synapses between diffe rent neurons occur.
2- The external granular layer:
- This layer contains large numbe rs of small pyramidal cells and stellate
cells
- The dendrites of these cells terminate in the molecular layer, and the
axons enter deeper layers, where they terminate or pass on to enter the
white matte r of the cerebral hemisphere.
3- The external pyramidal layer:

This layer is composed of pyramidal cells, whose cell body size increases
from the superficial to the deeper borders of the layer
- The apical dendrites pass into the molecular layer, and the axons enter
the white matter as projection, association, or commissural fibers.
4- The internal granular layer:
This layer is composed of closely packed stellate cells
There is a high concentration of horizontally arranged fibers known collectively
as the external band of Baillarger.
5- The ganglionic layer (internal pyramidal layer):
- This layer contains very large and medium-size pyramidal cells
- Scattered among the pyramidal cells are stellate cells and cells of
Martinotti.
- In addition, the re are a large numbe r of horizontally arranged fibers that
form the inne r band of Baillarger
- In the motor cortex of the precentral gyrus, the pyramidal cells of this
layer are very large and are known as Betz cells.
- These cells account for about 3% of the projection fibers of the
corticospinal or pyramidal tract.
6- The multiform layer (polymorphic cells layer):
- Although the majority of the cells are fusiform, many of the cells are
modified pyramidal cells, whose cell bodies are triangular or ovoid
- The cells of Martinotti also are conspicuous in this layer.
- Many nerve fibers are present that are entering or are leaving the
underlying white matter