The Cerebral Cortex
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The limbic system | Main Anatomy Index | Localisation of function of the cerebral cortex
Last updated 30 March 2006
The Cerebral Cortex
This is a sheet of neurons and their interconnections.
It plates the corrugated surface of the cerebral hemispheres
in a layer that is only a few millimetres thick.
This thin layer of grey matter is estimated to contain 30 billion neurons.
Histology of the Cerebral Cortex
- > 90% our total cortical area.
- 6-layered structure.
- Referred to as homogentic cortex.
- Covers some restricted parts of the base of the telencephalon
- Along with the archicortex, it is refereed to as heterogenic cortex.
- It does not have a 6-layered structure.
There are 2 principal cell types in the neocortex; the stellate (granule)
cells and the pyramidal cells.
Other neocortical cell types include: horizontal cells
(of Cajal), fusiform cells, and cells of Martinotti.
Stellate (Granule) Cells
These come in a wide assortment of shapes.
They are typically small (< 10 micrometres) multipolar neurons.
Their short axons do not leave the cortex.
- Stellate cells are the principal interneurons of the
These cells are shaped as they are named.
- Pyramidal cells range in size from 10 micrometres in
diameter to 70-100 micrometres of the giant
pyramidal cells (Betz cells) of the motor cortex.
- A long apical dendrite leaves the top
of each pyramidal cell and ascends vertically to the cortical
- A series of basal dendrites emerges from nearer the base of the cell and spreads out horizontally.
- The apical dendrites of pyramidal cells are studded
with dendritic spines.
- These are numerous small projections that are the preferential site of synaptic contact.
- It has been suggested that dendritic spines may be the sites of synapses that are
selectively modified as a result of learning.
- Most or all pyramidal ells have long axons that leave the cortex to reach either other
cortical areas or to various subcortical sites.
- Therefore, pyramidal cells are the principal output neurons.
Horizontal Cells (of Cajal)
These ramify within the most superficial cortical layer.
These are prominent during development but most disappear after birth.
These are found in the deepest cortical layer.
They are spindle-shaped with a tuft
of dendrites emerging from each end of the spindle.
They are, however, otherwise like pyramidal cells with an
axon that leaves the cortex.
Cells of Martinotti
These cells are found in all cortical layers but are
more abundant in the deeper cortical layers.
They are unusual in the fact that they have an axon
that ascends to the surface.
||External granular layer
||External pyramidal layer
||Internal granular layer
||Internal pyramidal layer
||Multiform (polymorphic layer)
- The cells of the neocortex are arranged as a 6-layered structure
though these layers are more prominent in some areas that others.
- The most superficial is the cell-poor molecular layer and the deepest
is the multiform (polymorphic) layer,
which is populated largely by fusiform cells.
- Between these two layers are 4 layers that are
alternatively mostly populated by stellate or pyramidal cells.
- The six neocortical layers are not equally prominent everywhere.
- In the motor cortex, layers II-V are dominated by large pyramidal cells to the extent that individual layers are no
- Because of the apparent lack of stellate cells, this type of cortex is known as agranular.
- In contrast, the primary
sensory cortex projects mainly to cortical regions
and does not give rise to many long axons.
- Layers II-V are thus dominated by small
stellate cells and pyramidal
- This type of cortex is thus known as granular cortex or
koniocortex (G. konia, dust).
- The agranular and granular
types of cortex are collectively called heterotypical.
- There is a continuum of structure types ranging between the thick (4.5 mm) agranular cortex to the think (1.5 mm) granular
- The intermediate types of cortex in which the 6-layered structure can be seen is called homotypical cortex.
- Afferents from other cortical sites may arise in:
- The same hemisphere (association
- Or in the contralateral hemisphere (commissural fibres).
Most efferents to the cortex
of the contralateral hemisphere pass through the corpus callosum.
Those interconnecting parts of the temporal
lobes traverse the anterior commissure.
- The efferents to the ipsilateral
cortical areas come in all lengths:
- There are very short ones that never
leave the cortex;
- Or U-shaped fibres that dip under
one sulcus to reach the next gyrus;
- And longer association
fibres that travel to a different lobe.
This is the largest fibre bundle in the human brain.
It contains more than 300 million axons.
- Most of these fibres interconnect mirror-image sites,
but a substantial number end in different areas from
those in which they arise.
- Nearly all cortical areas receive commissural fibres.
- There are a few notable exceptions to this including:
- The hand area of the somatosensory cortex;
- And all of area 17 not representing areas adjacent to
the vertical midline.
The commissural fibres to and from much of the temporal lobe pass through this,
especially those of the middle and inferior
Click here for a diagram of the association fibres.
Click here for a diagram of the association
fibres in a coronal section.
- The longer association fibres collection into well-defined bundles
that can be found in gross dissections.
- None of the association bundles are discrete, point-to-point pathways but rather fibres enter and leave them all along
- The most prominent of these bundles are:
- The superior longitudinal fasciculus;
- The superior and inferior occipitofrontal fasciculi;
- And the cingulum.
Superior Longitudinal Fasciculus
This is also known as the arcuate fasciculus.
Superior Occipitofrontal Fasciculus
It runs between the frontal and occipital lobes parallel to the corpus callosum and the caudate nucleus.
It is thus also known as the subcallosal bundle.
Inferior Occipitofrontal Fasciculus
This passes below the insula.
It is from the frontal lobe, passes through the temporal
lobe, and back to the occipital lobe.
Its fibres fan out at both ends
of the fasciculus.
Those fibres that hook around the lateral
sulcus to interconnect the orbital cortex and
the anterior temporal cortex are often considered
separately as the uncinate fasciculus (L. uncus,
This courses within the cingulate gyrus.
It also continues around the parahippocampal gyrus and
to nearly complete a circle.