The Cerebral Cortex

The limbic system | Main Anatomy Index | Localisation of function of 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.

• 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.

• 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 neocortex.

• 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 surface.
• 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.

• 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.

• 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.

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• 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 longer obvious.
• 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 cells.
• 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 cortex.
• The intermediate types of cortex in which the 6-layered structure can be seen is called homotypical cortex.

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• Afferents from other cortical sites may arise in:

1. The same hemisphere (association fibres);
2. Or in the contralateral hemisphere (commissural fibres).

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• 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:

1. There are very short ones that never leave the cortex;
2. Or U-shaped fibres that dip under one sulcus to reach the next gyrus;
3. 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:

1. The hand area of the somatosensory cortex;
2. 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 temporal gyri.

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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 their course.

• The most prominent of these bundles are:

1. The superior longitudinal fasciculus;
2. The superior and inferior occipitofrontal fasciculi;
3. And the cingulum.

• This is also known as the arcuate fasciculus.

• It sweeps along in a great arc above the insula.
• It travels from the frontal lobe to the posterior portions of the hemisphere where it fans out among the parietal, occipital, and temporal lobes.

• 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.

• 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, hook).

• This courses within the cingulate gyrus.
• It also continues around the parahippocampal gyrus and to nearly complete a circle.

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