Course Handout - Microanatomy of the
Cerebral Cortex
Copyright Notice: This material was
written and published in Wales by Derek J. Smith (Chartered Engineer). It forms
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First published online 14:12 GMT 19th February 2003,
Copyright Derek J. Smith (Chartered Engineer). This version [2.0 - copyright] 09:00 BST 4th July
2018.
An earlier version
of this material appeared in Smith (1997c; Chapter 2). It is repeated here with
minor amendments and supported with hyperlinks.
Microanatomy of
the Cerebral Cortex
The layered nature of the cerebral cortex was first noted by Gennari in 1776 and Baillarger in 1840, and a numbering system based upon this layering was developed by Korbinian Brodmann in 1909 [timeline]. He successfully departed from the traditional "naked eye" description of the brain, and turned instead to its "microanatomy" as a basis for cortical mapping. Under the microscope, the cerebral cortex contains six distinct layers (or "laminae"). These are numbered I to VI from the outside in as shown in Figure 1, and exist in this form because this is how the neurons migrated during neurogenesis. The apical dendrites of the pyramidal cells from the lower layers pass vertically upwards towards the surface, and in so doing tend to "organise" the smaller local cells into "columns". It is likely that these are major building blocks of neural computation. Nevertheless, the relative predominance of each layer is different in different areas of the cortex, as shown in Figure 2, apparently reflecting the different nature of the neural processing required. Brodmann allocated numbers - Brodmann's numbers - to these different areas, and his "cytoarchitectonic" numbering scheme is still in use today. The most important Brodmann's numbers are shown in Kleist (1934).
Another worker, Constantin Von Economo (1876-1931), identified over 100 discrete cortical fields, regarding these as very precisely delineated one from another by their microstructure. Others have argued for fewer fields, shading gradually into each other. Kaas (1987) reviews the arguments here and finds some very sharp boundaries (for example, that between Areas 17 and 18), as well as some less so. It is also worth noting that neocortex - all other things being equal - is thinner at the base of a sulcus than it is at the crowns of the gyri to either side. However the loss is not borne equally by all the layers, but by Layers V and VI selectively. Layers I and II are actually thicker at the base of a sulcus than at the crown. Von Economo (1929:20) has estimated the layer-on-layer percentages, and quotes 9-7-33-9-20-22 at the crown, and 25-15-30-12-10-8 at the base. Layer I in this instance is therefore nearly three times thicker at the base, and Layer VI nearly two thirds thinner!
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Figure 1 - Brodmann's Six Cortical Layers: Here is a schematic microscopic view of the
microstructure of the cerebral cortex. Note the six basic layers, but note
also how the relative proportion of each varies from one type of cortex to
another. Layer I is known as the plexiform (or molecular)
layer and contains mainly a densely matted network made up of dendrites from
the pyramidal cells in the lower layers. This network runs horizontally, thus
allowing information to flow from point to point across a given area
of cortex. Layer II is known as the external granular layer and
contains stellate cells and a large number of small pyramidal cells. Many of
the apical dendrites of the large pyramidal cells in Layer V (see below)
synapse here. Layer III is known as the external pyramidal layer
and contains stellate cells and both large and medium-sized pyramidal cells. Layer
IV is known as the internal granular layer, and contains primarily
stellate cells. Where this layer is particularly thick, then the resulting
cortex is known as granular cortex (or koniocortex) (see Figure
2). The primary auditory and visual areas are composed of granular cortex.
Where this layer is particularly devoid of stellate cells, then the resulting
cortex is known as agranular cortex.
The primary motor area and much of the upper frontal lobe are composed of agranular cortex. As with Layer I, there is a
concentration of horizontally arranged fibres, often referred to as the external
band of Baillarger. Layer V is known as
the internal pyramidal layer, and is rich in pyramidal cells. In the
primary motor area, these are exceptionally large and go by the name giant
cells of Betz. These Betz cells are important because their axons descend
to become the corticobulbar and corticospinal tracts. Their apical dendrites
are much shorter, however, ascending to Layers I and II before travelling off
horizontally in all directions. Layer V contains yet another horizontal
network of fibres, often referred to as the internal band of Baillarger. Layer VI is known as the fusiform
layer and contains a range of cell types, including callosal
neurons, namely those whose axons project into a commissural tract. If this diagram fails to load automatically,
it may be accessed separately at |
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Enhanced from a black and white original in Smith (1997; Figure 2.3). After Von Economo (1929:16), but with horizontal dendrite systems and vertical axon "columns" added. This version Copyright © 2003, Derek J. Smith. |
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Figure 2 -
Granular vs Agranular Cortex: Here are the lateral and medial aspects of a
typical left cerebral hemisphere, showing the broad distribution of the
various types of cortex. Note how widespread the various types of granular cortex
(types 2 - 5) are compared to the agranular type
(type 1). If this diagram fails to load automatically,
it may be accessed separately at |
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Enhanced from a black and white original in Smith (1997; Figure 2.4). After Von Economo (1929:18). This version Copyright © 2003, Derek J. Smith. |
References
Kaas, J.H. (1987). The organisation of neocortex in
mammals: Implications for theories of brain function. Annual Review of
Psychology, 38:129-151.
Smith, D.J.
(1997). Neuroanatomy for Students of Communication.
Cardiff: UWIC. [ISBN: 190066609X - out of print]
Von Economo, C. (1929). The Cytoarchitectonics of the Human Cerebral Cortex.
London: Oxford University Press. [Translated from the 1925
German original by S. Parker.]
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