Course Handout - Microanatomy of the Cerebral Cortex

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First published online 14:12 GMT 19th February 2003, Copyright Derek J. Smith (Chartered Engineer). This version [HT.1 - transfer of copyright] dated 18:00 14th January 2010

 

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!

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.

PICvoneconomo1929.gif

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.

 

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).

PICvoneconomo1929b.gif

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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