How Axons Relate To Cortical Layers And Columns

Two key elements of cortical architecture are layers and modules ("columns," "assemblages," or cell groups). Both layers and columns are complicated structures. Layers can be distinguished on the basis of cell sizes and density, and in many cases enzyme concentrations and other markers also show orderly laminar patterns.

As described previously, feedforward systems terminate heavily at the level of layer 4, and feedback systems preferentially target layer 1. Intrinsic collaterals of pyramidal neurons tend to arborize mostly in layers 3 and 5. It is not clear, however, whether laminar specificity implies, as it does in the hippocampal formation, dendritic stratification of inputs. Available evidence suggests, to the contrary, that extrinsic axons

Figure 11 Photomicrographs of axons labeled by BDA anterogradely transported from an injection site. [A and (higher magnification from arrow) B] A field of axons crossing in the corpus callosum. The histological sectioning results in axon fragments that can be identified and traced in sequential sections. Note the range of different fiber diameters. [C and (higher magnification from arrow) D] Field of corticocortical axons subjacent to their target region. Branched profile is evident at the arrows.

Figure 11 Photomicrographs of axons labeled by BDA anterogradely transported from an injection site. [A and (higher magnification from arrow) B] A field of axons crossing in the corpus callosum. The histological sectioning results in axon fragments that can be identified and traced in sequential sections. Note the range of different fiber diameters. [C and (higher magnification from arrow) D] Field of corticocortical axons subjacent to their target region. Branched profile is evident at the arrows.

synapse on many of the structures within their terminal domain.

The relationship between axon systems and cortical columns is likewise complex. For example, in visual cortex geniculocortical axons derived from the left or right eye segregate into alternating ocular dominance columns. These constitute the primary anatomical substrate for the functional columns that can be demonstrated by microelectrode recordings or by activity-driven imaging (with the metabolic tracer

2-deoxyglucose in animals or position emission tomography or functional magnetic resonance imaging in humans). It is important to remember, however, that the functional columns, that are visualized as extending from pia to white matter through the cortical depth, are actually a combination of direct thalamo-cortical connections to layer 4 and local, short interlaminar relays from layer 4 to the upper and lower layers.

In nonprimary cortices, the anatomical substrates of columnar architecture are still under investigation. Apparently different from the primary areas, in the association areas many connectional systems directly terminate in an extended column involving several layers. These projection columns, which can range from 0.2 to 0.5 mm in diameter, result from the convergence of hundreds (thousands?) of axons. The individual arbors vary in size and laminar distribution but are usually smaller in diameter than the parent column.

Thus, although cortical architecture is commonly compared to a brickwork of repetitive modules (''icecube'' model of the cortex), surprisingly few data are available about columnar microorganization, and the actual structure may be considerably more elaborate.

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