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Abstract

In the cerebral cortex two main types of anatomical parcellation have been proposed: cyto- and myeloarchitectonics. Cytoarchitectonics is based on differences in the distribution and the sizes of cell bodies. Myeloarchitectonics relies on the layering and packing density of intracortical myelinated fibres. Thereby attention is mainly focused on those horizontal fibres which are organized in bands known as the stripes of Baillarger. Cyto- and myeloarchitectonics must be somehow related: structural changes from area to area as revealed by the Nissl stain coincide with changes in the myelin picture. In this paper, it is demonstrated that two simple assumptions are sufficient to transform quantitative data on the Nissl picture of a certain area (i.e. layer thicknesses, neuron sizes, neuron densities) into the corresponding myelin picture. The first assumption is that large neurons contribute more to the intracortical myelin content than small ones, and that this relation can be represented by a sigmoid curve. The second assumption is that the average distribution of horizontal axon collaterals of pyramidal neurons with respect to the cell body can be quantified by a simple model. On the basis of these two assumptions, myelin patterns were computed for the whole spectrum of cortical variability, including the primary visual, somatosensory, auditory and motor cortices, the speech centres and a number of association areas. Comparison of the simulations with real myelin preparations revealed remarkable similarities. These findings support the assumption that the horizontal component of the myelin picture is mainly produced by axon collaterals of pyramidal cells, and that nonpyramidal neurons and afferent fibres play a minor role. Moreover, the results suggest that the distribution of horizontal axon collaterals of pyramidal neurons and the principles of their myelination are very similar in different areas. The function of intracortical myelin is discussed. The increase in conduction velocity gained by myelination seems negligible for most intracortical fibres. It is argued that myelination may be related to learning processes during the critical period.