Vertical and Tangential Organization in the Neocortex

Abstract

The neocortex is defined cytoarchitectonically as consisting of six laminae which can be distinguished by the presence of certain types of neurons and by their packing density. These laminae are arranged in parallel to the cortical surface (Fig.1, middle). The neocortex is also characterized by its pattern of fibre connections which can roughly be recognized from its fibre- and myeloarchitectonics (Fig.1, right), showing a number of tangentially arranged fibre bundles, which mainly serve intracortical connections, as well as radially oriented bundles, which represent input-output pathways. In layers II and IV predominate small granule cells, in layers III and V, pyramidal cells, and in layer VI, multiform cells, whereas layer I is cell sparse and consists mainly of a tangential fibre plexus. Through the neocortex is defined by these parameters, it shows a considerable variation with respect to cell types and cell density throughout its surface. Thus, Brodmann [2] could distinguish more than 50 different areas in the human neocortex, each one being defined by the uniformity of its cytoarchitectonic appearance. By comparing different cortical areas, five cytoarchitectonically different types of neocortex could be distinguished, three of which clearly show all neocortical laminae and are, therefore, called homotypical (frontal, parietal and polar cortex in Fig.2). In two of these types not every layer can be distinguished clearly. They are therefore called heterotypical (agranular and granular in Fig.2, von Economo et al. [3]).