The cortex appears curiously homogeneous given the extensive variety of its functionally diverse areas. Mapping the cortex and distinguishing between the functionally different areas has always been a challenge because of the large size and convolution of the cortical mantle and the subtlety of cyto- and mye-loarchitectonic differences between areas. Che-moarchitecture, on the other hand, is a reliable tool that can provide distinct characteristics distinguishing one cortical area from another on the basis of neuroactive content, which in itself is functional evidence. It must be acknowledged, however, that chemoarchitectonic boundaries between areas of the cortex are far less obvious than those in the subcortex.
Figure 9 Photograph of an autoradiograph of a coronal section through the visual cortex showing the distribution of 125I-salmon calcitonin binding. Large arrows indicate precise boundaries between V1 and V2. (B) A segment of the autoradiograph at higher magnification demonstrates 200-|mm periodicity of binding (small arrows) that may be related to ocular dominance columns or to cytochrome oxidase blobs, which are characteristic of V1. Note how the binding is confined to lamina 5 of the V1.
Therefore, in anatomical studies of the cortex che-moarchitecture is used as an aid to cytoarchitectonic guidelines rather than as a primary delineation criterion. Chemical differences, however subtle, between neuronal populations in different cortical areas and cortical layers are never the less pivotal criteria for understanding the structural organization of the human cortex.
Chemoarchitectonic boundaries often correspond to the boundaries of cortical areas as identified by functional studies. For example, the boundaries of the primary visual cortex (area V1) are apparent by the prominent acetylcholinesterase reactivity that abruptly stops on the border between V1 and V2. Such chemical characteristics bring credibility to the otherwise ambiguous boundaries between these cortical areas. Apart from differentiating areas of the cortex, chemoarchitecture also distinguishes between neuro-
nal populations in different cortical layers. For example, calcitonin receptor binding is abundant and exclusive for V1, but within that area it is confined to neurons of layer 5, which further characterizes these cells (Fig. 9).
As in the subcortex, the best results for structural differentiation of neuronal groups in the cortex are achieved through combining different chemical markers. Strong SMI32 immunoreactivity reveals a distinct population of large pyramidal cells in lower layer III and in layer V, calbindin immunoreactivity distinguishes pyramidal neurons in layers II and III, and immunoreactivity for the glutamate synthesizing enzyme glutaminase is characteristic of pyramidal cells in layers II, III, V, and VI. The combined use of histochemical markers for NADPH-d and the immu-nohistochemical technique for visualizing NPY revealed two populations of neurons that contain both markers. This finding prompted the distinction of these groups of nonpyramidal, peptidergic neurons in layers V and VI of the human cerebral cortex.
There are reports of several populations of nonpyramidal neurons containing peptides and characterized by specific morphology and laminar distribution. Chemoarchitectonic characteristics are of assistance in classifying these neuronal groups. For example, immunoreactivity for TH characterizes a population ofspindle-shaped neurons found to be numerous in the infragranular layers of the cortex. Cholecystokinin-immunoreactive neurons characterize a population of bipolar cells in the subgranular layers, whereas coex-pression of NPY and substance P characterizes another prominent population of multipolar neurons in the deep layers of cortex and in the white matter. It should be reiterated, however, that although che-moarchitecture is informative for differentiation of some areas of the cortex, the difference is generally less striking than in the subcortex.
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