Structural correlates of modular organization of signal transmission in the primate somatosensory cortex

Abstract

Introduction: Axonal connections of column size cortical regions exhibit patchy distribution all over the primate cerebral cortex. The axonal patches represent specific target sites as e.g. columns of similar orientation preference in the visual cortex. However, axonal connections also densely distribute outside of the patches without any apparent grouping. Instead, the out-patch axons exhibit a radial spreading from the origin towards probably distant target sites including the patches. Interestingly both patch and out-patch axons form axon terminal-like structures suggesting their role in synaptic transmission. Aim: The aim of our study is to know whether axons play a similar role in the propagation of activity and dissemination of information within and outside of the patches. To answer this question morphological properties of reconstructed axons within and outside of the patches were compared for intra- and inter-areal connections in the somatosensory cortex of squirrel monkeys. Methods: Distal finger pad representations [1] of six squirrel monkeys that were injected with BDA via iontophoresis in area 3b (3 cases) and area 1 (3 cases). Aldehyde fixation was made by transcardial perfusion after 10-20 days survival. Series of 50 μm thick horizontal, vibratome sections were collected at regular intervals from the flattened cortex. Standard ABC protocol was used to visualize BDA labelling with nickel intensified DAB as the chromogen. Sections were then osmicated and flat embedded in Durcupan. Intra and Inter areal axons [2] of areas 3b and 1 were studied. Axonal patches were mapped and outlined by using Neurolucida. Reconstruction of the BDA labelled axonal fibres was also made with Neurolucida at high magnification within and outside of the axonal patches. Tortuosity, Inter-varicosity distance, Thickness, Bouton convergence, and Density of varicosity thickness of the individual axonal segments within and out of the patches were compared. The abundance of axons bearing boutons with stalks were also studied within and outside of the patches. Summary: Based on data analysis our findings suggest that axons have similar tortuosity but different bouton density within and outside of the patches. Specifically, intrinsic connections exhibit higher bouton density within than outside of the patches. Moreover, we measure the Inter-varicosity distance a physical distance of an axon between two boutons which is significantly higher in out patch axons than the patch. Furthermore, the bouton convergence which is the distance from main axon bouton to neighbouring axon boutons is much higher within the patch than out patch. Finally, we measure the thickness of axonal processes within and out of the patches and we found that the out patch long-range axons are much thicker than in patch axons which may lead to faster signal transmission along the out-patch axons. Conclusion: The increased bouton density accompanied by extensive axonal convergence (Cris-cross) could result in a highly efficient way of signal transmission in terminal arborization patches of the cerebral cortex. In contrast, the long-range thick axons outside the patches could provide input to extra classical receptive field and form the structural correlate of cortical plasticity.

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