Synaptic organization of cortico-cortical communication in primates

In cortical circuitry synaptic communication across areas is based on two types of axon terminals, small and large, with modulatory and driving roles, respectively. In contrast, it is not known whether similar synaptic specializations exist for intra-areal projections. Using anterograde tracing and three-dimensional reconstruction by electron microscopy (3D-EM) we asked if large boutons form synapses in the circuit of somatosensory cortical areas 3b and 1. In contrast to observations in macaque visual cortex, light microscopy showed both small and large boutons not only in inter-areal pathways, but also in long-distance intrinsic connections. 3D-EM showed that correlation of surface and volume provides a powerful tool for classifying cortical endings. Principal component analysis supported this observation and highlighted the significance of the size of mitochondria as a distinguishing feature of bouton type. The larger mitochondrion and higher degree of perforated postsynaptic density associated with large rather than to small boutons support the driver-like function of large boutons. In contrast to bouton size and complexity, the size of the postsynaptic density appeared invariant across the bouton types. Comparative studies in human supported that size is a major distinguishing factor of bouton type in the cerebral cortex. In conclusion, the driver-like function of the large endings could facilitate fast dissemination of tactile information within the intrinsic and inter-areal circuitry of areas 3b and 1.Row data of a few boutons are missing in the dataset. The reason is those tracing work made in the past at my previous institution in my previouis computer, which was broken in the last year.The dataset contains two types of data: txt files for the light microscopic (LM) maps made by Neurolucida (e.g. Area1 ij_infra_M: injection was made into area 1 and the dataset contains only the infragranular layers). "Areal and granular distribution of large boutons" excel file includes the distribution of large boutons according to LM observations. The other main part of the dataset is electronmicroscopic data (EM). The txt files (e.g. VB3_A1_p3, ID of an individual bouton) were created by Reconstruct software and they contain tracing information by boutons with full series separately. "Bouton.data_full.series.selected" excel file contains all of the ultrastructural futures, which was extracted from Reconstruct and derived and measured data used for our analyses