MODULAR ORGANIZATION OF SIGNAL TRANSMISSION IN THE PRIMATE SOMATOSENSORY CORTEX
In primate cortex, axonal connections appear both as long-distance fibers and as terminal arborization patches. In the visual cortex, axonal patches represent specific target sites, such as columns of similar orientation preference. Both patch and out-patch axons form axon terminal-like structures suggesting their role in synaptic transmission. However, it is not known whether axons play a similar role in the propagation of activity and dissemination of information within and outside the patches. To answer this question we compared morphological properties of reconstructed axons responsible for the speed of signal transduction and convergence within and outside the patches for intra, inter-areal, feedforward and feedback connections in the somatosensory cortex of squirrel monkeys. Bouton convergence, the distance of a bouton from the nearest boutons of neighboring axons was higher within than outside patches permitting an increased probability of convergence to a postsynaptic site. Additionally, axonal convergence determined by the diversity of orientation of the axon segments was also higher in patches as opposed to the more similar orientation outside the patches further increasing the possibility of convergent input to a postsynaptic structure in patches. However, thicker average axon found outside patches allow higher speed of signal propagation than thinner average axons within patch. These findings were consistent across intra- and inter-areal connections. Our results provide evidence that terminal arborizations are sites of synaptic convergence whereas long-range axons are more involved in quick propagation of information in the cerebral cortex. Supported by NIH NS093998 and OTKA NN118902.