Individual cortical neurons innervating large proportions of the neocortical areas in the MouseLight database

The general notion of the extent of cortical areal interconnectivity in the neuroscience community can in part be due to results from traditional neuroanatomical studies, which later insights have shown to systematically represent underestimates of the extent of the axonal arborizations. But any underestimate of this interconnectivity could in turn be a factor in strengthening the notion of functional localization in the cortex, i.e. the idea that there are circumscribed cortical areas with specific functions that do not to a large extent depend on information being processed in other such cortical areas. Recent advances in neuroanatomical techniques have greatly improved the possibilities to follow the axonal projections of individual neurons in full, and many reconstructed neurons are made available by the MouseLight database. We sampled individual cortical neurons with the majority of their axons within the neocortex and explored the extent of their axonal connections. We found that the efferent axon of individual cortical neurons can commonly cover as much as 30-40% of all types of cortical areas, with a coverage of up to 80% for a single neuron also being demonstrated. Combining the distributions of the axonal trees of more than one cortical neuron, we found that as few as three neurons within one area could reach 100% of the other cortical areas.