Neocortical pyramidal neurons with axons emerging from dendrites are frequent in non-primates, but rare in monkey and human

The canonical view of neuronal function is that inputs are received by dendrites and somata, become integrated in the somatodendritic compartment and upon reaching a sufficient threshold, generate axonal output with axons emerging from the cell body. The latter is not necessarily the case. Instead, axons may originate from dendrites. The terms "axon carrying dendrite" (AcD) and "AcD neurons" have been coined to describe this feature. Here, we report on the diversity of axon origins in neocortical pyramidal cells. We found that in non-primates (rodent, cat, ferret, pig), 10-21% of pyramidal cells of layers II-VI had an AcD. In marked contrast, in macaque and human, this proportion was lower, and it was particularly low for supragranular neurons. Unexpectedly, pyramidal cells in the white matter of postnatal cat and aged human cortex exhibit AcDs to much higher percentages. In rodent hippocampus, AcD cells are functionally privileged, since inputs here can circumvent somatic integration and lead to immediate action potential initiation in the axon. Our findings expand the current knowledge regarding the distribution and proportion of AcD cells in neocortial regions of non-primate taxa, which strikingly differs from primates where these cells are mainly found in deeper layers and white matter.