The research team studied a variety of animals, including rodents (mice, rats), ungulates (pigs), carnivores (cats, ferrets), as well as zoological primates, macaques and humans. Using five different staining techniques and an assessment of more than 34,000 neurons, the scientists concluded that species differences exist between non-primates and primates.
There are significantly fewer axon-carrying dendrites on excitatory pyramidal neurons in outer layers II and III of the primate cerebral cortex compared to non-primate excitatory pyramidal neurons. Furthermore, for inhibitory interneurons, large differences in the percentage of axon-carrying dendritic cells were found between cat and human species. No quantitative differences were observed when comparing macaque cortical regions with primary sensory and higher brain functions.
High-resolution microscopy is particularly important in research, the researchers say, allowing detection of axonal sources to be tracked accurately at the micrometer scale, which is sometimes not so easy with conventional microscopes. Typically, a neuron integrates excitatory and inhibitory inputs reaching its dendrites, a process known as somatic integration. The neuron then decides whether the input is strong and important enough to be transmitted to other neurons and brain regions via action potentials.
Axon-carrying dendrites are considered privileged because depolarizing input from these dendrites is capable of directly evoking action potentials without involvement in somatic integration and somatic inhibition. Why this species difference evolved, and the potential advantages it might have over primate neocortical information processing, remain unclear.