There are differences in the neuronal architecture of primates and non-primates.
A multinational research team has now been able to increase the understanding of species-specific variations in the architecture of cortical neurons thanks to high-resolution microscopy.
Researchers from the Developmental Neurobiology research group at Ruhr-Universität Bochum, led by Professor Petra Wahle, have demonstrated that primates and non-primates differ in an important aspect of their architecture: the origin of the axon, which is the process responsible for transmission . of electrical signals known as action potentials. The results were recently published in the journal eLife.
Axons may arise from dendrites
Until now, it was considered textbook knowledge that axons always, with few exceptions, arise from the cell body of a neuron.
However, it can also come from dendrites, which serve to collect and integrate incoming synaptic signals. This phenomenon has been called “axon-bearing dendrites”.
Different mammalian species and high-resolution microscopy reveal variable axonal origins
“A unique aspect of the project is that the team worked with archived tissue and slide preparations, which included material that has been used for years to teach students,” explains Petra Wahle.
A variety of animals were also studied, including rodents (mouse, rat), ungulates (pig), carnivores (cat, ferret), and zoological primates macaques and humans. The scientists concluded that there is a species difference between non-primates and primates by using five distinct staining techniques and evaluating more than 34,000 neurons.
There are significantly fewer axon-bearing dendrites in excitatory pyramidal neurons in external layers II and III of the primate cerebral cortex than in excitatory pyramidal neurons in non-primates. Furthermore, for inhibitory interneurons, significant differences in the percentage of axon-bearing dendritic cells were detected between cat and human species.
No quantitative differences were observed when comparing macaque cortical areas with primary sensory and higher brain functions. High-resolution microscopy was of particular importance, as Petra Wahle describes: “This allowed the detection of axonal origins precisely traced at the micrometer level, which is sometimes not so easy with conventional light microscopy.”
Evolutionary advantage still enigmatic
Little is known about the function of axon-bearing dendrites. Typically, a neuron integrates excitatory inputs arriving at dendrites with inhibitory inputs, a process called somatodendritic integration. The neuron then decides whether the inputs are strong and important enough to be transmitted via action potentials to other neurons and brain areas.
Axon-bearing dendrites are considered privileged because the depolarizing inputs of these dendrites are able to evoke action potentials directly without the involvement of somatic integration and somatic inhibition. Why this species difference has evolved, and the potential advantage it may have for neocortical information processing in primates, is still unknown.
Reference: “Neocortical pyramidal neurons with axons emerging from dendrites are frequent in non-primates but rare in monkeys and humans” by Petra Wahle, Eric Sobierajski, Ina Gasterstädt, Nadja Lehmann, Susanna Weber, Joachim HR Lübke, Marenauharst, and Gundela Meyer, April 20, 2022, eLife.
The study was funded by the German Research Foundation.