Primates and non-primates differ in the architecture of their neurons — ScienceDaily
High-resolution microscopy has now enabled an international research team to expand knowledge of species-specific differences in cortical neuron architecture.
Researchers from the Developmental Neurobiology research group at Ruhr-Universität Bochum around Professor Petra Wahle, in collaboration with partners from Mannheim and Jülich, Germany, and Linz, Austria, and La Laguna, Spain, have shown that primates and non-primates differ in one important aspect of their architecture: the origin of the axon which is the process responsible for transmitting electrical signals called action potentials. The results are published on April 20, 2022 in the Journal eLife.
Axons can emerge from dendrites
Until now, it was taken as theoretical knowledge that the axon always originates, with few exceptions, from the cell body of a neuron. However, it can also originate from dendrites, which serve to collect and integrate incoming synaptic signals. This phenomenon has been called “axon-bearing dendrites”.
Various mammalian species and high-resolution microscopy reveal variable axonal origin
“A unique aspect of the project is that the team worked with archived tissue and slide preparations, which included material that had been used for years to teach students,” explains Petra Wahle. In addition, a range of species have been studied, including rodents (mouse, rat), ungulates (pig), carnivores (cat, ferret), macaques and human primates of the zoological order. Using five different staining methods and evaluating more than 34,000 neurons led the group to conclude that there is a species difference between non-primates and primates. Excitatory pyramidal neurons especially from outer layers II and III of the primate cerebral cortex have significantly fewer axon-bearing dendrites than non-primate pyramidal neurons. Furthermore, quantitative differences in the proportion of axon-bearing dendritic cells were found within cat and human species for inhibitory interneurons. No quantitative differences were observed when comparing the cortical areas of macaques with the primary and higher sensory functions of the brain. High-resolution microscopy was of particular importance, as described by Petra Wahle: “It allowed the detection of precisely tracked axonal origins at the micrometer level, which is sometimes not so easy with conventional optical microscopy .”
Evolutionary advantage still enigmatic
Little is known about the function of axon-bearing dendrites. Usually, a neuron integrates excitatory inputs arriving at the dendrites with inhibitory inputs, a process called somatodendritic integration. The neuron then decides if the inputs are strong and large enough to be transmitted via action potentials to other neurons and areas of the brain. 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 evolved, and the potential benefit it may have for neocortical information processing in primates, is still unknown.