Friday, June 3, 2022

Primates and non-primates differ in the construction of the nerve cells

The researchers worked exclusively with archived fabrics and preparations, including preparations that have been and are used for the training of students for decades.
Credit: RUB, Kramer

Using high-resolution microscopy, an international research team was able to significantly expand knowledge about the development of nerve cells of various types.

Researcher of the Development Neurobiology Working Group at the Ruhr University Bochum (RUB) around Prof. Dr. In cooperation with partners from Mannheim, J├╝lich, Linz, Austria, and La Laguna, Spain, Petra Wahle have shown that primates and non-primates differ in the architecture of their cortical neurons. The differences lie in where the nerve cell originates from the extension called Axon, which is responsible for the transmission of electrical potential. The team reports in the journal eLife.

When the axon comes out of the dendrite

So far, it was considered textbook knowledge that, with a few exceptions, this axon arises from the cell body of the nerve cell. However, the axon can also arise from a dendrite. Dendrites are processes that collect synaptic inputs. The phenomenon was described with the name "Axon carrying dendrite", in German "Axon-bearing dendrite".

Different animal species and high-resolution microscopy clarify the variable origin of axons

The video shows a neuron (marked in green) of a macaque from which an axon (in red) originates from the dendrite. The cell nuclei can be seen in blue.
Credit: Developmental neurobiology

"A special aspect of this research project is that only archived fabrics and preparations were used, including preparations that have been and are used for the training of students for decades," explains Petra Wahle. In addition, various types of mammals were examined, such as representatives of the zoological orders rodents (mouse, rat), ungulates (pig), predators (cat, ferret) as well as macaque and humans from the primates order.

By using five different methods to mark the dendrites and the axon and after counting more than 34,000 nerve cells, the researchers came to the conclusion that there is a species-dependent difference between non-primates and primates. The exciting pyramidal neurons, especially the outer layers II and III of the cerebral cortex of primates, have significantly fewer axon-bearing dendrites than pyramidal neurons of non-primates. Just as clear differences can be found within a species (cat, human) between the different types of inhibitory interneurons. On the other hand, there were no differences in primates between primarily sensory cortical areas and areas for higher brain functions.

High-resolution microscopy was particularly important, as Petra Wahle describes: “The detection of the axonal origin could be determined precisely at the micrometer level, which is sometimes not so easy with conventional light microscopy."

Evolutionary advantage is still unknown

Little is known about the biological function of the axon-bearing dendrites. The neuron usually calculates the excitating inputs on the dendrites with synaptic inhibition. This process is called somatodendritic integration. Only then does the neuron decide whether the inputs are strong enough and important enough to pass them on to downstream neurons and brain areas through action potential. Axon-bearing dendrites are considered privileged because exciting entrances to these dendrites are able to directly trigger action potential. It is unclear why this species-dependent difference in evolution arose and what advantage it could have for neocortical information processing in primates.

Source/Credit: Ruhr University Bochum

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