Scientists discover key brain difference between primates and other animals

Scientists discover key brain difference between primates and other animals

Brain illustration

Researchers have learned about species-specific differences in neural architecture.

There are differences in the neural architecture of primates and non-primates.

A multinational research team has now been able to improve their understanding of species-specific variation 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 the transmission of electrical impulses. signals known as action potentials. The results were recently published in the journal eLife.

Researchers from Ruhr University

The researchers worked exclusively with archived tissues and specimens, including specimens that have been and continue to be used for decades to teach students. Credit: RUB, Kramer

Axons can exit the dendrites

Until now, it was considered textbook that the axon always, with few exceptions, arises from the cell body of a neuron.

However, it can also originate from dendrites that serve to collect and integrate incoming synaptic signals. This phenomenon is 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 is working with archived tissue preparations and slides that include material used for years to teach students,” Petra Valle explains.

Various animals have also been studied, including rodents (mouse, rat), ungulates (pig), carnivores (cat, ferret), macaques, and humans from the primate order. The scientists concluded that there is a species difference between non-primates and primates by using five different staining techniques and evaluating more than 34,000 neurons.

There are markedly fewer axon-bearing dendrites on excitatory pyramidal neurons in external layers II and III of the primate cerebral cortex than on nonprimate excitatory pyramidal neurons. Furthermore, for inhibitory interneurons, significant variation in the percentage of axon-bearing dendritic cells was found between feline 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 Valle describes: “This allowed the detection of axon origins precisely traced at the micrometer level, which is sometimes not so easy with conventional light microscopy.”

The evolutionary advantage is still mysterious

Little is known about the function of axon-bearing dendrites. Normally, the neuron integrates excitatory inputs arriving at the dendrites with inhibitory inputs, a process called somatodendritic integration. The neuron then decides whether the inputs are strong enough and important enough to be relayed via action potentials to other neurons and areas of the brain.

Axon-bearing dendrites are considered privileged because depolarizing inputs to 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 common in non-primates but rare in monkeys and humans” by Petra Wahle, Eric Sobierajski, Ina Gasterstädt, Nadja Lehmann, Susanna Weber, Joachim HR Lübke, Maren Engelhardt , Claudia Distler and Gundela Meyer, 20 Apr 2022, eLife.
DOI: 10.7554/eLife.76101

The study was funded by the German Research Foundation.

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