Scientists discover startling neural differences between primates and non-primates

Scientists discover startling neural differences between primates and non-primates

Scientists looking very closely at the architecture of neuronal cells in the brain have discovered a key structural difference between primates and non-primates in cortical neurons – cells that are part of the brain.

The results give us greater insight into this most complex organ and how the shape and function of neurons can differ between species. We can also learn about human and animal evolution through research.

The key to this difference between neurons is the axonal fiber: a thin part of the neuron that carries electrical impulses. Previously, these axons were thought to almost always grow outside the cell body, but the new study shows they can also arise from dendrites – extensions that connect nerve cells together.

These axon-bearing dendrites are much more common in non-primates like cats and pigs than in primates, the team found. The study was based on existing tissue and archived specimens and included analysis of more than 34,000 individual neurons.

“A unique aspect of the project is that the team worked with archived tissue and slide preparations, which included material used for years to teach students,” says neurobiologist Petra Wahle, from Ruhr University. in Bochum, Germany.

The researchers examined samples spanning mice, rats, pigs, cats, ferrets, macaque monkeys and humans. While dendrites bearing axons were found in all species, there were many more in non-primates.

A crucial part of the research has been the use of the latest high-resolution microscopy techniques to gain close insight into cell development, further illuminated by the use of five different staining methods on the cells studied.

“This allowed the detection of axonal origins to be tracked precisely at the micrometer level, which is sometimes not so easy with conventional light microscopy,” says Wahle.

Further research will be needed to understand why some species have a higher percentage of axon-bearing dendrites than others, and what their evolutionary advantage might be for animals that use them.

Neurons generally act as gatekeepers when it comes to deciding which signals are passed on and which are not, based on other inputs they also receive. This is called somatodendritic integration. One difference that axon-bearing dendrites seem to have is the ability to bypass this gate and independently choose the messages that flow through the brain network.

At this time, we don’t know exactly what this means for brain processing, but we should get more clues over time. The researchers found that domestication in animals did not appear to affect the number of these axon-bearing dendrites, with pigs and boars having similar proportions. Moreover, animals of various species seem to be born with them, rather than developing them as they age.

With so many neurons to monitor – tens of billions in some cases – the brain is not an easy part of the body to study, although that does not discourage scientists. We are continually learning more about how neurons are arranged and how they work.

“Our findings expand current knowledge regarding the distribution and proportion of axon-bearing dendritic cells in the neocortex of non-primate taxa, which are strikingly different from primates where these cells are found primarily in deeper layers and white matter,” write the researchers in their published paper.

The research has been published in eLife.

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