A new type of neuron has been discovered, and it could help explain what makes the human brain unique compared to other animals.
Charmingly named the "rosehip neuron" due to the way its many axons resemble a rose that has lost its petals, the cells appear to exist only in the human cortex – the outermost layer of the brain that is comprised of the frontal, parietal, temporal, and occipital lobes. These areas are the origin of higher thought, self-awareness, and sensory processing, and are proportionally much larger in us Homo sapiens than any other species.
"It's the most complex part of the brain, and generally accepted to be the most complex structure in nature," researcher Ed Lein said in a statement. Lein and his colleagues at the Allen Institute for Brain Science teamed up with Gábor Tamás and his researchers at the University of Szeged, Hungary, to tease out the mysteries of the rosehip neuron after both groups realized they were hot on the trail of the same cell.
You see, although neuroscientists have made significant advances in unraveling the ironically mind-boggling inner workings of the mammalian brain in the past several decades, most of the findings have come from experiments and observations in mice. And while it appears that many aspects of our brains' anatomy and physiology are indeed similar to those of mice, very little work has been done comparing the two species’ cortices.
“With the mouse cortex as the dominant model for understanding human cognition, it is essential to establish whether the cellular architecture of the human brain is conserved or whether there are specialized cell types and system properties that cannot be modeled in rodents,” the collaborative team explained in their paper, now published in the journal Nature Neuroscience.
Prior to joining their efforts, both groups had made progress toward this goal by identifying the rosehip neuron; yet they had done so using two different techniques. Lein’s lab was analyzing differences in genes and gene expression between mouse and human brain cells, whereas Tamás' lab was using the classical approach of studying cell structure and electrical signaling properties.
In their joint research, the scientists employed these complementary methods to analyze the cortical tissue from two deceased men who had donated their bodies to science.
The results reveal that rosehip neurons are a type of GABA inhibitory neuron – cells that regulate the signaling pathways of excitatory neurons by blocking the propagation of action potentials. They do this by releasing the neurotransmitter GABA from their axons, which makes it harder for nearby cells to depolarize their membranes. Without this inhibitory mechanism, electrical signals would spread and spread, overwhelming the brain’s circuits. With it on hand, the brain can deliver messages with exacting precision.
The experiments also showed that rosehip neurons form synapses with pyramidal neurons, the most common type of excitatory cortical neuron, but they only attach to one specific location on these cells, suggesting a special form of information control.
Moving forward, the two teams will look for rosehip neurons in other parts of the human brain and plan to assess whether they play a role in psychiatric disorders. The researchers concede that other animals may possess these neurons as well, as they have only been confirmed to be lacking in rodents. Either way, the findings are yet another nail in the coffin of the traditional practice of using mouse brains to model human brain diseases.
"Our brains are not just enlarged mouse brains," said Allen Institute co-author Dr Trygve Bakken. "People have commented on this for many years, but this study gets at the issue from several angles."