Single human neurons may be much more powerful computational devices than once thought, according to a new study that identifies previously unknown electrical activity in neural dendrites.
At the end of a neuron, tree-like appendages called dendrites send and receive electrochemical signals, which play a critical role in how the brain compiles information to determine its next actions. The results published in the Jan. 3 issue of Science unveil unexpectedly complex electrical activity in the dendrites of human pyramidal neurons, which may help uniquely boost the processing power of the human brain, allowing us to understand and solve complicated problems.
Neurologically speaking, the physiology that makes the human brain so particularly special and capable remains poorly understood. One possibility may lie in the thickness of the human brain's cortical layers, particularly layers 2 and 3, which contain a disproportionate amount of brain matter compared to other species as well as numerous neurons with large and elaborate dendritic trees.
"The dendrites are central to understanding the brain because they are at the core of what determines the computational power of single neurons," said study co-author Matthew Larkum, a neuroscientist at Humboldt University of Berlin. According to Larkum, recording the activity of dendrites in living rodents is rather challenging — and nearly impossible in humans. As a result, almost all that is known about active dendrites has been gleaned from the brains of rodents.
To address this, the researchers directly probed the active properties of layer 2 and 3 dendrites in slices of human brain tissue and revealed several new classes of electrical activity unique to pyramidal neurons in these layers, unknown and far more complex than in all other neurons studied to date.
By modeling these unique electrical properties, Larkum and his colleagues demonstrated that the properties allowed single neurons to solve computational problems which were considered to require multi-layer neural networks.
"There was a 'eureka' moment when we saw the dendritic action potentials for the first time," said Larkum. "The experiments were very challenging, so to push the questions past just repeating what has been done in rodents already was very satisfying."
Larkum notes, however, that almost nothing is known about these dendrites in other species and it remains to be seen if this particular dendritic activity is uniquely complex in humans or uniquely simple in rodents, or something in the middle.
"We are missing the information about how they operate when the whole brain is active, which can help in answering this question," said Larkum.
Gidon et al. (2020) Dendritic action potentials and computation in human layer 2/3 cortical neurons. Science. DOI: https://doi.org/10.1126/science.aax6239