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In the human brain, thousands of neurons continuously transmit electrical signals, and the varying lengths of dendrites (extensions of the neuron cell body) play a key role in the integration of neuronal information, allowing our brain cells to respond and function normally.
This time, MIT neuroscientists discovered from precious human brain tissue samples that the electrophysiological properties of human dendrites differ from those of other biological species. This study reveals that the intensity of electrical signals weakens more rapidly along human dendrites, resulting in a higher degree of electrophysiological compartmentalization, indicating that small regions of dendrites can conduct physiological activities relatively independently from other parts of the same neuron.
Researchers state that this difference may be a significant reason why human brain computational efficiency surpasses that of other organisms.
Mark Harnett, an assistant professor at MIT’s Department of Brain and Cognitive Sciences, shared his insights: “Humans are intelligent not just because we have more neurons and a larger cortical area. Fundamentally, the behavior of neurons is also different. The neurons in the human brain have more electrophysiological compartments, and these small units are relatively more independent, which potentially increases the computational capacity of a single neuron.”
(Source: Cell)
This research was published on October 18 in the journal Cell, with the corresponding authors being Assistant Professor Harnett at MIT’s McGovern Institute for Brain Research, and Assistant Professor Sydney Cash at Harvard Medical School’s Department of Neurobiology and Massachusetts General Hospital (MGH). Graduate student Lou Beaulieu-Laroche from MIT’s Department of Brain and Cognitive Sciences is the first author of this article.
Neural Computing
Dendrites perform simple calculations using electrical signals like transistors in computers and transmit information inputs from other neurons to the cell body. If the stimuli received are strong enough, the neuron will generate an action potential—an electrical impulse sufficient to further stimulate other neurons. It is through this large network of interconnected neurons that we generate thoughts and behaviors.
The structure of a single neuron resembles a tree, receiving signals from the complex branching of dendrites and transmitting them to the distant cell body. Past research has shown that the strength of electrical signals received by the cell body partly depends on the length of the dendrites along the way, weakening the signal gradually in the process, so signals coming from farther away have a relatively weaker impact on the cell body.
The dendrites in the human brain cortex are much longer than those in other species like rats because the human cortex has evolved to be much thicker than that of other species. Compared to the 30% cortical volume of a rat’s brain, humans have a staggering 75% cortical volume.
Despite being 2-3 times thicker than that of rats, the human cortex retains a similar organizational structure, consisting of 6 layers of neurons, just like the rat cortex. Notably, the synapses of layer V neurons can reach layer I, indicating that the dendrites relevant to the development of the human brain must extend sufficiently long, and electrical signals must also be transmitted over equal distances.
Image | MIT neuroscientists can record the electrical signal activity of human neuronal dendrites (Source: Lou Beaulieu-Laroche & Mark Harnett)
The goal of the MIT team’s research is to explore how dendrite length affects its electrophysiological properties. They compared the electrical activity of dendrites in brain tissue surgically removed from epilepsy patients’ frontal lobes with that of rat brains. Additionally, to access the pathological brain region, surgeons had to remove a small piece of tissue from the anterior temporal lobe.
With the help of collaborators from MGH, including Cash, Matthew Frosch, Ziv Williams, and Emad Eskandar, Harnett’s lab was able to obtain precious human anterior temporal lobe samples, each about the size of a fingernail.
Harnett noted that under neuropathological techniques, the anterior temporal lobe showed no signs of being affected by epilepsy, and the tissue appeared normal. Normally, this brain region indeed participates in regulating various functions such as language and visual processing, but these are not critical functions; even if the patient had this area removed, they could still perform related functions normally.
After the tissue was removed, the researchers transferred it to an oxygenated artificial cerebrospinal fluid, which can keep the tissue active for up to 48 hours, providing researchers the opportunity to measure the electrical signals of pyramidal neurons (the most common type of excitatory neurons in the cortex) dendrites using electrophysiological patch-clamp techniques.
The above experiments were primarily conducted under the leadership of Beaulieu-Laroche. Multiple labs, including Harnett’s lab, have previously used this technique to study dendrites in rodents; however, Harnett’s team is the first to use this technique to investigate the electrophysiological properties of human dendrites.
Video | Using precious human brain tissue samples, researchers from McGovern and MGH discovered the electrophysiological properties of human dendrites differ from those of other species, which may be a significant reason why human brain computational efficiency surpasses that of other organisms. (Source: Cell)
Unique Properties
Researchers found that due to the longer distances covered by human dendrites, the degree of weakening when signals travel from layer I dendrites to layer V cell bodies is much greater than the weakening observed in rat cortices.
Moreover, human and rat dendrites have a similar number of ion channels (responsible for regulating neural currents), but due to the longer human dendrites, the density of dendrites is correspondingly lower. Harnett’s team also proposed a biophysical model to explain that the difference in dendrite density is one reason for the differences in electrophysiological activity between human and rat dendrites.
Nelson Spruston, a research project leader at the Howard Hughes Medical Institute’s Janelia Research Campus, described this study as “an outstanding achievement.”
“This is the most meticulous and careful study of human neuronal physiological properties to date. Such studies require extremely high technical demands, even for mice and rats; achieving these results in humans is truly remarkable,” Spruston said.
However, we still have questions to resolve: how do these differences affect human cognitive abilities? Harnett’s hypothesis is that these electrophysiological differences in neurons allow more regions of dendrites to influence incoming signals, enabling individual neurons to perform more complex information computations.
“For a small piece of human or rodent cortex, the structure of the human brain can complete more computations more quickly compared to the rodent brain,” Harnett explained.
He further added that there are many other differences between human neurons and those of other species, making it more challenging to analyze the effects of synaptic electrophysiological properties. In the future, Harnett hopes to further explore the impact of electrophysiological properties and how they interact with other characteristics of human neurons to produce efficient computational power.
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Editor: Chen Shuqing Editor-in-Chief: Dai Qing
References:
http://news.mit.edu/2018/dendrites-explain-brains-computing-power-1018
