Enhanced Dendritic Compartmentalization in Human Cortical Neurons
What is the science
Dr. Beaulieu-Laroche and colleagues did a novel study looking at the differences between human and rat neurons in the neocortex. Specifically they were comparing the electrical differences between Layer 5 pyramidal neurons. L5 neurons are much bigger in humans than rats and are much more "compartmentalized" than rats. Compartmentalization means that the large dendritic tree of human neurons attenuates voltage so much that the distal part of the dendritic tree is electrically isolated from the soma and distal synaptic input has a hard time eliciting an action potential. Physically, larger neurons need not be compartmentalized. If they have a low axial resistance, they can be electrically compact. That is, the electronic distance is small.
What did they do/ what are the results.
Beaulieu-Laroche and colleagues took both rat L5 neurons and human L5 neurons and compared them side by side. They did several experiments that showed that not only are human neurons physically larger, but are more compartmentalized.
The first experiment they did was compare how many spikes each neuron would fire in a stereotypical burst. Human neurons would fire less spikes on average. One possible hypothesis was that the large dendritic tree of the human neurons was acting as a current sink and drained current out of the soma. Thus limiting the number of spikes the neuron could fire.
However it could be that the two neuron types have different conductances. With that in mind, they did several experiments where they were able to isolate the soma from the dendrites and show that the two neurons would have similar burst patterns. This means that the dendritic tree is electrically large in humans.
Next, they tested the effects of nonlinear voltage spikes in the distal sections of the neurons. In human neurons it was much harder to elicit dendritc spikes distally. This is becaues of the large electronic distance between soma and dendritic tips in the human L5 neuron. Again, when isolating the soma and dendrite, Beaulieu-Laroche and colleagues were able to make both rat and human neurons act the same.
What does this mean?
Most immediately, this means that human neurons much more compartmentalized than rats. And this is one reason that our cortex is physically larger. More importantly however, because dendrites are eletrically isolated from somas, this allows the dendrites to perform more local computations outside the influence of action potential back propagation. This could lead to more complex repertoire of nonlinear operations in human dendrites.
Author: Alexander J. White
Source: Lou Beaulieu-Laroche, Enrique H.S. Toloza, Marie-Sophie van der Goes, Mathieu Lafourcade, Derrick Barnagian, Ziv M. Williams, Emad N. Eskandar, Matthew P. Frosch, Sydney S. Cash, Mark T. Harnett. Enhanced Dendritic Compartmentalization in Human Cortical Neurons. Cell 175(3) P643-651 (2018)
https://www.sciencedirect.com/science/article/pii/S0092867418311061
Dr. Beaulieu-Laroche and colleagues did a novel study looking at the differences between human and rat neurons in the neocortex. Specifically they were comparing the electrical differences between Layer 5 pyramidal neurons. L5 neurons are much bigger in humans than rats and are much more "compartmentalized" than rats. Compartmentalization means that the large dendritic tree of human neurons attenuates voltage so much that the distal part of the dendritic tree is electrically isolated from the soma and distal synaptic input has a hard time eliciting an action potential. Physically, larger neurons need not be compartmentalized. If they have a low axial resistance, they can be electrically compact. That is, the electronic distance is small.
What did they do/ what are the results.
Beaulieu-Laroche and colleagues took both rat L5 neurons and human L5 neurons and compared them side by side. They did several experiments that showed that not only are human neurons physically larger, but are more compartmentalized.
The first experiment they did was compare how many spikes each neuron would fire in a stereotypical burst. Human neurons would fire less spikes on average. One possible hypothesis was that the large dendritic tree of the human neurons was acting as a current sink and drained current out of the soma. Thus limiting the number of spikes the neuron could fire.
However it could be that the two neuron types have different conductances. With that in mind, they did several experiments where they were able to isolate the soma from the dendrites and show that the two neurons would have similar burst patterns. This means that the dendritic tree is electrically large in humans.
Next, they tested the effects of nonlinear voltage spikes in the distal sections of the neurons. In human neurons it was much harder to elicit dendritc spikes distally. This is becaues of the large electronic distance between soma and dendritic tips in the human L5 neuron. Again, when isolating the soma and dendrite, Beaulieu-Laroche and colleagues were able to make both rat and human neurons act the same.
What does this mean?
Most immediately, this means that human neurons much more compartmentalized than rats. And this is one reason that our cortex is physically larger. More importantly however, because dendrites are eletrically isolated from somas, this allows the dendrites to perform more local computations outside the influence of action potential back propagation. This could lead to more complex repertoire of nonlinear operations in human dendrites.
Author: Alexander J. White
Source: Lou Beaulieu-Laroche, Enrique H.S. Toloza, Marie-Sophie van der Goes, Mathieu Lafourcade, Derrick Barnagian, Ziv M. Williams, Emad N. Eskandar, Matthew P. Frosch, Sydney S. Cash, Mark T. Harnett. Enhanced Dendritic Compartmentalization in Human Cortical Neurons. Cell 175(3) P643-651 (2018)
https://www.sciencedirect.com/science/article/pii/S0092867418311061
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