Electrical Properties of
Dendrites help Explain
our Brain’s unique Computing Power
Neurons in human and rat brains carry electrical signals in different ways, scientists find
By Anne Trafton | MIT News Office
our Brain’s unique Computing Power
Neurons in human and rat brains carry electrical signals in different ways, scientists find
By Anne Trafton | MIT News Office
October 18, 2018 -- Neurons in the human brain receive electrical signals from
thousands of other cells, and long neural extensions called dendrites play a
critical role in incorporating all of that information so the cells can respond
appropriately.
Using
hard-to-obtain samples of human brain tissue, MIT neuroscientists have now
discovered that human dendrites have different electrical properties from those
of other species. Their studies reveal that electrical signals weaken more as
they flow along human dendrites, resulting in a higher degree of electrical
compartmentalization, meaning that small sections of dendrites can behave
independently from the rest of the neuron.
These
differences may contribute to the enhanced computing power of the human brain,
the researchers say.
“It’s
not just that humans are smart because we have more neurons and a larger
cortex. From the bottom up, neurons behave differently,” says Mark Harnett, the
Fred and Carole Middleton Career Development Assistant Professor of Brain and
Cognitive Sciences. “In human neurons, there is more electrical
compartmentalization, and that allows these units to be a little bit more
independent, potentially leading to increased computational capabilities of
single neurons.”
Harnett,
who is also a member of MIT’s McGovern Institute for Brain Research, and Sydney
Cash, an assistant professor of neurology at Harvard
Medical School
and Massachusetts General
Hospital
Neural computation
Dendrites
can be thought of as analogous to transistors in a computer, performing simple
operations using electrical signals. Dendrites receive input from many other
neurons and carry those signals to the cell body. If stimulated enough, a
neuron fires an action potential — an electrical impulse that then stimulates
other neurons. Large networks of these neurons communicate with each other to
generate thoughts and behavior.
The
structure of a single neuron often resembles a tree, with many branches
bringing in information that arrives far from the cell body. Previous research
has found that the strength of electrical signals arriving at the cell body
depends, in part, on how far they travel along the dendrite to get there. As
the signals propagate, they become weaker, so a signal that arrives far from
the cell body has less of an impact than one that arrives near the cell body.
Dendrites
in the cortex of the human brain are much longer than those in rats and most
other species, because the human cortex has evolved to be much thicker than
that of other species. In humans, the cortex makes up about 75 percent of the
total brain volume, compared to about 30 percent in the rat brain.
Although
the human cortex is two to three times thicker than that of rats, it maintains
the same overall organization, consisting of six distinctive layers of neurons.
Neurons from layer 5 have dendrites long enough to reach all the way to layer 1,
meaning that human dendrites have had to elongate as the human brain has
evolved, and electrical signals have to travel that much farther.
In
the new study, the MIT team wanted to investigate how these length differences
might affect dendrites’ electrical properties. They were able to compare
electrical activity in rat and human dendrites, using small pieces of brain
tissue removed from epilepsy patients undergoing surgical removal of part of
the temporal lobe. In order to reach the diseased part of the brain, surgeons
also have to take out a small chunk of the anterior temporal lobe.
With
the help of MGH collaborators Cash, Matthew Frosch, Ziv Williams, and Emad
Eskandar, Harnett’s lab was able to obtain samples of the anterior temporal
lobe, each about the size of a fingernail.
Evidence
suggests that the anterior temporal lobe is not affected by epilepsy, and the
tissue appears normal when examined with neuropathological techniques, Harnett
says. This part of the brain appears to be involved in a variety of functions,
including language and visual processing, but is not critical to any one
function; patients are able to function normally after it is removed.
Once
the tissue was removed, the researchers placed it in a solution very similar to
cerebrospinal fluid, with oxygen flowing through it. This allowed them to keep
the tissue alive for up to 48 hours. During that time, they used a technique
known as patch-clamp electrophysiology to measure how electrical signals travel
along dendrites of pyramidal neurons, which are the most common type of
excitatory neurons in the cortex.
These
experiments were performed primarily by Beaulieu-Laroche. Harnett’s lab (and
others) have previously done this kind of experiment in rodent dendrites, but
his team is the first to analyze electrical properties of human dendrites.
Unique features
The
researchers found that because human dendrites cover longer distances, a signal
flowing along a human dendrite from layer 1 to the cell body in layer 5 is much
weaker when it arrives than a signal flowing along a rat dendrite from layer 1
to layer 5.
They
also showed that human and rat dendrites have the same number of ion channels,
which regulate the current flow, but these channels occur at a lower density in
human dendrites as a result of the dendrite elongation. They also developed a
detailed biophysical model that shows that this density change can account for
some of the differences in electrical activity seen between human and rat
dendrites, Harnett says.
Nelson
Spruston, senior director of scientific programs at the Howard Hughes Medical
Institute Janelia Research Campus, described the researchers’ analysis of human
dendrites as “a remarkable accomplishment.”
“These
are the most carefully detailed measurements to date of the physiological
properties of human neurons,” says Spruston, who was not involved in the
research. “These kinds of experiments are very technically demanding, even in
mice and rats, so from a technical perspective, it’s pretty amazing that
they’ve done this in humans.”
The
question remains, how do these differences affect human brainpower? Harnett’s
hypothesis is that because of these differences, which allow more regions of a
dendrite to influence the strength of an incoming signal, individual neurons
can perform more complex computations on the information.
“If
you have a cortical column that has a chunk of human or rodent cortex, you’re
going to be able to accomplish more computations faster with the human
architecture versus the rodent architecture,” he says.
There
are many other differences between human neurons and those of other species,
Harnett adds, making it difficult to tease out the effects of dendritic
electrical properties. In future studies, he hopes to explore further the
precise impact of these electrical properties, and how they interact with other
unique features of human neurons to produce more computing power.
Link
(includes another link to a video): http://news.mit.edu/2018/dendrites-explain-brains-computing-power-1018
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