This
is a computer simulation showing color-coded voltage (in millivolts) in a
portion of a neuron’s dendritic tree. Computer-generated spines have been
attached to the dendrites, and synapses on seven spines near the center have
been activated, raising the voltage at those locations. The simulation
quantifies the spread of electric charge and the accompanying voltage rise in
neighboring parts of the dendritic tree a short time (1-1/3 milliseconds) after
the synapses have been activated.
It's the least understood organ
in the human body: the brain, a massive network of electrically excitable
neurons, all communicating with one another via receptors on their tree-like
dendrites. Somehow these cells work together to enable great feats of human
learning and memory. But how?
Researchers know dendritic spines
play a vital role. These tiny membranous structures protrude from dendrites'
branches; spread across the entire dendritic tree, the spines on one neuron
collect signals from an average of 1,000 others. But more than a century after
they were discovered, their function still remains only partially understood.
A Northwestern University
researcher, working in collaboration with scientists at the Howard Hughes
Medical Institute (HHMI) Janelia Farm Research Campus, has recently added an
important piece of the puzzle of how neurons "talk" to one another.
The researchers have demonstrated that spines serve as electrical compartments
in the neuron, isolating and amplifying electrical signals received
at the synapses, the sites at which neurons connect to one another.
The key to this discovery is the
result of innovative experiments at the Janelia Farm Research Campus and computer simulations performed
at Northwestern University that can measure electrical responses on spines
throughout the dendrites.
A paper about the findings,
"Synaptic Amplification by Dendritic Spines Enhances Input
Cooperatively," was published November 22 in the journal Nature.
"This research conclusively
shows that dendritic spines respond to and process synaptic inputs not just
chemically, but also electrically," said William Kath, professor of
engineering sciences and applied mathematics at Northwestern's McCormick School
of Engineering, professor of neurobiology at the Weinberg College of Arts and
Sciences, and one of the paper's authors.
Dendritic spines come in a
variety of shapes, but typically consist of a bulbous spine head at the end of
a thin tube, or neck. Each spine head contains one or more synapses and is
located in very close proximity to an axon coming from another neuron.
Scientists have gained insight
into the chemical properties of dendritic spines: receptors on their surface
are known to respond to a number of neurotransmitters, such as glutamate and
glycine, released by other neurons. But because of the spines' incredibly small
size—roughly 1/100 the diameter of a human hair—their electrical properties have
been harder to study.
In this study, researchers at the
HHMI Janelia Farm Research Campus used three experimental techniques to assess
the electrical properties of dendritic spines in rats' hippocampi, a part of
the brain that plays an important role in memory and spatial navigation. First,
the researchers used two miniature electrodes to administer current and measure
its voltage response at different sites throughout the dendrites.
They also used a technique called
"glutamate uncaging," a process that involves releasing glutamate, an
excitatory neurotransmitter, to evoke electrical responses from specific
synapses, as if the synapse had just received a signal from a neighboring
neuron. A third process utilized a calcium-sensitive dye—calcium is a chemical
indicator of a synaptic event—injected into the neuron to provide an optical
representation of voltage changes within the spine.
At Northwestern, researchers used
computational models of real neurons—reconstructed from the same type of rat
neurons—to build a 3D representation of the neuron with accurate information
about each dendrites' placement, diameter, and electrical properties. The
computer simulations, in concert with the experiments, indicated that spines'
electrical resistance is consistent throughout the dendrites, regardless of where on
the dendritic tree they are located.
While much research is still
needed to gain a full understanding of the brain, knowledge about spines' electrical processing could
lead to advances in the treatment of diseases like Alzheimer's and Huntington's
diseases.
"The brain is much more
complicated than any computer we've ever built, and understanding how it works
could lead to advances not just in medicine, but in areas we haven't considered
yet," Kath said. "We could learn how to process information in ways
we can only guess at now."
"After 100 years,
understanding the electrical role of dendritic spines." December 5th,
2012. http://medicalxpress.com/news/2012-12-years-electrical-role-dendritic-spines.html
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