Wise Young
11-24-2006, 02:19 PM
http://www.hhmi.org/news/yuste20061120.html
November 20, 2006
The Calculating Brain: New Studies Suggest That Neurons Are Built To Perform Simple Arithmetic
The tiniest wires that link neurons to one another probably serve a critical role in the brain's computational function. New data about how these wires, or dendritic spines, modulate their electrical properties and receive incoming signals is giving scientists a more complete view of their knack for acting as efficient mathematical calculators. Moreover, the findings also hint that these dendritic spines could make the human brain a far more efficient learning machine than that of other animals.
Howard Hughes Medical Institute investigator Rafael Yuste and professor of chemistry Kenneth B. Eisenthal, both at Columbia University, collaborated on the studies on dendritic spines. Yuste, Eisenthal, and colleagues published their findings about dendritic spines in a trio of papers in the Proceedings of the National Academy of Sciences (PNAS).
<snip> It has been difficult for researchers to study the function of dendritic spines, said Yuste, because the length of the microscopic structures is only about a hundredth the diameter of a human hair. Neurobiologists have traditionally used microelectrodes to explore electrical properties of whole neurons - but dendritic spines are far too small for insertion of even the finest of these.
To overcome that barrier, Yuste and his colleagues developed an entirely new imaging technique. Rather than using electrodes, they chose to fill neurons with a dye whose optical properties vary according to voltage. By measuring the dye's optical "second harmonic" generation, the researchers could determine voltages at any point along a neuron. Moreover, using this optical method, the team was able to measure, for the first time in history, the membrane potential in a dendritic spine. These new imaging technique was published in a paper in the January 17, 2006, issue of PNAS. Yuste's co-authors on the paper were Mutsuo Nuriya, Boaz Nemet, Jiang Jiang, and Kenneth Eisenthal.
In addition, the researchers also used a second optical technique to precisely trigger an electrical impulse in a dendritic spine. To do so, the scientists bathe brain tissue in the neurotransmitter glutamate, which has been chemically modified so that it is "caged," meaning it is not recognizable to the cell. The researchers then use an extremely precise laser beam to zap the head of a single spine, unleashing the glutamate only there and triggering an electrical signal restricted to the zapped spine.
Using these two optical techniques, the researchers then turned their attention to measuring how the length of a spine's neck affects its propagation of electrical impulses. They found that the spine neck acts as an electrical filter for signals from the spine head. Longer spine necks attenuate these signals more than do shorter necks, and the longest spines produced “silent” spines that did not propagate signals down their necks at all, even when triggered by glutamate. These silent spines, they said, could play a significant role in learning as they become active with experience. Overall, they concluded that spines are electrical devices and that their necks keep them electrically isolated from the rest of the spines in the neuron.
Those findings were published in a paper in the November 21, 2006, issue of PNAS. Yuste's co-authors on the paper were Roberto Araya, Jiang Jiang, and Kenneth Eisenthal.
In a third PNAS paper, the researchers studied the effects of stimulating two spines at the same time in order to understand the functional reason behind the electrical isolation of spines. They found that the spines allow a cell to detect each incoming signal individually. Signals from multiple spines are then added to one another to generate a summation signal in the dendrite. Interestingly, the summation of spine signals was linear. The neuron summed activated inputs just the way schoolchildren are taught: one plus one equals two. If, on the other hand, two activated inputs were located directly on the dendrite directly, instead of on spines, they interacted with each other: one plus one equaled less than two.
Therefore, Yuste explained, it appears that by making connections onto spines, neurons can add their inputs according to a simple arithmetic sum. This linear arithmetic of excitatory inputs had been observed before by Yuste and others, but this paper finally explained why it happened and in doing so, provided a functional logic for the spines. That paper was published online the week of November 20, 2006, in the PNAS Early Edition. Araya, Eisenthal, and Yuste were the authors of that paper.
November 20, 2006
The Calculating Brain: New Studies Suggest That Neurons Are Built To Perform Simple Arithmetic
The tiniest wires that link neurons to one another probably serve a critical role in the brain's computational function. New data about how these wires, or dendritic spines, modulate their electrical properties and receive incoming signals is giving scientists a more complete view of their knack for acting as efficient mathematical calculators. Moreover, the findings also hint that these dendritic spines could make the human brain a far more efficient learning machine than that of other animals.
Howard Hughes Medical Institute investigator Rafael Yuste and professor of chemistry Kenneth B. Eisenthal, both at Columbia University, collaborated on the studies on dendritic spines. Yuste, Eisenthal, and colleagues published their findings about dendritic spines in a trio of papers in the Proceedings of the National Academy of Sciences (PNAS).
<snip> It has been difficult for researchers to study the function of dendritic spines, said Yuste, because the length of the microscopic structures is only about a hundredth the diameter of a human hair. Neurobiologists have traditionally used microelectrodes to explore electrical properties of whole neurons - but dendritic spines are far too small for insertion of even the finest of these.
To overcome that barrier, Yuste and his colleagues developed an entirely new imaging technique. Rather than using electrodes, they chose to fill neurons with a dye whose optical properties vary according to voltage. By measuring the dye's optical "second harmonic" generation, the researchers could determine voltages at any point along a neuron. Moreover, using this optical method, the team was able to measure, for the first time in history, the membrane potential in a dendritic spine. These new imaging technique was published in a paper in the January 17, 2006, issue of PNAS. Yuste's co-authors on the paper were Mutsuo Nuriya, Boaz Nemet, Jiang Jiang, and Kenneth Eisenthal.
In addition, the researchers also used a second optical technique to precisely trigger an electrical impulse in a dendritic spine. To do so, the scientists bathe brain tissue in the neurotransmitter glutamate, which has been chemically modified so that it is "caged," meaning it is not recognizable to the cell. The researchers then use an extremely precise laser beam to zap the head of a single spine, unleashing the glutamate only there and triggering an electrical signal restricted to the zapped spine.
Using these two optical techniques, the researchers then turned their attention to measuring how the length of a spine's neck affects its propagation of electrical impulses. They found that the spine neck acts as an electrical filter for signals from the spine head. Longer spine necks attenuate these signals more than do shorter necks, and the longest spines produced “silent” spines that did not propagate signals down their necks at all, even when triggered by glutamate. These silent spines, they said, could play a significant role in learning as they become active with experience. Overall, they concluded that spines are electrical devices and that their necks keep them electrically isolated from the rest of the spines in the neuron.
Those findings were published in a paper in the November 21, 2006, issue of PNAS. Yuste's co-authors on the paper were Roberto Araya, Jiang Jiang, and Kenneth Eisenthal.
In a third PNAS paper, the researchers studied the effects of stimulating two spines at the same time in order to understand the functional reason behind the electrical isolation of spines. They found that the spines allow a cell to detect each incoming signal individually. Signals from multiple spines are then added to one another to generate a summation signal in the dendrite. Interestingly, the summation of spine signals was linear. The neuron summed activated inputs just the way schoolchildren are taught: one plus one equals two. If, on the other hand, two activated inputs were located directly on the dendrite directly, instead of on spines, they interacted with each other: one plus one equaled less than two.
Therefore, Yuste explained, it appears that by making connections onto spines, neurons can add their inputs according to a simple arithmetic sum. This linear arithmetic of excitatory inputs had been observed before by Yuste and others, but this paper finally explained why it happened and in doing so, provided a functional logic for the spines. That paper was published online the week of November 20, 2006, in the PNAS Early Edition. Araya, Eisenthal, and Yuste were the authors of that paper.