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|Иной способ связи нейронов - электрическое поле
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|Автор:||mark [ 06 березня 2011, 20:56 ]|
|Тема повідомлення:||Иной способ связи нейронов - электрическое поле|
Коммуникация нейронов в мозге осуществляется не только через синапсы, но и посредством электрических полей. Это значит, что взаимодействовать могут нейроны, не связанные друг с другом физически (аксонами-дендритами).
Прочно засевшая в сознании картинка устройства головного мозга – сеть, по которой проходят импульсы – вероятно, будет пересмотрена и дополнена новым механизмом общения нейронов. Про присутствие в голове электрических полей разной интенсивности известно давно, но они рассматривались, как правило, в качестве побочного эффекта возбуждений нервных клеток. Этим полям не приписывалось полезной функции (кроме как сообщать ученым об активности мозга по EEG).
Новый эксперимент, проведенный нейрофизиологами из Калифорнийского Технологического института (Caltech), показал, что внеклеточные электрические поля, генерируемые нейронами, изменяют характеристики потенциалов действия других нейронов. Фактически речь идет об открытии другого типа коммуникации поверх сети, независимого от синаптических соединений.
Coordinated behavior occurs whether or not neurons are actually connected via synapses
Pasadena, Calif.—The brain—awake and sleeping—is awash in electrical activity, and not just from the individual pings of single neurons communicating with each other. In fact, the brain is enveloped in countless overlapping electric fields, generated by the neural circuits of scores of communicating neurons. The fields were once thought to be an "epiphenomenon" similar to the sound the heart makes—which is useful to the cardiologist diagnosing a faulty heart beat, but doesn't serve any purpose to the body, says Christof Koch, the Lois and Victor Troendle Professor of Cognitive and Behavioral Biology and professor of computation and neural systems at the California Institute of Technology (Caltech).
New work by Koch and neuroscientist Costas Anastassiou, a postdoctoral scholar in biology, and his colleagues, however, suggests that the fields do much more—and that they may, in fact, represent an additional form of neural communication.
"In other words," says Anastassiou, the lead author of a paper about the work appearing in the journal Nature Neuroscience, "while active neurons give rise to extracellular fields, the same fields feed back to the neurons and alter their behavior," even though the neurons are not physically connected—a phenomenon known as ephaptic (or field) coupling. "So far, neural communication has been thought to occur almost entirely via traffic involving synapses, the junctions where one neuron connects to the next one. Our work suggests an additional means of neural communication through the extracellular space independent of synapses."
Ephaptic coupling leads to coordinated spiking of nearby neurons,
as measured using a 12-pipette electrophysiology setup developed
in the laboratory of coauthor Henry Markram.
[Credit: Image from Figure 4 in Anastassiou et.,Nature Neuroscience, 2011]
Extracellular electric fields exist throughout the living brain. Their distant echoes can be measured outside the skull as EEG waves. These fields are particularly strong and robustly repetitive in specific brain regions such as the hippocampus, which is involved in memory formation, and the neocortex, the area where long-term memories are held. "The perpetual fluctuations of these extracellular fields are the hallmark of the living and behaving brain in all organisms, and their absence is a strong indicator of a deeply comatose, or even dead, brain," Anastassiou explains.
Previously, neurobiologists assumed that the fields were capable of affecting—and even controlling—neural activity only during severe pathological conditions such as epileptic seizures, which induce very strong fields. Few studies, however, had actually assessed the impact of far weaker—but very common—non-epileptic fields. "The reason is simple," Anastassiou says. "It is very hard to conduct an in vivo experiment in the absence of extracellular fields," to observe what changes when the fields are not around.
To tease out those effects, Anastassiou and his colleagues focused on strong but slowly oscillating fields, called local field potentials (LFP), that arise from neural circuits composed of just a few rat brain cells. Measuring those fields and their effects required positioning a cluster of tiny electrodes within a volume equivalent to that of a single cell body—and at distances of less than 50 millionths of a meter from one another; this is approximately the width of a human hair.
"Because it had been so hard to position that many electrodes within such a small volume of brain tissue, the findings of our research are truly novel," Anastassiou says. Previously, he explains, "nobody had been able to attain this level of spatial and temporal resolution."
An "unexpected and surprising finding was how already very weak extracellular fields can alter neural activity," he says. "For example, we observed that fields as weak as one volt per meter robustly alter the spiking activity [firing] of individual neurons, and increase the so-called 'spike-field coherence'"—the synchronicity with which neurons fire. "Inside the mammalian brain, we know that extracellular fields may easily exceed two to three volts per meter. Our findings suggest that under such conditions, this effect becomes significant."
What does that mean for brain computation? At this point we can only speculate, Koch says, "but such field effects increase the synchrony with which neurons become active together. This, by itself, enhances the ability of these neurons to influence their target and is probably an important communication and computation strategy used by the brain."
Can external electric fields have similar effects on the brain? "This is an interesting question," Anastassiou says. "Indeed, physics dictates that any external field will impact the neural membrane. Importantly, though, the effect of externally imposed fields will also depend on the brain state. One could think of the brain as a distributed computer—not all brain areas show the same level of activation at all times.
"Whether an externally imposed field will impact the brain also depends on which brain area is targeted," he says. "During epileptic seizures, the hypersynchronized activity of neurons can generate field as strong as 100 volts per meter, and such fields have been shown to strongly entrain neural firing and give rise to super-synchronized states." And that suggests that electric field activity—even from external fields—in certain brain areas, during specific brain states, may have strong cognitive and behavioral effects.
Ultimately, Anastassiou, Koch, and their colleagues would like to test whether ephaptic coupling affects human cognitive processing, and under which circumstances. "I firmly believe that understanding the origin and functionality of endogenous brain fields will lead to several revelations regarding information processing at the circuit level, which, in my opinion, is the level at which percepts and concepts arise," Anastassiou says. "This, in turn, will lead us to address how biophysics gives rise to cognition in a mechanistic manner—and that, I think, is the holy grail of neuroscience."
The work in the paper, "Ephaptic coupling of cortical neurons," published January 16 in the advance online edition of the journal, was supported by the Engineering Physical Sciences Research Council, the Sloan-Swartz Foundation, the Swiss National Science Foundation, EU Synapse, the National Science Foundation, the Mathers Foundation, and the National Research Foundation of Korea.
Written by Kathy Svitil
Статья опубликована в свежем Nature Neuroscience, возглавлял исследование Christof Koch, один из лидеров современной нейрофизиологии. Эксперимент потребовал довольно ювелирной работы: чтобы зарегистрировать эффект локальных полевых потенциалов, необходимо было разместить в нервной ткани крысы 12 электродов, расстояние между которыми не превышало толщину человеческого волоса. При этом электроды помещались внутри и снаружи нейронов. Это, в общем, объясняет, почему механизм удалось продемонстрировать только сейчас. По словам исследователей, наличие полей позволяет достичь синхронизации активности групп из тысяч нейронов.
Первый автор, Costas Anastassiou, говорит:
"I firmly believe that understanding the origin and functionality of endogenous brain fields will lead to several revelations regarding information processing at the circuit level, which, in my opinion, is the level at which percepts and concepts arise. This, in turn, will lead us to address how biophysics gives rise to cognition in a mechanistic manner—and that, I think, is the holy grail of neuroscience." -- press release
C.A. Anastassiou, R. Perin, H. Markram, C. Koch (2011) Ephaptic communication in cortical neurons. - Nature Neuroscience [Abstract], [PDF]
Таким образом, картина взаимодействия нейронов серьезно усложняется. При моделировании процессов прохождения импульсов в мозге придется учитывать не только синаптические связи, но и наложение полей – что представляется практически неподъемной задачей. Энтузиастов эмуляции мозга на компьютере искренне жаль.
З.Ы. Практически следом еще одна новость, ломающая традиционные представления о работе нейрона! Rewrite the textbooks: Findings challenge conventional wisdom of how neurons operate -- PhysOrg
(PhysOrg.com) -- Neurons are complicated, but the basic functional concept is that synapses transmit electrical signals to the dendrites and cell body (input), and axons carry signals away (output). In one of many surprise findings, Northwestern University scientists have discovered that axons can operate in reverse: they can send signals to the cell body, too.
It also turns out axons can talk to each other. Before sending signals in reverse, axons can perform their own neural computations without any involvement from the cell body or dendrites. This is contrary to typical neuronal communication where an axon of one neuron is in contact with another neuron's dendrite or cell body, not its axon. And, unlike the computations performed in dendrites, the computations occurring in axons are thousands of times slower, potentially creating a means for neurons to compute fast things in dendrites and slow things in axons.
A deeper understanding of how a normal neuron works is critical to scientists who study neurological diseases, such as epilepsy, autism, Alzheimer's disease and schizophrenia.
The findings are published in the February issue of the journal Nature Neuroscience.
"We have discovered a number of things fundamental to how neurons work that are contrary to the information you find in neuroscience textbooks," said Nelson Spruston, senior author of the paper and professor of neurobiology and physiology in the Weinberg College of Arts and Sciences. "Signals can travel from the end of the axon toward the cell body, when it typically is the other way around. We were amazed to see this."
He and his colleagues first discovered individual nerve cells can fire off signals even in the absence of electrical stimulations in the cell body or dendrites. It's not always stimulus in, immediate action potential out. (Action potentials are the fundamental electrical signaling elements used by neurons; they are very brief changes in the membrane voltage of the neuron.)
Similar to our working memory when we memorize a telephone number for later use, the nerve cell can store and integrate stimuli over a long period of time, from tens of seconds to minutes. (That's a very long time for neurons.) Then, when the neuron reaches a threshold, it fires off a long series of signals, or action potentials, even in the absence of stimuli. The researchers call this persistent firing, and it all seems to be happening in the axon.
Spruston and his team stimulated a neuron for one to two minutes, providing a stimulus every 10 seconds. The neuron fired during this time but, when the stimulation was stopped, the neuron continued to fire for a minute.
"It's very unusual to think that a neuron could fire continually without stimuli," Spruston said. "This is something new -- that a neuron can integrate information over a long time period, longer than the typical operational speed of neurons, which is milliseconds to a second."
This unique neuronal function might be relevant to normal process, such as memory, but it also could be relevant to disease. The persistent firing of these inhibitory neurons might counteract hyperactive states in the brain, such as preventing the runaway excitation that happens during epileptic seizures.
Spruston credits the discovery of the persistent firing in normal individual neurons to the astute observation of Mark Sheffield, a graduate student in his lab. Sheffield is first author of the paper.
The researchers think that others have seen this persistent firing behavior in neurons but dismissed it as something wrong with the signal recording. When Sheffield saw the firing in the neurons he was studying, he waited until it stopped. Then he stimulated the neuron over a period of time, stopped the stimulation and then watched as the neuron fired later.
"This cellular memory is a novelty," Spruston said. "The neuron is responding to the history of what happened to it in the minute or so before."
Spruston and Sheffield found that the cellular memory is stored in the axon and the action potential is generated farther down the axon than they would have expected. Instead of being near the cell body it occurs toward the end of the axon.
Their studies of individual neurons (from the hippocampus and neocortex of mice) led to experiments with multiple neurons, which resulted in perhaps the biggest surprise of all. The researchers found that one axon can talk to another. They stimulated one neuron, and detected the persistent firing in the other unstimulated neuron. No dendrites or cell bodies were involved in this communication.
"The axons are talking to each other, but it's a complete mystery as to how it works," Spruston said. "The next big question is: how widespread is this behavior? Is this an oddity or does in happen in lots of neurons? We don't think it's rare, so it's important for us to understand under what conditions it occurs and how this happens."
More information: The title of the paper is “Slow Integration Leads to Persistent Action Potential Firing in Distal Axons of Coupled Interneurons.” http://www.nature.com/neuro/journal/v14/n2/full/nn.2728.html
Provided by Northwestern University
Сигнал по аксону идет в обратном направлении, нейроны возбуждаются в отсутствие стимуляции. И тоже в Nature Neuroscience.
Источник: http://nature-wonder.livejournal.com/195030.html, http://media.caltech.edu/press_releases/13401
|Автор:||bopa [ 27 листопада 2012, 13:31 ]|
|Тема повідомлення:||Re: Иной способ связи нейронов - электрическое поле|
Не слід впадати в песимізм!
По-перше, питання суперпозиції електромагнітних хвиль не нове і достатньо вивчене, як прямих (вивчення поля) так і обернених задач (по заданому полю знайти характеристики об'єкту). Наприклад, принцип роботи приладів томографії (МРТ і т.п.) в медицині. Цікавіше практичне використання відомих розробок, хоча б для інвалідів по зору, наприклад, (сліпих). І тут є певні успіхи на експериментальному (лабораторному) рівні. Росіяни розробляють ситеми управління РС думкою. Так що потрібно шукати своє місце в цьому перспективному напрямку!
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