Posts tagged action potential
Articles and news from the latest research reports.
Brain Noise Found to Nurture Synapses
A study has shown that a long-overlooked form of neuron-to-neuron communication called miniature neurotransmission plays an essential role in the development of synapses, the regions where nerve impulses are transmitted and received. The findings, made in fruit flies, raise the possibility that abnormalities in miniature neurotransmission may contribute to neurodevelopmental diseases. The findings, by researchers at Columbia University Medical Center (CUMC), were published today in the online edition of the journal Neuron.
The primary way in which neurons communicate with each another is through “evoked neurotransmission.” This process begins when an electrical signal, or action potential, is transmitted along a long, cable-like extension of the neuron called an axon. Upon reaching the axon’s terminus, the signal triggers the release of chemicals called neurotransmitters across the synapse. Finally, the neurotransmitters bind to and activate receptors of the neuron on the other side of the synapse. Neurotransmitters are packaged together into vesicles, which are released by the hundreds, if not thousands, with each action potential. Evoked neurotransmission was first characterized in the 1950s by Sir Bernard Katz and two other researchers, who were awarded the 1970 Nobel Prize in Physiology or Medicine for their efforts.
“Dr. Katz also found that even without action potentials, lone vesicles are released now and then at the synapse,” said study leader Brian D. McCabe, PhD, assistant professor of pathology and cell biology and of neuroscience in the Motor Neuron Center. “These miniature events — or minis — have been found at every type of synapse that has been studied. However, since minis don’t induce neurons to fire, people assumed they were inconsequential, just background noise.”
Recent cell-culture studies, however, have suggested that minis do have some function and even their own regulatory mechanisms. “This led us to wonder why there would be such complicated mechanisms for regulating something that was just noise,” said Dr. McCabe.
To learn more about minis, the CUMC team devised new genetic tools to selectively up- or down-regulate evoked and miniature neurotransmission in fruit flies (a commonly used model organism for neuronal function and development). This was the first study to identify a unique role for minis in an animal model.
The researchers found that when both types of neurotransmission were blocked, synapse development was abnormal. However, inhibiting or stimulating evoked neurotransmission alone had no effect on synaptic development. “But when we blocked minis, synapses failed to develop,” said Dr. McCabe, “and when we stimulated the release of more minis, synapses got bigger.”
The study also showed that minis regulate synapse development by activating a signaling pathway in neurons involving Trio and Rac1 proteins in presynaptic neurons. These proteins are also found in humans.
It remains to be seen exactly how minis are exerting their effects. “Parallel communication occurs in computer networks,” Dr. McCabe said. “Computers communicate primarily by sending bursts of data bundled into packets. But individual computers also send out pings, or tiny electronic queries, to determine if there is a connection to other computers. Similarly, neurons may be using minis to ping connected neurons, saying in effect, ‘We are connected and I am ready to communicate.’”
The researchers are currently looking into whether minis have a functional role in the mature nervous system. If so, it’s possible that defects in minis could contribute to neurodegenerative disease.
Listen!
How nerve cells flexibly adapt to acoustic signals: Depending on the input signal, neurons generate action potentials either near or far away from the cell body. This flexibility improves our ability to localize sound sources.
(Image caption: A neuron in the brain stem, that processes acoustic information. Depending on the situation, the cell generates action potentials in the axon (thin process) either close to or far from the body. Photo: Felix Felmy)
In order to process acoustic information with high temporal fidelity, nerve cells may flexibly adapt their mode of operation according to the situation. At low input frequencies, they generate most outgoing action potentials close to the cell body. Following inhibitory or high frequency excitatory signals, the cells produce many action potentials more distantly. This way, they are highly sensitive to the different types of input signals. These findings have been obtained by a research team headed by Professor Christian Leibold, Professor Benedikt Grothe, and Dr. Felix Felmy from the LMU Munich and the Bernstein Center and the Bernstein Focus Neurotechnology in Munich, who used computer models in their study. The researchers report their results in the latest issue of The Journal of Neuroscience.
Did the bang come from ahead or from the right? In order to localize sound sources, nerve cells in the brain stem evaluate the different arrival times of acoustic signals at the two ears. Being able to detect temporal discrepancies of up to 10 millionths of a second, the neurons have to become excited very quickly. In this process, they change the electrical voltage that prevails on their cell membrane. If a certain threshold is exceeded, the neurons generate a strong electrical signal — a so-called action potential — which can be transmitted efficiently over long axon distances without weakening. In order to reach the threshold, the input signals are summed up. This is achieved easier, the slower the nerve cells alter their electrical membrane potential.
Input signals are optimally processed
These requirements — rapid voltage changes for a high temporal resolution of the input signals, and slow voltage changes for an optimal signal integration that is necessary for the generation of an action potential — represent a paradoxical challenge for the nerve cell. “This problem is solved by nature by spatially separating the two processes. While input signals are processed in the cell body and the dendrites, action potentials are generated in the axon, a cell process,” says Leibold, leader of the study. But how sustainable is the spatial separation?
In their study, the researchers measured the axons’ geometry and the threshold of the corresponding cells and then constructed a computer model that allowed them to investigate the effectiveness of this spatial separation. The researchers’ model predicts that depending on the situation, neurons produce action potentials with more or less proximity to the cell body. For high frequency or inhibitory input signals, the cells will shift the location from the axon’s starting point to more distant regions. In this way, the nerve cells ensure that the various kinds of input signals are optimally processed — and thus allow us to perceive both small and large acoustic arrival time differences well, and thereby localize sounds in space.
(Source: en.uni-muenchen.de)