"This cellular memory is a novelty," Spruston said. Then he stimulated the neuron over a period of time, stopped the stimulation and then watched as the neuron fired later. When Sheffield saw the firing in the neurons he was studying, he waited until it stopped. The researchers think that others have seen this persistent firing behavior in neurons but dismissed it as something wrong with the signal recording. 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. 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. This unique neuronal function might be relevant to normal process, such as memory, but it also could be relevant to disease. "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." "It's very unusual to think that a neuron could fire continually without stimuli," Spruston said. The neuron fired during this time but, when the stimulation was stopped, the neuron continued to fire for a minute. Spruston and his team stimulated a neuron for one to two minutes, providing a stimulus every 10 seconds. The researchers call this persistent firing, and it all seems to be happening in the axon. (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. 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. (Action potentials are the fundamental electrical signaling elements used by neurons they are very brief changes in the membrane voltage of the neuron.) It's not always stimulus in, immediate action potential out. 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. "Signals can travel from the end of the axon toward the cell body, when it typically is the other way around. "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. The findings are published in the February issue of the journal Nature Neuroscience. 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.Ī 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. 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. Before sending signals in reverse, axons can perform their own neural computations without any involvement from the cell body or dendrites. It also turns out axons can talk to each other.
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