The conventional view of neurons is that synaptic inputs are integrated on a timescale of milliseconds to seconds in the dendrites, with action potential initiation occurring in the axon initial segment. We found a much slower form of integration that leads to action potential initiation in the distal axon, well beyond the initial segment. In a subset of rodent hippocampal and neocortical interneurons, hundreds of spikes, evoked over minutes, resulted in persistent firing that lasted for a similar duration. Although axonal action potential firing was required to trigger persistent firing, somatic depolarization was not. In paired recordings, persistent firing was not restricted to the stimulated neuron; it could also be produced in the unstimulated cell. Thus, these interneurons can slowly integrate spiking, share the output across a coupled network of axons and respond with persistent firing even in the absence of input to the soma or dendrites.
_Abstract NatureNeuroSci
Our brains contain about 100 billion neurons, with about 10 billion of those in the neocortex. There are perhaps 40 million neurons in the hippocampus, naturally decreasing with age. Each neuron in the brain is a computer in itself. Connected together in cortical columns and short, medium, and long-range networks, the collection of neurons in a human brain possesses complexity of behaviour beyond comprehension. Science is still learning new things about how neurons function individually and in small groups.
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. _PO
The real complexity does not even arise until you go up at least a couple of logical levels of brain function from the neuron. So if science is still learning basic facts about neuronal function, it is likely that there is quite a bit left to learn at multiple levels.
Mammalian brains -- particularly primate and cetacean brains -- are amazing universes where spontaneous order is created out of chaos. The most adventurous of these brains wants to not only understand itself and its world: it wants to know what else is out there.
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