The researchers also looked at how far away from the soma the axon began, with particular emphasis on how long the initial segment region was. They found that the AcDs could generate spikes at lower threshold and were also more synaptically excitable than dendrites without axons. When they hit particular regions with a low intensity laser, glutamate could be released while they listend to the cell with a patch clamp electrode. They used a precision technique called two-photon uncaging to activate just a few spines at a time in a select dendritic region of the cell. The authors suggest there might be some method to this madness, namely that the particular dendrite that presumably controls the axon has certain useful "privileges". Unfortunately for the purely electrophysiological interpreation of cell structure, it now seems that a neuron placing its axon off in the electrically isolated world of a single basal dendrite would be the equivalent of growing your head out from your elbow. There are a few neural eunuchs floating around the invertebrate population (and in special places like the retina) with no axon to speak of, but for the most part they are a given in neurons. There's no astrocytes in their models because astrocytes don't have axons. Until now the axon was the one constant on which modelers could depend. Not only that, but where to keep the axon once established, or where to move it should the grass look greener, becomes even trickier. If the whole dendritic tree, or at least the local parts are now fair game, the question of where exactly a neuron should put its axon just got real. There is an inexhaustable supply of possible proteins to implicate in the process, and the game is never really over until it is over. Watching developing neurons under the time-lapse microscope as they pick one dendrite among many to crown as the axon is a source of wonderment for many neurobiologists. But using better imaging techniques the researchers now found that it is the basal dendrites below where the AcDs are more likely to be found. In other words, the upper regions of the extended cell. It had been previously shown by other researchers that around 5% of CA1 pyramidal cells had axons shooting out of their apical dendrites. The researchers had a look at the other parts of the hippocampus as well and found that about a quarter of the pyramidal cells there had AcDs too. While it probably shouldn't be applied to situations where the majority is only 51 percent, if 20 out of the 36 cells you checked on had AcDs, it's probably at least a trend. Many would argue about what percent of something the word "most" actually means. Rather than mere quirk or curiousity the axon carrying dendrite, or AcD as the authors call it, is found on most neurons-at least among pyramidal cells in the CA1 area of the mouse hippocampus. A paper recently published in Neuron now suggests that minimizing this fundamental neural character has been a mistake. The existence of these anomalies is an inconvenient truth for all neuron modelers and typically they choose to ignore them. Certain interneurons, dopaminergic cells, or neuroendicrine cells for example, have been shown to grow their axon out from a lucky dendrite instead of the soma. ![]() There have always been exceptions to this neat and tidy picture of a neuron. The electrical energy of these signals is generally believed to be integrated at the cell body and converted into pulses at the axon initial segment (this is the region where the axon sprouts, the AIS). (Medical Xpress)-The well-behaved neuron receives signals through its many dendrites to generate spikes on a single axon.
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