Why can’t neurons regenerate themselves like other cells?
In: Biology
Well the proximal reason is simple, they’re just quiescent cells, they do not have preprogrammed responsiveness to proliferative stimuli. In English: their identity or the part of the genetic code they apply does not permit them to respond to signals that normally tell cells to divide. Of course it’s not so simple, when we get into olfactory neurons and others, but let’s keep it simple.
The ultimate reason is a different story. That is, why are neurons not supposed to divide? Well, the same reason you can’t regenerate an arm, kind of. You can technically repair/regenerate many parts of your arm, like blood vessels, muscles, bones, etc. Why not the whole thing? Because it’s too complicated. It’s like origami, if you tear a piece of it off, why can’t you just repair it? You built it afterall. Because the final shape came from so many gradual processes that were built over one another. You need to start from scratch. Now that’s why you can’t grow an arm, but how is that similar to neurons?
Well, the functionality of the brain does not come from any one or even all of its neuron, it comes from the sum of the neurons organized in a particular architecture and fine tuned to operate at the edge of chaos. If we look at a simple peripheral neuron, that is one going from the spinal column to your finger for instance, this neuron when it was getting generated, it grew from the spinal column and took a very particular path to make a connection with a particular target. This process is called axonal guidance, whereby a neuron is attracted to chemicals trailing in front of it and repelled by chemicals surrounding it, sort of like the road and the sidewalk. And this allows it to find its way to the target. This neuron never had any programming to tell it to go exactly to that one muscle or sensory cell in the finger, it just followed whatever path was in front of it, and the path changed the programming of the neuron. As the human grows, the brain learns ahh if I send a signal in this particular neuron, I move this finger. So the identity of the neuron came first from actually fulfilling its role in the future and from the farther future, where the brain used it to represent something. So now of you kill this neuron, how’s it gonna regenerate? There’s no more highway, there’s no more path, that was a property of the developing human, at a very particular stage in time. If this neuron wasn’t all just killed at once, but rather severed, it can still regenerate, because (some) neurons are surrounded by a sheathe, and if that sheathe is partially intact, the neuron can regrow in it to find its other half. Look up Wallerian degeneration.
So now we talked about a simple peripheral neuron. Brain neurons are MUCH more complex. Having a target to reach is one of the simpler aspects of such neurons, their real complexity comes from their interactions. That neuron that went to your finger, a higher up neuron in the brain connects with it, and that neuron is also integrated in a circuit related to proprioceprors (those sensors that sense tension in muscles, tendons, etc, to tell position or bodily orientation with respect with a force like gravity), related to mechanoreceptors (that facilitate sensing touch and heat and so on), etc. Now if you kill THAT neuron, well.. The new neuron has to not only know where to go and what to connect with, but also what to get connected to. Everything in the brain is representation by association, so the entire function and identity of a central neuron comes from its position in the whole network, and that’s far far too complex to fix. Even if you do end up evolving a way to fix such damage, it’s too risky. Like I said, the brain operates on the edge of chaos, it is so fine tuned, it’s a self organized criticality, you cannot afford even tiny mistakes. Despite all that, the brain is terrific at reorganizing itself to compensate for functional damage, it just cannot directly replace physical damage.
Latest Answers