Discovery rewriting the evolutionary history of the nervous system

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A component of vertebrate neurons – known as the axon initial segment (AIS) – that is responsible for regulating the nerve cell’s output has long been thought by scientists to have evolved relatively recently, and specifically in vertebrates, in order to enable rapid, precise signaling in the complex circuitry of the vertebrate nervous system.But a discovery made by the labs of Penn State researchers Melissa Rolls and Tim Jegla is bringing about a rewriting of a fundamental aspect of that story.
“One of the things I’ve been doing for a very long time,” said Jegla, “is studying nervous system evolution, and in general what we find is that most fundamental features of neurons – from the molecules that control the signaling to various cellular structures – tend to be old and conserved and shared between many extant species, like all the bilaterians, and the signaling molecules are even in very early nervous systems in cnidarians.”
“But one of the exceptions to this in the literature – coincidentally, the story we’re rewriting,” he said, “is that this very special area of a neuron called the axon initial segment, which is the beginning of the output end of the neuron, is a unique vertebrate adaptation for rapid signaling. The axon initial segment provides a barrier to separate the membrane of the cell body and the input end – the dendrites – from the axon, which is critical because this membrane has a different role, which is to initiate and then carry the action potential – the output of the neuron. You’ve got all these inputs coming in from the cell body and the dendrites, and it’s at the axon initial segment where the decision is made whether all those inputs warrant signaling the next cell, or not. So it’s the decision point for neurons.”
Jegla further explains that the structure and function of ...

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