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I was reading a paper investigating using MRI as a way to determine peripheral nerve regeneration: https://sci-hub.do/10.1016/j.expneurol.2009.10.012

They mention that

... nerve conduction studies are widely used but these studies are limited only to the distal parts of peripheral nerves and not suitable to measure regeneration early on and in response to proximal lesions (Kimura, 1999). Furthermore,after complete axonal injury, particularly as it relates to traumatic nerve injuries or proximal plexus lesions, the distal portions of nerves are often inexcitable, which limits electrodiagnostic testing's utility to assess the degree of regeneration in proximal nerve trunks.

I understand this because normal nerve conduction studies wrap an electrode on your finger (where current goes in) and reads it down stream at your wrist.

enter image description here

I totally get why this wouldn't work if the injury occurred very high up at say the elbow, because the signal could not travel from your finger upwards since it's all degenerated.

However, what if we instead stimulated the nerve a bit above the elbow (maybe using a needle) and measured the signal downstream. We could move closer and closer until a signal was detected. This is where the nerve has regenerated to - thus this method CAN detect peripheral nerve regeneration. What is wrong with my reasoning?

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  • $\begingroup$ I think that theoretically there's no fault in your reasoning. However, technically, to access the regenerating axon at the elbow level could be almost impossible. I also think that the labels in your draw should be: (in -> recording; out -> stimulation) $\endgroup$
    – heracho
    Feb 5 '21 at 15:28
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Remember that peripheral nerves come all the way from the spinal cord. When they write of

traumatic nerve injuries or proximal plexus lesions

they are talking about injuries to the trunk, nerves running more or less around the collarbone. They're not talking about elbow vs wrist. See for example https://en.wikipedia.org/wiki/Brachial_plexus and the image there from Gray's:

enter image description here

They're also not necessarily saying that it is physically impossible to do a nerve conduction study in these areas with traditional means, they are talking about practical matters and use in-practice, and their paper is touting their method using noninvasive imaging.

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  • $\begingroup$ Great points but I don't see what's so impractical to stick a few pads on someone's collar bones? Is it that we would actually need to stick deep needles into the person? $\endgroup$
    – John Hon
    Feb 6 '21 at 1:46
  • $\begingroup$ @JohnHon Yeah the surface isn't anywhere near those nerves. $\endgroup$
    – Bryan Krause
    Feb 6 '21 at 2:25

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