Dear colleagues!
Tell me, please, why does the velocity of nerve impulse conduction along the nerve decrease when the nerve is stretched?
Maybe there are some mathematical models that describe this effect?
Best regards, Vladislav
Hi Vladislav, I am not a biologist so I am probably not the best person to answer such a biological question; however, from my knowledge in math modelling of neurons I can try to provide possible explanations and observations.
First, I would have expected that the velocity of the nerve signal INCREASES as the axon is stretched. In fact, the electric signal "jumps" from one node of Ranvier to the next one: that's why we speak about "saltatory conduction" for the impulse (the nodes of Ranvier are the only parts of the axon that are not covered by a myelin sheath). Then, if the axon is stretched the nodes of Ranvier become more distant from each other and so the signal would make "longer" jumps and thus travel faster.
However, it could be that as the axon is stretched, it is also partly damaged, and that could affect the efficiency in generating an action potential at each node of Ranvier...
From what source did you get the information that the nerve impulse is slower when the axon is stretched? Perhaps some other experimental settings could explain this phenomenon...
Dear, Arianna! A neurologist and I conducted experiments on abduction of the foot, in which the tibial nerve should seem to stretch, since in the area of the ankle joint the nerve passes in the tarsal canal (flexor retinaculum), which limits the possibility of nerve displacement so that its length does not change. Perhaps, this is not a stretching of the nerve, but its compression. We want to create a biomechanical model of this process. When the nerve is lengthened, the width (length) of the nodes of Ranvier may increase, i.e. the area (surface) of the nerve that is not covered by myelin increases. This possibly brings the conduction of the nerve impulse closer to continuous, i.e. as for unmyelinated nerve fibers.
I see what you mean by making nodes of Ranvier "longer": that is a possibility; but this would have to be compared with possible elongation of myelin sheaths as well (i.e. the signal could be slowed down in longer nodes of Ranvier, but at the same time make longer "jumps").
I also thought about another possible explanation of the phenomenon that you observe: could it be that the axon diameter is decreased as you stretch it? We know that the propagation velocity is proportional to the (square root of the) axon diameter, so if the diameter is decreased, so will be the velocity.
I think it is important, in any case, to know what is happening to the myelin sheath: is it stretching as well? Is it by any chance getting thinner? A thinner myelin sheath could also imply a smaller membrane resistance...
I don't know what in particular you want to focus on in your model, but it could be relevant for you to consider time and length constants (see for instance https://nba.uth.tmc.edu/neuroscience/m/s1/chapter03.html).
Hope this is useful! :-)
Dear, Arianna!
Thank You very much for Your detailed answer and the link to the materials.
I wrote to Dr. Gabriel Corfas from Boston who probably knows almost everything about Schwann cells and their attachment to the axon.
I really hope that he will tell you where to go and what we see in practice, i.e. the Schwann cages are stretched and the nodes of Ranvier remain unchanged, or the Schwann cages are not stretched, and the nodes of Ranvier become wider, or both of these cases occur simultaneously.
I understand that the diameter of the axon probably decreases when the nerve fiber is stretched, and this leads to an increase in resistance (similar to electricity through a wire), but I also want to understand how Schwann cells (myelin layer) behave.
I also agree with Your opinion, that when the nerve fiber is stretched, the myelin sheath becomes thinner, which means that the resistance of the myelin sheath decreases. Accordingly, greater value of current begins to flow through the myelin layer, which means that the current along the nerve fiber decreases than in norm.
Best regards, Vladislav.
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