Recording membrane voltage along an axon

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Recording membrane voltage along an axon

Post by edaneshi »

Dear Ted,

I have made a simple model of an axon with 21 nodes. I use Clamp() to stimulate the first node of my axon. All nodes (and the same idea for internode) have similar properties (Passive + HH mechanisms). I recorded membrane voltage at all nodes (node[1].v(0.5),node[2].v(0.5),...). Surprisingly, after importing my data to Matlab and plotting them, I see that the membrane voltage increases at the last node and the one before that. I also downloaded a model from Model DB and repeated my experiment. Again, I observed the same thing even when stimulation happened in the middle of the axon. I was wondering if this is something rated to cable theory and/or the way NEURON processes signal propagation in a finite cable model. I would really appreciate it if you could kindly help me understand this issue better.
Once again, thank you for your kind support and help.

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Re: Recording membrane voltage along an axon

Post by ted »

Good observation, and also good that you didn't just shrug and say "whazzat?" before turning to something else. Read about "sealed end effect," sometimes called the "closed end effect." The original experimental papers about this date back maybe 60-80 years to authors like Katz, Grundfest, and Rushton, so you're unlikely to find them by googling or searching pubmed, or in practically any contemporary neuroscience coloring book--oops, textbook.

Sealed end effect falls right out cable theory and the partial differential equation that describes the spread of charge and potential along a cable. del Castillo and Katz mention "closed end effect" almost in passing in their article
The effect of magnesium on the activity of motor nerve endings.
J Physiol. 1954 Jun 28;124(3):553-9
which shows that this was already settled science by the early 1950s. You might try some searches for this term at the Journal of Physiology (London) web site. Most of those old articles are available at no charge, and most are quite readable and informative. Not a lot of me-too science in those days.
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