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Nerve stimulation by E field

Timothy E Vaughan tvaughan at athena.mit.edu
Tue Nov 15 13:58:27 EST 1994

I am trying to find out the magnitude of the electric field
that is required to stimulate sensory nerves in hairy skin.
I am interested in two different perspectives on this problem:

Direct, empirical perspective:

People have place electrodes on the surface of the skin and
applied pulses (for example in evoked potential or nerve
conduction tests?).  These people must know the threshold of
field (or voltage or current) that can be applied to the
surface before pain occurs.

Indirect, theoretical perspective:

It is known that receptors are activated when their membranes
are depolarized.  Therefore, given information about the size,
depth, and distribution of the receptors, one could estimate
the field strength that one would have to apply to the surface
in order to activate the receptors.

I have explored both of these perspectives in the literature,
but I have been a little frustrated.  In the first case, I have
found little quantitative information about the stimulating
pulse.  The articles I have found are only quantitative about
the evoked potential.

In the second case, I have found the following factoids
(please correct any misinformation):

1)  Pain is first "triggered" by the polymodal nociceptors.

2)  These nociceptors are the least differentiated of the sensory
    receptors.  They exist as free (or bare) nerve endings that
    do not have peripheral structures to transduce and filter

3)  These free nerve endings reach to about the epidermal-dermal

4)  The nociceptors are the "Type C" afferent fibers, unmyelinated,
    with a diameter of 0.2-1.5 microns.  (However, they may have
    some electrically insulating covering?)

[Note that most of my factoids are condensed from Kandel &
Schwartz, which I am told is the definitive neural science
textbook.  I have quoted freely from it, but any mistakes
are probably my own.]

I have not been able to find anything about the spatial
density of the free nerve endings (i.e., the number per
square centimeter of skin surface).  I am also not 100%
sure that "Type A" fibers cannot be receptors for pain.

So!  If you have read this far, you are a wonderful person for
being so interested in my problem.  Does anyone have a reference
where I can find a QUANTITATIVE discussion of these issues of
nerve distribution and/or field thresholds?  Please don't say
"It's in any neuro book" or "Look up evoked potential stuff".
I have done this, and traced back through lots of references.
Can someone please provide SPECIFIC references!

Thanks in advance for any help!

Tim Vaughan
Postdoctoral Associate
Harvard-MIT Division of Health Science and Technology

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