Hi Tania and Richard:
> When you find out any papers on this subject, please share with us.
I stumbled across a short, authoritative discussion
about why a mechanical vibration can feel like a
small electric shock. Actually, it is the other
way around. At small voltages and currents, the
stimulation is a mechanical stimulation, not an
electrical one.
This quote is from:
Applied electricity from electrical stimulation
to electropathology
by J. Patrick Reilly
Springer Verlag New York
ISBN 0-387-98407-0
*****
7.1 Introduction
Sensory sensitivity to electrical stimulation depends on a host of
factors associated with the stimulus waveform, its method of delivery,
and subjective variables. In most situations involving electrical
safety or acceptability, current is applied to the body by cutaneous
electrodes. There are also practical applications in which electric
current may be applied subcutaneously or induced internally by external
electromagnetic fields. Although the emphasis in this chapter is on
electrocutaneous stimulation, many of the principles discussed may be
applied to other modes of stimulation. The reader is directed to
Chapter 9 for additional discussion of peripheral nerve stimulation by
time-varying magnetic field effects or by induced shock within intense
electric field environments. In addition to sensory effects described
in this chapter, stimulation by electric current and electromagnetic
fields can also elicit visual and auditory sensations. These will be
treated in Sect. 9.8.
7.2 Mechanisms of Electrical Transduction
Current of a fraction of a microampere can be detected when the finger
is gently drawn across a surface charged with small AC potentials
(Grimnes, 1983b). Such levels are roughly 100 times less than commonly
tested electrical thresholds. Detection of such small current results
from electromechanical forces arising from electrostatic compression
across the stratum corneum (the outermost layer of dead skin cells). As
analyzed by Grimnes, the electrostatic force K is
AEV**2
K = ---------------
2d**2
(7.1)
where A is the contact area, E is the dielectric constant of the
corneum, d is its thickness, and v is the instantaneous voltage. The
compression of the corneum would not normally be sensed. But when the
skin is moved along the charged surface, there is a vibratory frictional
force on the finger that is maximized on each half-cycle of the
alternating voltage. This vibrational force stimulates mechanoreceptors
and is responsible for the detection of microampere currents. Grimnes
estimates that the minimum voltage contributing to a detectable
vibration is about 1.5V at 5OHz.
The detection of microampere currents through mechanical vibration is,
for most purposes, of passing interest, although it may be important for
a researcher to know about it when designing perception tests. Of
greater significance is the mode of detection when the current level is
raised to roughly 0.1 mA or above. At that point, perception can be
initiated by the electrical excitation of neural structures, according
to the mechanisms discussed in Chapters 3 and 4.
Exactly what is excited with electrocutaneous stimulation, and what is
the specific site of initiation? At the lowest levels of stimulation,
it is likely that peripheral structures are involved, because these are
closest to the surface electrode. Among fiber classes, the larger-
diameter myelinated fibers have the lowest electrical thresholds, and
circumstantial evidence presented in this chapter points to the
involvement of one or another class of mechanoreceptor. The precise
site of cutaneous electrical stimulation is unknown; whether stimulation
is initiated at the axon proper, at the site of the generator potential
of sensory receptors, or along free nerve endings has not been
demonstrated. Some evidence, however, exists, as noted in Chapter 4,
that the site of initiation is near neural end structures, including
receptors or free nerve endings. Electrocutaneous perception is a local
phenomenon; subjects typically report sensation occurring locally at the
electrode site rather than remotely as might be supposed if the
excitation occurred on the axons of deeper-lying nerves. It is only
when the current is raised substantially above the perception level that
distributed sensations are felt.
If the current is raised sufficiently above the threshold of perception,
excitation of unmyelinated nociceptors becomes possible. Because of
their higher electrical thresholds and generally deeper sites, these
structures are not likely to be involved at perception threshold levels.
At still higher current levels (some tens of milliamperes for
long-duration stimuli), thermal detection due to tissue heating becomes
possible. Neuroelectric thresholds may exceed thermal thresholds if the
waveform of the electric current is inefficient for electrical
stimulation, such as with sinusoidal currents of very high frequency
(>10 Hz).
*****
The reference to Grimnes is:
Grimnes, S. (1983a). Dielectric breakdown of human skin in vivo.
Med. Biol. Eng. Comp. 21: 379-381.
*****
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