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The Journal of Neuroscience, November 15, 1999, 19(22):10184-10190
Functional Attributes Discriminating Mechano-Insensitive and
Mechano-Responsive C Nociceptors in Human Skin
C.
Weidner1,
M.
Schmelz1,
R.
Schmidt2,
B.
Hansson2,
H. O.
Handwerker1, and
H. E.
Torebjörk2
1 Department of Physiology and Experimental
Pathophysiology, University of Erlangen/Nürnberg, D-91054
Erlangen, Germany, and 2 Department of Clinical
Neurophysiology, University of Uppsala, S-75185 Uppsala, Sweden
Microneurography was used in healthy human subjects to record
action potentials from unmyelinated nerve fibers (C units) in cutaneous
fascicles of the peroneal nerve.
Activity-dependent slowing (n = 96) and
transcutaneous electrical thresholds (n = 67) were
determined. Eight units were sympathetic efferents according to their
responses to sympathetic reflex provocations. Mechano-heat-responsive C
units (CMH) (n = 56) had thresholds to von Frey
hair stimulation 90 mN (6.5 bar). Mechano-insensitive C units
(n = 32) were unresponsive to 750 mN (18 bar).
Twenty-six mechano-insensitive units responded to heat (CH), and the
remaining six units did not respond to physical stimuli but were proven to be afferent by their response to intracutaneous capsaicin
(CMiHi).
Mechano-insensitive units had significantly slower conduction velocity
(0.81 ± 0.03 m/sec), and CH units had higher heat thresholds (48.0 ± 0.6°C) compared with CMH units (1.01 ± 0.01 m/sec; 40.7 ± 0.4°C). Transcutaneous electrical thresholds were
<9 mA for CMH units and >35 mA for CH and
CMiHi units. Activity-dependent slowing was
much more pronounced in mechano-insensitive than in mechano-responsive
units, without overlap. Sympathetic efferent C units showed
intermediate slowing, significantly different from CMH, and completely
separate from CH and CMiHi units. The
activity-dependent slowing of conduction provides evidence for
different membrane attributes of different classes of C fibers in humans.
Key words:
mechanosensitivity; human C fibers; activation-dependent slowing; microneurography; afterhyperpolarization; pain
Copyright © 1999 Society for Neuroscience 0270-6474/99/192210184-07$05.00/0
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