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The Journal of Neuroscience, August 1, 2002, 22(15):6704-6712

Neural Signal Processing: The Underestimated Contribution of Peripheral Human C-Fibers

Christian Weidner¹, Martin Schmelz¹, Roland Schmidt², Björn Hammarberg², Kristin Ørstavik³, Marita Hilliges⁴, H. Erik Torebjörk², and Hermann O. Handwerker¹

¹ Department of Physiology and Experimental Pathophysiology, University of Erlangen/Nürnberg, 91054 Erlangen, Germany, ² Department of Clinical Neurophysiology, University of Uppsala, 75185 Uppsala, Sweden, ³ Department of Neurology, Ullevål Hospital, 0407 Oslo, Norway, and ⁴ Department of Basic Oral Sciences, Karolinska Institutet, 14104 Huddinge, Sweden

The microneurography technique was used to analyze use-dependent frequency modulation of action potential (AP) trains in human nociceptive peripheral nerves. Fifty-one single C-afferent units (31 mechano-responsive, 20 mechano-insensitive) were recorded from cutaneous fascicles of the peroneal nerve in awake human subjects. Trains of two and four suprathreshold electrical stimuli at interstimulus intervals of 20 and 50 msec were applied to the receptive fields of single identified nociceptive units at varying repetition rates. The output frequency (interspike interval) recorded at knee level was compared with the input frequency (interstimulus interval) at different levels of accumulated neural accommodation.

At low levels of use-dependent accommodation (measured as conduction velocity slowing of the first action potential in a train), intervals between spikes increased during conduction along the nerve. At increasing levels of neural accommodation, intervals decreased because of a relative supernormal period (SNP) and asymptotically approached the minimum "entrainment" interval of the nerve fiber (11 ± 1.4 msec) corresponding to a maximum instantaneous discharge frequency (up to 190 Hz).

For neural coding, this pattern of frequency decrease at low activity levels and frequency increase at high levels serves as a mechanism of peripheral contrast enhancement. The entrainment interval is a good minimum estimate for the duration of the refractory period of human C-fibers.

At a given degree of neural accommodation, all afferent C-units exhibit a uniform pattern of aftereffects, independent of fiber class. The receptive class of a fiber only determines its susceptibility to accommodate. Thus, the time course of aftereffects and existence or absence of an SNP is fully explained by the amount of preexisting accommodation.

Key words: microneurography; C-fibers; neural coding; refractory period; accommodation; post-excitatory effects

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Copyright © 2002 Society for Neuroscience  0270-6474/02/22156704-09$05.00/0

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