Conduction properties distinguish unmyelinated sympathetic efferent fibers and unmyelinated primary afferent fibers in the monkey

Matthias Ringkamp, Lisa M. Johanek, Jasenka Borzan, Timothy V. Hartke, Gang Wu, Esther M. Pogatzki-Zahn, James N. Campbell, Beom Shim, Raf J. Schepers, Richard A. Meyer

Research output: Contribution to journalArticlepeer-review

Abstract

Background:Different classes of unmyelinated nerve fibers appear to exhibit distinct conductive properties. We sought a criterion based on conduction properties for distinguishing sympathetic efferents and unmyelinated, primary afferents in peripheral nerves. Methodology/Principal Findings: In anesthetized monkey, centrifugal or centripetal recordings were made from single unmyelinated nerve fibers in the peroneal or sural nerve, and electrical stimuli were applied to either the sciatic nerve or the cutaneous nerve endings, respectively. In centrifugal recordings, electrical stimulation at the sympathetic chain and dorsal root was used to determine the fiber's origin. In centrifugal recordings, sympathetic fibers exhibited absolute speeding of conduction to a single pair of electrical stimuli separated by 50 ms; the second action potential was conducted faster (0.61 ∀ 0.16%) than the first unconditioned action potential. This was never observed in primary afferents. Following 2 Hz stimulation (3 min), activity-dependent slowing of conduction in the sympathetics (8.6 ∀ 0.5%) was greater than in one afferent group (6.7 ∀ 0.5%) but substantially less than in a second afferent group (29.4 ∀ 1.9%). In centripetal recordings, most mechanically-insensitive fibers also exhibited absolute speeding to twin pulse stimulation. The subset that did not show this absolute speeding was responsive to chemical stimuli (histamine, capsaicin) and likely consists of mechanically-insensitive afferents. During repetitive twin pulse stimulation, mechanosensitive afferents developed speeding, and speeding in sympathetic fibers increased. Conclusions/ Significance: The presence of absolute speeding provides a criterion by which sympathetic efferents can be differentiated from primary afferents. The differences in conduction properties between sympathetics and afferents likely reflect differential expression of voltage-sensitive ion channels.

Original languageEnglish (US)
Article numbere9076
JournalPloS one
Volume5
Issue number2
DOIs
StatePublished - Feb 5 2010

ASJC Scopus subject areas

  • General

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