Thermosensitivity of Acutely Axotomized Sensory Nerve Fibers.
BLENK, KARL-HEINZ, MARTIN MICHAELIS, CAROLA VOGEL, AND WILFRID J[umlaut]aNIG.
Physiologisches Institut, Christian-Albrechts-Universitat, 24098 Kiel,
APStracts 3:0048N, 1996.
SUMMARY AND CONCLUSIONS
1. Thermosensitivity of axotomized myelinated and unmyelinated fibers in the
sural nerve was examined in anaesthetized rats within 24 hours after nerve
ligation and transection. Activity in single fibers was recorded
extracellularly from small filaments dissected from the nerve about 20-30 mm
proximal to the lesioned site. Cold and warm stimuli were applied to the
ligated and cut nerve end or 5-10 mm further proximal. 2. Among 871
unmyelinated fibers tested 40 were excited by cooling of the cut and ligated
nerve end (8-15 degrees C), 44 were excited by warming (35-65 degrees C) and
16 were both cold and warm sensitive. None of the 438 myelinated fibers
investigated were activated by either cold or warm stimuli. 3. Cold and warm
sensitive fibers responded in a graded fashion to thermal stimuli of variable
temperatures. Between 11 and 30 degrees C, the responses of cold sensitive
fibers increased when the temperature was reduced. Responses of warm sensitive
fibers increased between 40 and 65 degrees C, when the temperature was
increased. 4. Thermosensitive fibers displayed characteristic response
profiles. Responses to cold stimuli were maximal at the beginning and
decreased continuously to lower discharge frequencies as the stimuli were
maintained. The stimulus response curves to warm stimuli were bell-shaped and
discharge frequencies were maximal after 5-15s. 5. In some cold sensitive
fibers a spatial gradient of excitability emerged after axotomy. Maximal
responses to isothermal stimuli were observed at the transection site, whereas
the excitability declined when the thermal stimuli were applied 5 and 10 mm
further proximal. 6. After axotomy, one-hundred ninety unmyelinated fibers
were exposed to cold and warm stimuli at two sites 5 and 10 mm proximal of the
nerve stump, and 172 intact C fibers were stimulated at anatomically
comparable sites along the nerve. No statistically significant difference was
found between the prevalences of thermosensitivity of C fibers along their
axons in these two groups (7/172 vs. 11/190). 7. In conclusion, some
unmyelinated fibers can be ectopically excited by thermal stimuli within 24
hours after nerve cut and ligation. This ectopically evoked activity may
contribute to the generation of paresthesias, painful sensations and
associated changes following peripheral nerve lesion.
Received 4 October 1995; accepted in final form 29 February 1996.
APS Manuscript Number J661-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 20 March 96