1. Thermosensitivity of axotomized myelinated and unmyelinated fibers
in the sural nerve was examined in anesthetized rats within 24 h after
nerve ligation and transection. Activity in single fibers was recorde
d extracellularly from small filaments dissected from the nerve simila
r to 20-30 mm proximal to the lesioned site. Cold and warm stimuli wer
e 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. N
one 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 increa
sed. 4. Thermosensitive fibers displayed characteristic response profi
les. Responses to cold stimuli were maximal at the beginning and decre
ased continuously to lower discharge frequencies as the stimuli were m
aintained. The stimulus-response curves to warm stimuli were bell-shap
ed and discharge frequencies were maximal after 5-15 s. 5. In some col
d-sensitive fibers a spatial gradient of excitability emerged after ax
otomy. Maximal responses to isothermal stimuli were observed at the tr
ansection site, whereas the excitability declined when the thermal sti
muli were applied 5 and 10 mm further proximal. 6. After axotomy, 190
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 stati
stically significant difference was found between the prevalences of t
hermosensitivity of C fibers along their axons in these two groups (7
of 172 vs. 11 of 190). 7. In conclusion, some unmyelinated fibers can
be ectopically excited by thermal stimuli within 24 h after nerve cut
and ligation. This ectopically evoked activity may contribute to the g
eneration of paresthesias, painful sensations, and associated changes
following peripheral nerve lesion.