Thermosensitivity of acutely axotomized sensory nerve fibers

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 recorded extracellularly from small filaments dissected from the nerve approximately 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 5-15 s. 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, 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 statistically significant difference was found between the prevalences of thermosensitivity 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 generation of paresthesias, painful sensations, and associated changes following peripheral nerve lesion.