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Speed and Temperature Dependences of Mechanotransduction in Afferent Fibers Recorded From the Mouse Saphenous Nerve

Department of Neuroscience, Max-Delbrück Center for Molecular Medicine and Charité Universitätsmedizin Berlin, Berlin, Germany

Submitted 17 July 2008; accepted in final form 15 September 2008

Here we have systematically characterized the stimulus response properties of mechanosensitive sensory fibers in the mouse saphenous nerve. We tested mechanoreceptors and nociceptors with defined displacement stimuli of varying amplitude and velocity. For each sensory afferent investigated we measured the mechanical latency, which is the delay between the onset of a ramp displacement and the first evoked spike, corrected for conduction delay. Mechanical latency plotted as a function of stimulus strength was very characteristic for each receptor type and was very short for rapidly adapting mechanoreceptors (<11 ms) but very long in myelinated and unmyelinated nociceptors (49–114 ms). Increasing the stimulus speed decreased mechanical latency in all receptor types with the notable exception of C-fiber nociceptors, in which mean mechanical latency was not reduced 100 ms, even with very fast ramp stimuli (2,945 µm/s). We examined stimulus response functions and mechanical latency at two different temperatures (24 and 32°C) and found that stimulus response properties of almost all mechanoreceptors were not altered in this range. A notable exception to this rule was found for C-fibers in which mechanical latency was substantially increased and stimulus response functions decreased at lower temperatures. We calculated Q₁₀ values for mechanical latency in C-fibers to be 5.1; in contrast, the Q₁₀ value for conduction velocity for the same fibers was 1.4. Finally, we examined the effects of short-term inflammation (2–6 h) induced by carrageenan on nociceptor and mechanoreceptor sensitivity. We did not detect robust changes in mechanical latency or stimulus response functions after inflammation that might have reflected mechanical sensitization under the conditions tested.