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Robichaud, Daniel R. II, Zaccaria Del Prete, and Peter Grigg. Stretch sensitivity of cutaneous RA mechanoreceptors in rat hairy skin. J Neurophysiol 90: 2065-2068, 2003. First published June 4, 2003 10.1152/jn.00405.2003. Twenty-five rapidly adapting mechanoreceptor afferents were recorded in an in vitro preparation of rat skin and nerve. Single units were recorded while the skin was subjected to dynamic uniaxial stretch using a pseudo-Gaussian noise (PGN) input waveform. Force was the controlled variable in stretch stimuli. Measured loads and displacements were used to calculate tensile stresses, strains, and their rates of change. Associations between spike responses and individual stimulus components such as tensile stress or strain were determined in a reverse correlation design using multiple logistic regression. Spikes were strongly associated with stress, at memory times from 0 to 14 ms, and with the rate of change of stress, at a memory times between 6 and 18 ms. There was a strong interaction between stress and its rate of change, with a maximum value at a memory time of 10 ms. We found no relationship between spike responses and strain. I N T R O D U C T I O NIt is well known that many cutaneous mechanoreceptor afferents can be activated by stretch stimuli (Del Prete et al. 2003;Grigg 1996;Grigg and Hoffman 1996;Kumazawa and Perl 1977; Nordin 1994). However, stretching skin produces a number of mechanical states, such as tensile stress, tensile strain, strain energy density, and their rates of change, in the skin. In general, it has been unclear which of these states might act as the stimulus to the mechanoreceptor ending. Recently, however, several of us (Del Prete et al. 2003) described a method that allows for determining the association between spikes and multiple stimulus variables. This method, which is based on multiple logistic regression (Hosmer and Lemeshow 1989), also allows for determining whether there are memory effects in the association between variables and spikes. "Memory," in this sense, refers to the time between the production of a stimulus in the skin and the appearance of a response in the afferent.The use of logistic regression in applications such as this has been described in detail with reference to mechanoreceptors from mouse skin (Del Prete et al. 2003). In addition, the method was used to re-analyze previously published data from rapidly adapting (RA) afferents in rat skin (Grigg and Del Prete 2002). In both preparations, spikes were strongly associated with the rate of change of tensile stress. However, the two experiments found differences in the memory times at which the effect of that stimulus was observed. Responses in mouse mechanoreceptors were most strongly associated with the rate of change of stress 8 -10 ms before a spike. In contrast, the re-analyzed data from rat skin units showed that the rate of change of stress had its maximal effect on the order of 30 ms prior to a spike. In attempting to account for the differences between these two results, we note that...

We describe a method for evaluating the threshold for cutaneous mechanical sensation in rodents, based on a stimulator that drives a probe against the plantar surface of the foot. The stimulator applies loads that can be either constant or linearly increased. We describe withdrawal responses, including forms of movement that precede foot withdrawals. With constant stimuli, response latency declines in a nonlinear fashion as stimulus magnitude is increased. With ramped stimuli the effect of loading rate is complex, reflecting both the rate of change of the stimulus and the animal\u27s reaction time. We demonstrate the utility of using ramped stimuli in experiments that show that thresholds vary spatially across the foot and experiments that show that intradermal capsaicin injections cause allodynia but not hyperalgesia

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When skin is stretched, stimuli experienced by a cutaneous mechanoreceptor neuron are transmitted to the nerve ending through the skin. In these experiments, we tested the hypothesis that the viscoelastic response of the skin influences the dynamic response of cutaneous rapidly adapting (RA) neurons. Cutaneous RA afferent neurons were recorded in 3 species of mice (Tsk, Pallid, and C57BL6) whose skin has different viscoelastic properties. Isolated samples of skin and nerve were stimulated mechanically with a dynamic stretch stimulus, which followed a pseudo Gaussian waveform with a bandwidth of 0-60 Hz. The mechanical response of the skin was measured as were responses of single RA cutaneous mechanoreceptor neurons. For each neuron, the strength of association between spike responses and the dynamic and static components of stimuli were determined with multiple logistic regression analysis. The viscoelastic material properties of each skin sample were determined indirectly, by creating a nonlinear (Wiener-Volterra) model of the stress-strain relationship, and using the model to predict the complex compliance (i.e., the viscoelastic material properties). The dynamic sensitivity of RA mechanoreceptor neurons in mouse hairy skin was weakly related to the viscoelastic properties of the skin. Loss modulus and phase angle were lower (indicating a decreased viscous component of response) in Tsk and Pallid than in C57BL6 mice. However, RA mechanoreceptor neurons in Tsk and Pallid skin did not differ from those in C57 skin with regard to their sensitivity to the rate of change of stress or to the rate of change of incremental strain energy. They did have a decreased sensitivity to the rate of change of tensile strain. Thus the skin samples with lower dynamic mechanical response contained neurons with a somewhat lower sensitivity to dynamic stimuli.