Tensile and compressive responses of nociceptors in rat hairy skin.

Mechanically sensitive nociceptor afferents were studied in a preparation of isolated skin from rat leg. Each neuron was studied while the skin was subjected to tensile and compressive loading. The experiment was designed to create highly uniform states of stress in both tension and compression. Tensile loads were applied by pulling on the edges of the sample. Applied loads were used to determine the tensile stresses. Surface displacements were used to determine tensile strains. Compressive loads were applied by indenting the surface of the skin with flat indenter tips applied under force control. The skin was supported by a flat, hard substrate. Compressive stresses were determined from the applied loads and tip geometry. Compressive strains were determined from skin thickness and tip excursions. All nociceptors were activated by both tensile and compressive loading. There was no interaction between the responses to compressive and tensile stimuli (i.e., the responses were simply additive). Responses of nociceptors were better related to tensile and compressive stresses than to strains. Nociceptors responded better to tensile loading than to compressive loading. Response thresholds were lower and sensitivities were higher for tensile stress than for compressive stress. The response to compression was better related to compressive stress than to other stimulus parameters (i.e., load/circumference or simply load). Indentations of intact skin over a soft substrate such as muscle would be expected to cause widespread activation of nociceptors because of tensile stresses.