Encoding of location and intensity of noxious indentation into rat skin by spatial populations of cutaneous mechano-nociceptors

The ability of a spatial population of cutaneous, A delta, and C mechano-nociceptors to encode the location and intensity of a noxious, cutaneous indentation was examined using an isolated preparation in a rat model. Skin and its intact innervation were harvested from the medial thigh of the rat hindlimb and placed in a dish, with the cerium side down, containing synthetic interstitial fluid. The margins of the skin were coupled to an apparatus that could stretch and apply compression to the skin. The skin was suspended on top of a deformable platform whose bulk, nonlinear, compressive compliance emulated that found in vivo. The isolated preparation facilitated examination of the spatial population response by eliminating the nonlinear geometry and inhomogeneous compressive compliance present in-vivo. Spatial population responses (SPR) were formed from recordings of single neurons that were stimulated by compressing the skin with an indenter (flat cylinder, 3-mm diam) at discrete intervals from the center of their receptive fields. SPR were composed of the neural responses (z axis) at each indentation location (x, y plane), and were analyzed quantitatively using nonlinear regression to fit an equation of a Gaussian surface. Both A delta and C SPR accurately encoded the location and intensity of noxious indentation. The intensity of the stimulus was encoded in the peak neural response of the SPR, which had a nonlinear relationship to the compressive force. The location of the stimulus was encoded in the x, y position of the peak of the SPR. The position of the peak remained constant with increasing magnitudes of compressive force. The overall form of the SPR also remained constant with changes of compressive load, suggesting a possible role for encoding in the SPR some aspects of shape of a noxious stimulus.