Responses of rabbit corneal nociceptors to mechanical and thermal stimulation

We recorded 116 units from dissected strands of the long ciliary nerve of rabbit and tested them with punctate mechanical, cool, or warm stimulation. Seventy-two percent of the units responded only to mechanical stimulation, 17% responded only to cooling, and 11% were bimodal, responding to both warming and mechanical stimulation. Mechanical responses were rapidly adapting and decreased on successive stimulation at the same locus. Conduction velocities determined for 43 mechanical units were in the same low range of the A-delta group (mean = 2.71 m/s). Receptive fields for all classes of units found here overlapped and did not extend beyond the center of the cornea. Precise determination of field size for 20 mechanical units showed that most included 5 to 20% of the corneal surface and a few extended to about 40%. Activation of units responding to stimulation below the adapting temperature (33 degrees C) showed them to be sensitive to cooling steps of 0.5 degrees C. The average conduction velocity of five cold units was 0.94 m/s. Bimodal units were not active at 33 degrees C and responded to warming above 39 degrees C with an increased discharge frequency; however, the response did not include an initial high-frequency discharge as did that of the cool units.