Responses of cat corneal sensory receptors to mechanical and thermal stimulation.

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Abstract
The afferent responses evoked by mechanical and thermal stimulation of the cat cornea were recorded extracellularly from strands of long and mixed ciliary nerves under deep anesthesia. Of the units studied, 94% (n = 53) responded consistently to both stimuli. Conduction velocities, measured by electrical stimulation of the receptive field, corresponded to the lower range of the A-delta fiber group (average = 5.4 m/s). Receptive fields covered approximately a quadrant of the corneal surface and showed continuous sensitivity and overlapping. Units were silent in the absence of stimulation but an ongoing activity was commonly present after repeated mechanical and thermal stimulation. Mechanical responses were evoked at low thresholds and consisted of a dynamic and static response that paralleled the amplitude of the stimulus. The pattern of the discharge was irregular and fatigue was easily developed by repeated stimulation. Thresholds to heating were above 38.degree. C and the response increased monotonically with the temperature over the range from threshold to 50.degree. C. The heat response could be sensitized by repeated long suprathreshold stimulation while variable changes in the response was induced by briefer stimuli. Also depression was observed in some circumstances. A weak response to cooling was present in 50% of the units tested. Damaging mechanical stimulation or the application of a strong acid solution evoked a vigorous response followed by an after discharge that persisted for hours. The relation of these receptors to other polymodal nociceptors and corneal sensation is considered.