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Y. Feng, T.L. Simpson; Corneal suprathreshold mechanical and chemical stimulation inhibits conjunctival sensory channels . Invest. Ophthalmol. Vis. Sci. 2004;45(13):3796.
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Purpose: To compare how humans respond to mechanical and chemical stimulation on the ocular surface at suprathreshold levels. Methods: Ten healthy human subjects participated. The Belmonte pneumatic esthesiometer was used to present mechanical and chemical stimuli to the apical cornea and temporal conjunctiva of the left eye. Stimuli were presented to cornea and conjunctiva in paired and unpaired ways with conjunctival stimulation immediately following corneal stimulation in the former while conjunctival stimulation occurred alone in the latter. The magnitude of the evoked sensation from cornea and conjunctiva was reported by subjects using Visual Analogue Scales. The transducer functions (relationships between the sensation magnitude and stimulus intensity) were estimated using non–linear regression. Results: The transducer functions were very well fit with ogives (e.g., all r's >0.93 for logistic functions). Both conjunctival mechanical and chemical stimulation induced lower sensation magnitudes compared to equal corneal stimulation (conjunctival transducer functions were lower than corneal ones (sign test, all p's <0.024)). Corneal and conjunctival transducer functions diverged at higher stimulus intensities and these shifts were more apparent for conjunctival transducer functions with paired presented stimuli than with unpaired stimuli for both mechanical and chemical stimulation. At every stimulus level for both mechanical and chemical stimuli, the unpaired ratings were higher than the paired ratings (sign tests, p=0.041 and p=0.023 for mechanical and chemical transducer functions respectively). Conclusions: For a specific stimulus, the relatively strong discomfort evoked from the cornea appears to partially suppress the relatively weaker conjunctival stimulation. This is manifest as the conjunctival transducer functions being shallower (less intense sensation) when immediately preceded by corneal stimulation than when the conjunctival transducer functions were measured alone (unpaired). The underlying mechanism could be an example of "diffuse noxious inhibitory control (DNIC)" which possibly involves descending inhibitory effects of reticular structures of the brainstem. At some level therefore, corneal and conjunctival sensory channels are not independent.
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