Contribution of the different subclasses of corneal nociceptive neurons to sensory experiences evoked by selective mechanical, thermal and chemical stimulation of the cornea

Abstract

[Purpose] To correlate the activation of the different types of sensory neurons innervating the cornea in cats with the quality of pain sensations evoked in humans during selective mechanical, chemical and thermal stimulation of the cornea.

[Methods] Pulses of gas (3s) of controlled flow and temperature were applied on the cornea of humans and cats. Mechanical stimulation was produced with suprathreshold flows of air reaching the cornea at 33°C. For chemical stimulation, a subthreshold flow of a gas mixture (air and CO at different concentrations) at 33°C was used. Thermal stimulation was obtained with air at a subthreshold flow rate, reaching the cornea at temperatures below (cold) or over (heat) 33°C. In young human volunteers, the different parameters of the evoked comeal sensation (magnitude, degree of irritation, burning pain, itching pain, thermal component) were quantified using a visual analogue scale, In anesthetized cats, the discharge of single afferent fibers was recorded from filaments of the ciliary nerves.

[Results] Selective mechanical, thermal or chemical stimuli elicited always a sensation of irritation in humans, with differences in the quality of pain and in the thermal component of the sensation. Polymodal nociceptors responded weakly to 3s-mechanical pulses and fired proportionally to CO concentrations and to temperatures over 40°C but not below 33°C. Mechano-nociceptors were not recruited by 3s suprathreshold mechanical pulses and responded to mechanical stimuli of a longer duration but not to C03 or to temperature changes. Cold nociceptors fired vigorously in response to air pulses at temperatures below 33°C and were activated inconsistently by CO or mechanical pulses.

[Conclusions] Differences in the quality of comeal sensation evoked in humans by mechanical, chemical and thermal stimuli may be explained by the variable recruitment of the distinct populations of sensory neurons innervating the cornea by each type of stimulus.