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CO 2 Stimulation of the Cornea: A Comparison Between Human Sensation and Nerve Activity in Polymodal Nociceptive Afferents of the Cat
Author(s) -
Chen Xiaojie,
Gallar Juana,
Pozo Miguel A.,
Baeza Manuel,
Belmonte Carlos
Publication year - 1995
Publication title -
european journal of neuroscience
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.346
H-Index - 206
eISSN - 1460-9568
pISSN - 0953-816X
DOI - 10.1111/j.1460-9568.1995.tb01105.x
Subject(s) - stimulation , nociception , nociceptor , sensation , receptive field , cornea , tonic (physiology) , anesthesia , chemistry , bursting , sensory system , threshold of pain , cats , afferent , anatomy , medicine , neuroscience , ophthalmology , psychology , biochemistry , receptor
Excitation of nociceptors by low pH has been proposed as a cause of pain following tissue injury. Here we have studied the effect of pH reductions caused by application of CO 2 pulses to the cornea on the activity of corneal afferent nerves of the cat and on the magnitude of pain sensations in humans. Single‐unit activity was recorded from corneal afferent fibres in anaesthetized cats. The corneal receptive field of A‐delta or C polymodal nociceptive units was exposed for 30 s to a gas mixture with different concentrations of CO 2 in air (0, 35, 50, 65, 80 and 98.5%). Responses to CO 2 were evoked at a mean threshold concentration of 40 ± 3% CO 2 . They consisted of a discharge of impulses that decayed gradually to a tonic level. In 15% of the units the initial burst was absent. The CO 2 concentration and firing frequency data could be fitted to a power function with an exponent of 1.12. Pulses of CO 2 were also applied to the cornea of 16 human volunteers. Sensations experienced were measured by means of a visual analogue scale and a verbal descriptor scale. Flow was adjusted below the mechanical stimulation threshold (2.8 ± 0.5 mg). When mixtures containing 10‐90% CO 2 in 5% steps were applied as 3 s pulses, threshold sensation, described as a mild stinging pain, was evoked at 33.5 ± 4.0% CO 2 . This sensation became overtly painful with higher CO 2 concentrations (47.5 ± 3.6% CO 2 ). For the same subject the sensory threshold was remarkably constant, though it changed with longer exposure times. The relationship between CO 2 concentration and magnitude of pain could be adjusted to a power function with a power exponent of 1.12. Curves of CO 2 concentration versus neural discharges in the cat and versus psychophysical sensation in humans were very similar. These results show that corneal polymodal nociceptors respond to protons, and encode changes in CO 2 concentration presumably reflecting pH changes. The same stimulus evokes corneal pain sensations in humans, thus opening the possibility of using CO 2 as an effective stimulus for corneal aesthesiometry.