Response of sensory units with unmyelinated fibres to mechanical, thermal and chemical stimulation of the cat's cornea.

1. In the cat anaesthetized with sodium pentobarbitone, electrical activity was recorded from single unmyelinated sensory fibres innervating the cornea. 2. Based on their response to mechanical (calibrated aesthesiometer), chemical (10 mM acetic acid or 616 mM NaCl) and thermal (cooling from 35 to 5 degrees C; heating to 51 degrees C) stimuli, corneal unmyelinated fibres were classified as polymodal (71%) or 'cold' nociceptors (29%). 3. Polymodal units responded to mechanical indentation of the cornea and developed fatigue after repeated stimulation. They were excited by temperatures over 37 degrees C and exhibited sensitization to repeated heating. 4. Corneal polymodal units were also activated by topical application of 10 mM acetic acid and hypertonic NaCl (616 mM). Capsaicin (0.33 mM) elicited a discharge of impulses that was followed by inactivation to mechanical, chemical and thermal stimuli. 5. 'Cold' nociceptors had small receptive fields, preferentially located at the periphery of the cornea. They were excited by small temperature decreases of the corneal surface in a range between 30 and 8 degrees C, but were not responsive to noxious heat. 6. 'Cold' nociceptors encoded temperature changes between 35 and 23 degrees C. The discharge was proportional to the velocity of the temperature drop; sustained temperatures were not signalled by changes in static frequency values. 'Cold' nociceptive fibres responded to hypertonic NaCl (616 mM) and weakly to 10 mM acetic acid. Capsaicin (0.33 mM) first excited and then inactivated 'cold' nociceptors. 7. Thermoreceptive fibres were found in the episclera. They fired in bursts and responded to small temperature decreases, but were insensitive to irritant chemical and capsaicin.