Sensory receptors in the anterior uvea of the cat's eye. An in vitro study.

Purpose: To identify electrophysiologically the functional types of sensory fibers innervating the iris and the ciliary body of the cat's eye.

Methods: The uveal tract tract of cat's eye was excised and placed in a superfusion chamber. Recordings were made from single afferent units of ciliary nerve branches responding to mechanical stimulation of the iridal surface, the ciliary body, and the choroid with a nylon filament or a glass rod. Chemical sensitivity was explored by applying acetic acid, hypertonic NaCl, and bradykinin. Warm (60 degrees C) and cold (4 degrees C) saline and a servocontrolled thermode were used for thermal stimulation.

Results: Thirty per cent of the studied population of sensory units (n = 95) were spontaneously active when the recording was started. Approximately 30% of the fibers conducted in the lowest range of the A-delta group; the remaining 70% were C fibers. Sustained mechanical stimulation of the receptive area elicited a tonic response in approximately 60% of the units, and a phasic response in the remaining 40%. Exposure of the receptive field of mechanosensitive fibers to 600 mM NaCl evoked a long-lasting discharge in 50% of the units; application of 1 to 10 mM acetic acid elicited a short discharge in 30% of the fibers, often followed by inactivation. Bradykinin (1 to 100 microMs) produced a long-lasting response in almost 50% of the units. Warming the receptive field recruited 20% of the explored units, whereas 17% were activated by low temperature.

Conclusions: Two main functional types of sensory fibers innervating the iris and the ciliary body were distinguished: (1) mechanoreceptors, corresponding to afferent units sensitive only to mechanical stimuli were generally silent at rest, had relatively higher force thresholds, and discharged phasically in response to long-lasting mechanical stimulation; (2) polymodal nociceptors, which were activated by mechanical as well as by chemical and/or thermal stimuli, usually displayed spontaneous activity, had lower force thresholds, and fired tonically upon sustained mechanical stimulation.