Structural and functional specialization of Aδ and C fiber free nerve endings innervating rabbit corneal epithelium

Abstract

An in vitro preparation of rabbit cornea was used to compare anatomical specialization and electrophysiological responses of Aδ and C fiber sensory afferents which terminate as free nerve endings. Living nerve endings were visualized using epifluorescence microscopy and the vital dye 4-di2 ASP, and response properties were determined using microstimulation and recording of fiber discharge activity. Fiber type was determined based on conduction velocity measurement and preferred stimulus energy (modality) of each fiber. Four modality-specific fiber populations were identified: (1) slowly adapting C fiber cold receptors (conduction velocity of 0.25-1.6 m/sec), (2) C fiber chemosensitive units with mixed phasic and tonic activity (1.1-1.8 m/sec), (3) rapidly adapting mechanosensitive AS fibers (1.5-2.8 m/sec), and (4) high-threshold mechano/heat (>350 dyne or >40°C) phasic AS afferents (3.5-4.4 m/sec). In addition to these physiological differences, anatomical specialization was also noted. AS fiber nerve endings were distinguished from those of C fibers by thin, elongated sensory endings that ran parallel to the corneal surface; C fiber endings formed short, branching clusters that ran mostly perpendicular to the surface. The elongated structure of Aδ nerve endings was associated with directional selectivity for mechanical stimuli. These results substantiate previous suggestions that free nerve endings can exhibit both structural and functional specialization.