Free nerve ending terminal morphology is fiber type specific for A delta and C fibers innervating rabbit corneal epithelium.

1. A delta and C fibers are the smallest diameter and most numerous axons in peripheral nerve bundles. They have been thought to terminate as "free" nerve endings lacking organized structure. The present study used a vital fluorescent dye to selectively visualize living free nerve endings innervating rabbit corneal epithelium, allowing structure to be correlated with electrophysiological and functional characteristics. 2. Conduction velocity measurement of visually identified nerve endings were used to discriminate between C and A delta fibers. C fiber sensory endings terminated as short (< 50 microns) vertically directed processes clustered within the epithelium. A delta fibers terminated as long (0.1-1.2 mm) horizontal processes running parallel to the epithelial surface. 3. Only A delta fiber endings were mechanoreceptive, and the unique elongated structure imparted directional selectivity. Comparison of physiological and electrical activation indicated that mechanical stimuli were transduced in < 600 microseconds. This study confirms previous suggestions of structural and functional specialization for "free" nerve endings.