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Journal of Neuroscience, Vol 13, 4511-4524, Copyright © 1993 by Society for Neuroscience
Structural and functional specialization of A delta and C fiber free nerve endings innervating rabbit corneal epithelium
MB MacIver and DL Tanelian
Department of Anesthesia, Stanford University School of Medicine, California 94305-5117.
An in vitro preparation of rabbit cornea was used to compare anatomical
specialization and electrophysiological responses of A delta 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 A delta
fibers (1.5-2.8 m/sec), and (4) high- threshold mechano/heat (> 350 dyne
or > 40 degrees C) phasic A delta afferents (3.5-4.4 m/sec). In addition
to these physiological differences, anatomical specialization was also
noted. A delta 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 A delta 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.
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