Absolute corneal sensitivity and corneal trigeminal nerve anatomy in normal dogs
Barrett, P.M.; Scagliotti, R.H.; Merideth, R.E.; Jackson, P.A.; Alarcon, F.L.
Progress in Veterinary and Comparative Ophthalmology 1(4): 245-254
The sensory nerves in the cornea of normal dogs was investigated through sensitivity testing with a CochetBonnet aesthesiometer, and histological examination. The mean corneal touch threshold (CTT) â€” or minimum stimulation to the corneal surface to elicit a blink reflex â€” for 60 normal dogs was 96 mg/0.0113 mm2 in the central cornea and 116, 148, 154 and 159 mg/0.0113 mm2 in the nasal, temporal, dorsal and ventral corneal regions, respectively. Evaluation by skull type revealed significant differences in the corneal sensitivity values. Dolichocephalic skull types had the most sensitive corneas; the mean CTT was 74, 89, 121, 131 and 135 mg/0.0113 mm2 for the central, nasal, temporal, dorsal and ventral corneal regions, respectively. The mean CTT for mesaticephalic skull types was 93, 113, 151, 153 and 160 mg/0.0113 mm2 for the central, nasal, temporal, dorsal and ventral corneal regions, respectively. Corneal sensitivity was lowest in brachycephalic skulltype dogs; the mean CTT was 141, 150, 170, 178 and 186 mg/0.0113 mm2 for the central, nasal, temporal, dorsal and ventral corneal regions, respectively. A modified gold chloride technique was used to stain the neural elements of 14 canine corneas. Whole flat cornea mounts and sectioned histological mounts revealed a mean of 11.5 major stromal nerve trunks entering the midposterior corneal stroma at various perilimbal sites. Stromal nerve trunks extended to the center of the cornea. Repeated branching of the stromal nerve trunks was identified to the level of a subepithelial plexus, which gave rise to fine nerve bundles that extended into the corneal epithelium. Free nerve endings were noted in the corneal epithelial wing cell layer as terminal expansions. The peripheral cornea was innervated by several smaller nerve fibers that penetrated the limbus at the subepithelial level, at various sites encircling the cornea. These smaller nerves branched rapidly and sent extensions into the corneal epithelial wing cell layer to end in terminal expansions.