Nerves and sensations from the eye surface.

Because vision plays a critical role in obtaining information from the external world, evolutionary development has provided the structures that sustain this function with special protection against injury. Thus, the cornea possesses the richest sensory innervation of the body to detect noxious stimuli. The trigeminal sensory neurons that innervate the eye vary in their chemical composition and electrophysiological properties, and can be classified according to the stimuli that activate them preferentially: mechanical forces, temperature, or irritant chemicals. Different classes of noxious stimuli (mechanical injuries, heat, extreme cold) activate to a different degree the various populations of sensory fibers of the ocular surface and evoke unpleasant sensations of distinct quality. When injured either accidentally or following ocular surgery, sensory nerve fibers of the ocular surface may form neuromas that develop abnormal activity and become the source of unpleasant sensations, such as pain, dryness, grittiness, etc. In parallel, their response to natural stimuli is diminished. The possibility of hypesthesia and dysaesthesias must be considered in the assessment of the risks of therapeutic procedures that involve damage to ocular sensory nerves.