Cold thermoreceptors, unexpected players in tear production and ocular dryness sensations.

Physical and chemical agents acting on the ocular surface (extreme environmental temperatures, wind, foreign bodies, and chemicals) elicit conscious sensations and reflex motor and autonomic responses (blinking, lacrimation, conjunctival vasodilation) aimed at protecting the eye from further injury. Sensory nerve terminals of trigeminal ganglion neurons innervating the cornea and conjunctiva are the origin of the peripheral neural information that reaches higher central nervous system areas, evoking ultimately these protective neural responses. The innervation of the eye surface is provided by functionally distinct types of trigeminal ganglion neurons, whose peripheral nerve endings specialize in the preferential detection of various modalities of physical (mechanical, thermal) and chemical (exogenous irritants and endogenous mediators) stimuli, encoding their spatial and temporal characteristics (intensity and duration) into a discharge of nerve impulses. Most ocular trigeminal ganglion neurons belong to the general group of nociceptor sensory neurons whose peripheral nerve endings are activated by injurious or near injurious stimuli that initiate normal pain sensations.