NEUROBIOLOGY OF OCULAR PAIN

Ocular irritation and pain are associated with many clinical situation s (e.g. accidental injury, eye diseases, surgery and contact lens wear ing). Pain and related ocular sensations begin with stimulation by inj urious stimuli of first-order sensory neurons uf the trigeminal gangli on. Neurons responding solely to application in their receptive field of noxious mechanical forces (mechanonociceptive neurons), or of irrit ant chemicals and heat (polymodal nociceptive neurons), have been iden tified electrophysiologically in the conjunctiva, cornea, sclera, iris , ciliary body and choroid. The cornea is additionally innervated by n eurons responding to low temperatures, which may account for corneal d iscomfort caused by cold. Also, low-threshold mechanoreceptive and col d-sensitive neurons supply the conjunctiva and sclera, possibly mediat ing touch and thermal sensations aroused by innocuous stimuli in the f ront of the eye. Ocular sensory information is transmitted from the tr igeminal ganglion to specific higher-order neurons located in the trig eminal brainstem nuclear complex, the thalamus and the cerebral fortes . Local ocular inflammatory responses enhance injury-induced neural ac tivity both in ocular nociceptive terminals and in higher order neuron s. In addition to signalling acute lesions, ocular primary sensory neu rons participate in post-injury processes, contributing to local infla mmatory reactions (neurogenic inflammation) and io the repair of damag ed tissues. These effects are mediated at least in part, by substance P and CGRP, two neuropeptides contained in ocular sensory nerves cells that are released peripherally upon tissue damage. Ocular tissues hav e a trophic interdependence with their sensory neurons. Ocular tissues are the source of neurotrophic factors that are critical for the earl y development and survival of trigeminal sensory neurons. On the other hand, the morphofunctional integrity of some ocular tissues like the contra. appears to be dependent on the presence of an intact sensory i nnervation, Stimulation of ocular sensory pathways by noxious mechanic al, chemical and thermal stimulation of cornea, conjunctiva or of othe r eye structures, evokes distinct types of ocular sensations. Differen ces in the quality of pain sensation presumably result from the magnit udes of activation of the various sub-populations of ocular nociceptiv e neurons by different stimulus modalities. In addition to conscious s ensations, injurious stimuli evoke protective reflexes (blinking and l acrimation) aimed at protecting the rye and minimizing further ocular damage by noxious stimuli. Copyright (C) 1996 Elsevier Science Ltd.