An approach to the patient with photophobia.

1. Ganglion cells project light-related signaling to the olivary pretectal nucleus (OPN; light green). OPN projections activate superior salivatory nucleus (SSN; dark green), which via pterygopalatine ganglion, causes ocular vasodilation and activation of ocular trigeminal afferents (orange) which are heavily expressed on blood vessels. These afferents, with cell bodies in the trigeminal ganglion, project to trigeminal nucleus caudalis, thalamus and cortex. 2. Intrinsically photosensitive retinal ganglion cells (IPRGCs) project directly to thalamic neurons (blue) that also receive intracranial nociceptive afferent signal (yellow neurons in trigeminal ganglion and trigeminal nucleus caudalis. Thalamic neurons fire in response to light and pain stimuli. Their output projects diffusely to sensory and association cortex. 3. Melanopsin-containing, intrinsically photosensitive ganglion-like cells have been identified in rodent iris. These afferents may explain the fact that light can activate trigeminal blink reflex even after the optic nerve (through which circuits 1. and 2. pass) has been sectioned. Note that all three circuits may interact at different locations. (created from references 84–86, 91).