Light entering the eye strikes three different
photoreceptors in the
retina: the familiar
rods and
cones used in image forming and the more newly discovered
photosensitive ganglion cells. The ganglion cells give information about ambient light levels, and react sluggishly compared to the rods and cones. Signals from photosensitive ganglion cells have multiple functions including acute suppression of the hormone
melatonin,
entrainment of the body's
circadian rhythms and regulation of the size of the pupil.
The retinal photoceptors convert light stimuli into electric impulses. Nerves involved in the resizing of the pupil connect to the
pretectal nucleus of the high
midbrain, bypassing the
lateral geniculate nucleus and the primary
visual cortex. From the pretectal nucleus neurons send axons to neurons of the
Edinger-Westphal nucleus whose
visceromotor axons run along both the left and right
oculomotor nerves. Visceromotor nerve axons (which constitute a portion of
cranial nerve III, along with the somatomotor portion derived from the Edinger-Westphal nucleus) synapse on
ciliary ganglion neurons, whose parasympathetic axons innervate the
iris sphincter muscle, producing miosis.