Afferent Pathway of Pupillary Light Reflex (solid yellow above):
● Light enters the
pupil and stimulates the
retina.
● Retinal ganglion cells transmit the light signal to the
optic nerve
● The optic nerve enters the
optic chiasm where the nasal retinal fibers
cross to contralateral optic tract. while the temporal retinal fibers stay in
the ipsilateral optic tract
● Fibers from the optic tracts project and synapse in the
pretectal nuclei
in the dorsal midbrain in the collicular region
● The pretectal nuclei project fibers to the ipsilateral
Edinger-Westphal
nuclei and also to the contralateral Edinger-Westphal nucleus via the posterior
commissure
Efferent Pathway of Pupillary Light Reflex (black dotted above):
● The Edinger-Westphal nucleus projects pre-ganglionic parasympathetic
fibers, which exit the midbrain and travel along the oculomotor nerve (CN III)
and then synapse on post-ganglionic parasympathetic fibers in the
ciliary
ganglion
● Ciliary ganglion post-ganglionic parasympathetic fibers (short ciliary
nerves) innervate the sphincter muscle of the pupils resulting in pupillary
constriction.
The physiological result of the neuroanatomical pathways as described above
is that light shined in one eye will result in pupillary constriction in both
the ipsilateral pupil (direct pupillary light reflex) and the contralateral
pupil (consensual pupillary light reflex).
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