Pupillary pathway
The pupillary reflex can be thought of as an afferent limb, a parasympathetic efferent limb, and a sympathetic efferent limb.
Afferent limb- Light stimulates photoreceptors and the signal is conveyed to a special set of ganglion cells that send their nerve impulses through the axons in a similar topographic distribution as those carrying other signals in the optic nerve (arrow 1 in the figure, please enlarge). Decussation occurs at the optic chiasm (arrow 2 in the figure) for the nasal fibers (arrow 4) . The big difference is that the afferent fibers do not enter the lateral geniculate body(LGN) but instead exit and pass through the brachium of the superior colliculus (arrow 5) where they synapse on the pretectal nuclei (arrow 7). These nuclei project bilaterally to the Edinger Westphal nuclei (arrows 6 and 7) via internuncial neurons by a process that has not been fully elucidated.
Efferent parasympathetic response- The Edinger Westphal nuclei (EWN) send fibers to join the oculomotor CNIII (arrow 8) and follow that course on the dorsomedial surface of the nerve (arrow 9). This is important because this is an outer surface which lies adjacent to the posterior communicating artery and is exposed to forces of herniation. After coursing thru the cavernous sinus these fibers emerge to enter the orbit with the inferior oblique branch of CNIII. These fibers synapse at the ciliary ganglion (arrow 10) and then enter the eye thru short posterior ciliary nerves to distribute fibers to the choroid, iris (arrow 11), and ciliary body. The red nucleus is shown in red and substantia nigra in black. The medial geniculate body is dark gray and medial to the brachium of the superior colliculus. The lateral geniculate body where the fibers responsible for vision will synapse lie lateral to the pupillary fiber pathway in another plane of section.
Efferent sympathetic response- This is believed to start in the hypothalamus and project in an uncrossed fashion with synapses in the mesencephalon and pons. These neurons project to and synapse upon the intermediolateral cell column from C8-T2 in the spinal cord. These exit the spinal cord and pass thru the stellate ganglion to synapse in the superior cervical ganglion. These fibers travel with the internal carotid artery, enter the cavernous sinus and travel with CN VI in the cavernous sinus to enter the superior orbital fissue with cranial nerve V. The fibers travel with the nasociliary branch of V, and pass thru the ciliary ganglion without synapsing. The fibers pass thru the long ciliary nerves to terminate on the dilator muscle. Some fibers diverge in the superior orbital fissue to innervate Muller's muscle.
Clinical significance? These pathways are important for the clinician to understand the basis for a blown pupil with a third nerve palsy. The pupillary fibers are compressed and this is most likely due to an aneurysm that affects the 3rd nerve at the posterior communicating artery.
