Afferent pupillary pathway arises from the optic nerve, where nerve fibres that are not transmitted to the LGN will be sent to the ispilateral pretactal nucleus of the eye, at the midbrain region. From there, all the impulses will be further sent to the Edinger Westphal nucleus (EWN), which is the starting point for parasympathetic pathways.
PUPILLARY REFLEX PATHWAYS
(a) For example, when eye is subjected to bright conditions, efferent parasympathetic response occurs, whereby the EWN will transmit the nerve impulses through the lateral geniculate nucleus (LGN) to the cranial nerve III, oculomotor nerve.These impulses will then synapse at the ciliary ganglion. After which the impulses will be sent to the target muscle through the short posterior ciliary nerves to carry out an action. In this case, the impulses will be sent to the sphincter muscle, which results in constriction of the pupil.
(b) If the eye is now exposed to a dark illumination, impulses from the brain will be sent to the eye through the sympathetic pathway. Nerve fibres will be transmitted by the hypothalamus instead of the EWN, then it will synapse in the superior cervical ganglion.
These fibres travel with the internal carotid artery, enter the cavernous sinus to enter the superior orbital fissue with cranial nerve V, the trigeminal nerve.
Then, nerve fibres will travel with the nasociliary branch of V, and pass through the ciliary ganglion, but without synapsing.
Finally, the nerve fibres will pass through the long ciliary nerves to activate the dilator muscle. This explains why our pupil will dilate in dark illuminations after being exposed to bright light.
