Parasympathetic Pathway

Video overview

Light shined in the eye stimulates retinal photoreceptors and intrinsically photosensitive retinal ganglion cells
Signal travels in the retinal ganglion cell axons through the optic nerves, optic chiasm, and both optic tracts
Signal branches off in the brachium of the superior colliculus to synapse in the dorsal midbrain pretectal nuclei on both sides

Axons leave the pretectal nuclei and cross to nuclei on the other side of the dorsal midbrain, so that the afferent signal is distributed equally to both Edinger-Westphal nuclei

Pre-ganglionic axons from the Edinger-Westphal parasympathetic nuclei carry signals in both third cranial nerves

to the ciliary ganglia in both orbits

Post-ganglionic axons leaving the ciliary ganglia in the short ciliary nerves carry signals

to both iris sphincters for pupil constriction

and to both ciliary bodies for accommodation

Lesion of one optic nerve
- Produces an afferent pupil defect
- Reduces ipsilateral pupil constriction to light but preserves constriction to a target placed within reading distance because awareness of a near target stimulates a cerebral pathway that bypasses the dorsal midbrain and connects directly to the Edinger-Westphal nuclei (“afferent light-near dissociation”)
- Trap: anisocoria does not occur with lesions of the optic nerves because visual input is distributed equally to the iris sphincter muscles in both eyes

Lesion of both optic nerves
- Reduces pupil constriction in both eyes to light stimuli
- Trap: an afferent pupil defect will not occur when there is equal damage to both optic nerves
- Preserves pupil constriction to a near target (“afferent light-near dissociation”)

Lesion of one optic tract
- Produces a contralateral afferent pupil defect because optic chiasm crossing axons outnumber non-crossing axons
- Produces a contralateral homonymous hemianopia

Lesion of one brachium of the superior colliculus
- Produces a contralateral afferent pupil defect
- Trap: does not cause a visual field defect because a lesion here does not interrupt the visual pathway

Lesion of the dorsal midbrain
- Produces large pupils and impairs pupil constriction to light in both eyes (“tectal pupils”)
- Spares pupil constriction to a visual target placed near the eye (“tectal light-near dissociation”)

Lesion of the third nerve
- Produces ipsilateral mydriasis and impairment of pupil constriction to light, ptosis, and ocular ductional deficits
- Trap: third nerve lesions never produce mydriasis as an isolated clinical abnormality

Lesion of the ciliary ganglion or post-ganglionic ciliary nerves
- Produces a tonic (Adie) pupil
- Tip: a tonic pupil is always caused by a postganglionic (orbital) lesion; there should be no concern for an intracranial lesion if you make this diagnosis!