Expansion
In by two, out by three
Mnemonic
“II in, III out”:
- Afferent - optic nerve (II) to the pretectal nucleus
- The pretectal nucleus projects to both Edinger-Westphal nuclei, which is why the response is consensual
- Efferent - oculomotor nerve (III), parasympathetic, via the ciliary ganglion to sphincter pupillae
The bilateral projection is what makes the swinging light test work. A relative afferent pupillary defect (Marcus Gunn pupil) means that swinging the torch to the affected eye causes both pupils to dilate, because the afferent signal from that eye is weaker.
“An afferent defect affects both pupils equally; an efferent defect affects only one.”
The accommodation reflex uses a different pathway through the visual cortex, which is why light-near dissociation occurs: the pupil accommodates but does not react to light, as in an Argyll Robertson pupil (neurosyphilis) and Adie’s tonic pupil.
Expansion
- Afferent - CN II, optic nerve
- Efferent - CN III, oculomotor nerve