Overview

Afferent limb: light → retinal ganglion cells (especially melanopsin-containing ipRGCs) → optic nerve → chiasm (partial crossing) → optic tract → leaves the tract before the LGN via the brachium of the superior colliculus → olivary pretectal nucleus in the dorsal midbrain.

Interneurons: each pretectal nucleus projects bilaterally to the Edinger–Westphal (accessory oculomotor) nuclei — ipsilaterally and, via the posterior commissure, contralaterally. This double bilateral crossing (at the chiasm and in the midbrain) explains why both pupils constrict equally whichever eye is stimulated.

Efferent limb: preganglionic parasympathetic fibres from the Edinger–Westphal nucleus travel in the oculomotor nerve (superficially, in its dorsomedial periphery) and its inferior division (nerve to inferior oblique) → ciliary ganglion (in the orbit, lateral to the optic nerve) → postganglionic short ciliary nerves → sphincter pupillae (muscarinic M3) → miosis.

Because the LGN and visual cortex are not in the loop, a patient with cortical blindness still has normal pupillary light reflexes, whereas a complete optic nerve lesion abolishes the direct reflex in that eye (consensual from the other eye preserved).

Location: Retina → optic nerve/tract → pretectum and Edinger–Westphal nucleus (midbrain) → oculomotor nerve → ciliary ganglion → iris.

Parts & subdivisions

Afferent: optic nerve
Retinal ganglion cells (ipRGCs) → optic nerve, chiasm, optic tract.
Olivary pretectal nucleus
Integrating centre at the level of the superior colliculus; projects bilaterally.
Edinger–Westphal nucleus
Preganglionic parasympathetic nucleus of CN III in the midbrain.
Efferent: oculomotor nerve and ciliary ganglion
Parasympathetic preganglionic fibres synapse in the ciliary ganglion; postganglionic short ciliary nerves supply sphincter pupillae (and ciliary muscle).

Function

Regulate retinal illumination rapidly
Protect the retina from bright light
Clinical test of optic nerve, midbrain and oculomotor function

Clinical significance

Interpreting pupil responses

Afferent defect (e.g. left optic nerve): light in the left eye → neither pupil constricts well; light in the right eye → both constrict normally; swinging flashlight → both dilate when the light reaches the left eye (left RAPD). Pupils are equal at rest. Efferent defect (e.g. left CN III): left pupil is dilated and does not constrict to light in either eye, while the right pupil constricts to light in either eye — anisocoria greater in bright light.

Unilateral fixed dilated pupil in coma

Raised intracranial pressure with transtentorial (uncal) herniation compresses the oculomotor nerve against the tentorium/posterior cerebral artery; pupillomotor fibres on the nerve surface are affected first — "blown pupil", an emergency.

Argyll Robertson and Adie pupils

Argyll Robertson (neurosyphilis): small, irregular pupils that accommodate but do not react ("prostitute's pupil") — lesion near the pretectal nucleus spares the more ventral near-reflex fibres. Adie tonic pupil (young women; ciliary ganglion denervation): unilateral large pupil, poor light reaction, slow tonic near response, supersensitive to dilute (0.1%) pilocarpine; with absent tendon reflexes = Holmes–Adie syndrome.

Mnemonics

Argyll Robertson pupil

"ARP — Accommodation Reflex Present, Pupillary reflex absent" (accommodates but does not react).

Test yourself

Which statement about the pupillary light reflex is correct?

It requires an intact primary visual cortex
Its efferent limb is sympathetic via the superior cervical ganglion
Pretectal neurons project bilaterally to the Edinger–Westphal nuclei, producing a consensual response
Its postganglionic fibres travel in the long ciliary nerves

Answer: Pretectal neurons project bilaterally to the Edinger–Westphal nuclei, producing a consensual response

Optic tract fibres reach the olivary pretectal nucleus, which projects to both Edinger–Westphal nuclei (via the posterior commissure). Efferents: CN III → ciliary ganglion → short ciliary nerves → sphincter pupillae. Cortical blindness spares the light reflex.