Overview
This lecture covers the clinical significance, anatomy, and neural pathways of the pupillary light reflex, near reflex, and psychosensory reflex, with a focus on nerve supply to the sphincter and dilator pupillae muscles.
Clinically Significant Pupillary Reflexes
- There are three main clinically relevant pupillary reflexes: light reflex, near reflex, and psychosensory reflex.
- Light reflex: pupil constricts when light is shone into the eye (both direct and consensual response).
- Near reflex: pupil constricts when shifting gaze from far to near objects.
- Psychosensory reflex: pupil dilates in response to emotional or sensory stimuli.
Types of Light Reflex
- Direct light reflex: constriction of the pupil in the eye where light is shown.
- Consensual light reflex: constriction of the opposite eye’s pupil when light is shown in one eye.
- Proper testing conditions: low illumination, patient looking at distance, use a bright, focused light.
Light Reflex Pathway
- Afferent limb: from retina to pretectal nucleus in the midbrain (bypass lateral geniculate body).
- Pretectal nucleus sends fibers to both Edinger-Westphal nuclei (bilateral innervation).
- Efferent limb: parasympathetic fibers from Edinger-Westphal nucleus travel via oculomotor nerve (CN III) to ciliary ganglion, then via short ciliary nerves to sphincter pupillae.
Pupillary Muscles and Nerve Supply
- Sphincter pupillae: circular muscle causing constriction, supplied by parasympathetic fibers (short ciliary nerves).
- Dilator pupillae: radial muscle causing dilation, supplied by sympathetic fibers (long ciliary nerves).
Parasympathetic (Constriction) Pathway
- Originates from Edinger-Westphal nucleus (in midbrain) → oculomotor nerve (inferior division) → ciliary ganglion (preganglionic) → short ciliary nerves (postganglionic) → sphincter pupillae.
Sympathetic (Dilation) Pathway
- Originates from hypothalamic center → descends to ciliospinal center of Budge (C8–T4) → preganglionic fibers synapse at superior cervical ganglion.
- Postganglionic fibers form carotid plexus, join ophthalmic division of trigeminal nerve (V1), enter nasociliary branch, travel as long ciliary nerves → dilator pupillae.
- Sympathetic fibers do not relay in ciliary ganglion.
Key Terms & Definitions
- Direct Light Reflex — Pupil constriction in the stimulated eye.
- Consensual Light Reflex — Pupil constriction in the unstimulated (opposite) eye.
- Sphincter Pupillae — Circular muscle that constricts the pupil; parasympathetic control.
- Dilator Pupillae — Radial muscle that dilates the pupil; sympathetic control.
- Pretectal Nucleus — Midbrain structure relaying the light reflex pathway.
- Edinger-Westphal Nucleus — Parasympathetic nucleus controlling pupil constriction.
- Ciliary Ganglion — Ganglion for parasympathetic innervation of the eye.
- Ciliospinal Center of Budge — Spinal cord segment (C8–T4) for sympathetic outflow to eye.
- Short Ciliary Nerves — Parasympathetic/postganglionic fibers to sphincter pupillae.
- Long Ciliary Nerves — Sympathetic/postganglionic fibers to dilator pupillae.
Action Items / Next Steps
- Review and memorize the pathways for both parasympathetic and sympathetic innervation of the pupil.
- Practice drawing the pupillary light reflex pathway.
- Prepare for questions on clinical testing of pupillary reflexes.