Overview
This lecture explains the pupillary light reflex, outlining its pathway, key structures, and clinical significance in regulating pupil size in different lighting.
Function of the Pupillary Light Reflex
- The pupillary light reflex controls how much light enters the eye by changing pupil diameter.
- It allows vision to adapt to various light levels.
Pathway of the Pupillary Light Reflex
- Retinal ganglion cells detect light and send signals via the optic nerve.
- Some optic nerve fibers bypass the lateral geniculate nucleus and synapse at the pretectal nucleus.
- The pretectal nucleus sends bilateral projections to each Edinger-Westphal nucleus.
- Parasympathetic fibers from the Edinger-Westphal nuclei travel with the oculomotor nerve to the ciliary ganglia.
- Short ciliary nerves then stimulate the sphincter pupillae muscle to constrict the pupil.
Clinical Observation
- Shining light in one eye causes its pupil to constrict (direct reflex).
- The opposite pupil also constricts at the same time (consensual reflex).
- Parasympathetic action on the sphincter pupillae muscle causes pupil constriction.
Muscles Controlling Pupil Size
- The iris has two muscles: sphincter pupillae and dilator pupillae.
- Sphincter pupillae (circular muscle) constricts the pupil, controlled by the parasympathetic nervous system.
- Dilator pupillae (radial muscle) dilates the pupil, controlled by the sympathetic nervous system.
Key Terms & Definitions
- Pupillary Light Reflex — Reflex that adjusts pupil size in response to light intensity.
- Retinal Ganglion Cells — Neurons in the retina that transmit visual information.
- Optic Nerve — Nerve carrying visual signals from the eye to the brain.
- Pretectal Nucleus — Midbrain region involved in pupillary light reflex.
- Edinger-Westphal Nucleus — Parasympathetic nucleus mediating pupil constriction.
- Ciliary Ganglion — Cluster of nerve cells involved in eye muscle control.
- Sphincter Pupillae — Muscle that constricts the pupil.
- Dilator Pupillae — Muscle that widens the pupil.
Action Items / Next Steps
- Review the pathway and function of the pupillary light reflex.
- Study the roles of iris muscles and their innervation.