Overview
This lecture explains the pupillary mechanism of the eye, detailing how the iris muscles control pupil size in response to different light conditions.
Eye Structure Recap
- The iris is the colorful part of the eye and is made of two muscles: outer radial and inner circular muscles.
- The pupil is the black hole in the center of the iris that allows light to enter the eye.
Pupillary Mechanism Purpose
- The pupillary mechanism adjusts pupil size to regulate light entering the eye.
- Smaller pupil size prevents too much light (which can cause blinding brightness), while a larger pupil lets in more light in darkness.
Pupil Response in Dim Light
- Radial muscles contract, making the pupil larger (dilates).
- Circular muscles relax during this process.
- More light enters the eye when the pupil dilates.
Pupil Response in Bright Light
- Radial muscles relax and circular muscles contract, making the pupil smaller (constricts).
- Less light enters the eye when the pupil constricts.
Exam Tips and Reflex Arc
- The dim light and bright light mechanisms are opposites: contract/relax and more/less light.
- The pupillary mechanism is a reflex, often linked to exam questions about reflex arcs (e.g., response to a bright torch).
Key Terms & Definitions
- Pupil — Black opening in the iris; controls light entry into the eye.
- Iris — Colored part of the eye; contains the muscles that adjust pupil size.
- Radial Muscles — Outer, elongated iris muscles; contract to enlarge the pupil.
- Circular Muscles — Inner, ring-shaped iris muscles; contract to shrink the pupil.
- Pupillary Mechanism — Process controlling pupil size to regulate light entry.
Action Items / Next Steps
- Review the difference between radial and circular muscles' roles.
- Practice explaining the pupillary mechanism for both dim and bright light conditions.
- Prepare for possible exam questions linking the pupillary mechanism with reflex arcs.