## Structural and Functional Distinction Between Rods and Cones ### Photopigment Composition and Spectral Sensitivity **Key Point:** The most fundamental and clinically relevant discriminator is the **photopigment type and wavelength sensitivity**. - **Rods:** Contain **rhodopsin** (11-cis-retinal + opsin). Maximally sensitive at **~498 nm** (blue-green light). Monochromatic vision. - **Cones:** Contain **iodopsin** (three types: S, M, L cones). Peak sensitivities at **420 nm (S), 530 nm (M), and 560 nm (L)**. Enable color vision via trichromatic mechanism. ### Comparative Table | Feature | Rods | Cones | |---------|------|-------| | **Photopigment** | Rhodopsin | Iodopsin (3 types) | | **λ_max (nm)** | 498 | 420, 530, 560 | | **Vision type** | Scotopic (dim light) | Photopic (bright light) | | **Spatial location** | Peripheral retina | Fovea centralis (high density) | | **Threshold** | Low (sensitive) | High (less sensitive) | | **Acuity** | Poor | Excellent | | **Number** | ~120 million | ~6 million | ### Clinical Pearl **High-Yield:** The **wavelength of maximum absorption** is the single best discriminator used in physiology exams and is the basis for understanding color blindness (e.g., red-green color blindness involves M and L cone defects). ### Why This Matters The photopigment difference explains why: - Rods mediate night vision (scotopic) - Cones mediate day vision and color perception (photopic) - Patients with cone dystrophy lose color vision and bright-light acuity first - Rod-monochromat individuals (rare) have only rods and see in grayscale [cite:Guyton & Hall 14e Ch 51] 
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