Ophthalmology
Photoreceptors
Rods, cones, and the molecular cascade that turns light into neural signals
Photoreceptors convert light into electrical signals via phototransduction. Two types — rods (~120 million per retina, exquisitely sensitive, single-photon detection, peak rhodopsin sensitivity 498 nm, scotopic/night vision) and cones (~6 million, less sensitive, color vision in three types: S/blue 420 nm, M/green 534 nm, L/red 564 nm). Photoreceptors are unusual neurons — they are depolarized in darkness and hyperpolarize in response to light. Light isomerizes 11-cis-retinal to all-trans-retinal in opsin, activating transducin, which activates phosphodiesterase, hydrolyzing cGMP, closing CNG cation channels. Disease: retinitis pigmentosa (rod loss first, then cones), age-related macular degeneration (cone-rich macula), color blindness (X-linked).
- Rods~120 million; rhodopsin (498 nm peak)
- Cones~6 million; S/M/L opsins (420/534/564 nm)
- SensitivitySingle photon detection in dark-adapted rods
- PhototransductionLight → 11-cis-retinal isomerization → cascade
- Output to brainVia bipolar cells → ganglion cells → optic nerve
- Resting stateDepolarized in dark (unique among neurons)
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Why photoreceptors matter
- Vision. The first step of sight; everything downstream depends on photoreceptor function.
- Retinitis pigmentosa. Genetic photoreceptor diseases now have first gene therapies.
- Macular degeneration. Leading cause of central vision loss in elderly.
- Color blindness. Genetic cone variants affect 8% of males.
- Diabetic retinopathy. Disrupts blood supply to photoreceptors and RPE.
- Vitamin A deficiency. Causes night blindness from impaired retinal supply.
- Drug toxicity. Hydroxychloroquine, ethambutol, sildenafil affect photoreceptors.
Common misconceptions
- Photoreceptors fire when light hits them. They actually hyperpolarize and reduce neurotransmitter release.
- We have one type of cone. Three cone types, each tuned to different wavelengths.
- Color is in the light. Color is the brain's interpretation of cone activation ratios.
- Cone vision works at night. Cones need bright light; rods do scotopic vision.
- Eyes regenerate photoreceptors. Mammalian photoreceptors don't regenerate; lost cells are permanent.
- Reading in dim light damages eyes. Causes eye strain but not permanent damage.
Frequently asked questions
Why are photoreceptors depolarized in the dark?
This counterintuitive design means light hyperpolarizes rather than depolarizes. In darkness, cGMP levels are high, keeping cyclic nucleotide-gated (CNG) channels open. Na+ and Ca2+ flow in, depolarizing the cell to -40 mV. This 'dark current' triggers tonic glutamate release at the synapse. Light activates phosphodiesterase, dropping cGMP, closing CNG channels, hyperpolarizing the cell to -70 mV, and reducing glutamate release. Bipolar cells decode the change.
How does the phototransduction cascade work?
A single photon hits rhodopsin (or cone opsin), isomerizing 11-cis-retinal to all-trans-retinal. The activated opsin (metarhodopsin II) catalyzes GDP-GTP exchange on transducin (a G-protein) — one rhodopsin activates ~500 transducins. Each transducin activates phosphodiesterase, which hydrolyzes ~1000 cGMP molecules. Loss of cGMP closes CNG channels. Net amplification: one photon to 100,000+ closed channels, enabling single-photon sensitivity in rod vision.
How do we see color?
Three cone types with different opsins absorb different wavelengths — S-cones (blue, 420 nm), M-cones (green, 534 nm), L-cones (red, 564 nm). The brain compares activation ratios to perceive color. M and L genes are on X chromosome, explaining male predominance of red-green colorblindness (8% of males vs 0.5% of females). Trichromatic theory (Young-Helmholtz) plus opponent-process theory (Hering) together explain perception. Some women carry 4 cone types (tetrachromacy).
What is the visual cycle?
After photoactivation, all-trans-retinal is reduced to all-trans-retinol, transported to retinal pigment epithelium (RPE), re-isomerized to 11-cis-retinol by RPE65 enzyme, oxidized back to 11-cis-retinal, returned to photoreceptors, and recombined with opsin. Without this cycle, vision would fail in seconds. RPE65 mutation causes Leber congenital amaurosis — the first FDA-approved gene therapy (Luxturna, 2017) restores RPE65 function.
Why are rods more sensitive than cones?
Rods amplify each photon more strongly via convergence — many rods synapse onto one bipolar cell, summing weak signals (~100 rods per ganglion cell). Cones have less convergence (1:1 in fovea) for spatial resolution. Rods also have more rhodopsin per cell and longer integration times. Rods saturate in bright light (only 30 photons/sec) — that's why color vision dominates in daylight. Dark adaptation: cones recover in 5-7 minutes, rods take 30+ minutes for full sensitivity.
What causes retinitis pigmentosa?
A group of inherited retinal dystrophies (>50 genes — RHO, RPGR, USH2A) characterized by progressive photoreceptor death. Rods die first, causing night blindness and peripheral vision loss; cones follow, causing central vision loss. Bone-spicule pigmentation, attenuated retinal vessels, waxy disc pallor on fundoscopy. Onset: childhood to middle age. Treatment limited — vitamin A palmitate slows progression; gene therapy emerging; retinal implants (Argus II) for late-stage cases.
How does age-related macular degeneration affect cones?
AMD is the leading cause of blindness in adults > 65. The macula (cone-rich central retina) is preferentially affected. Dry AMD (90%): drusen accumulate beneath RPE, geographic atrophy of photoreceptors. Wet AMD (10%): choroidal neovascularization causes hemorrhage and rapid vision loss. Risk factors: age, smoking, genetics (CFH, ARMS2). Treatment: AREDS vitamins for dry AMD; intravitreal anti-VEGF (ranibizumab, aflibercept) for wet AMD.