Human Vision and Light

Study Notes

UV Radiation and Vision

UV radiation damages the cornea and anterior structures
Polycarbonate absorbs most UV radiation
Visible light has a wavelength of 400-700 nm

Photoreceptors in the Retina

There are two types of photoreceptors: cones and rods
Cones are responsible for color vision and are concentrated in the fovea centralis
Rods are responsible for peripheral and night vision

Rods

Absence of color discrimination
Poor acuity (~20/200)
Best discrimination about 30 minutes after darkness
Predominate in peripheral vision
Peripheral vision is more sensitive in dim conditions
Motion is better detected in peripheral vision
Rods shed and are recycled by the RPE (retinal pigment epithelium)

Retinitis Pigmentosa (RP)

A genetic disorder characterized by the death of photoreceptors
Bony spicules may be present in the retina
All disc waste remains in the eye and damages the retina
Some RP patients may have healthy-looking retinas but have trouble seeing; ERG (electroretinogram) is needed to diagnose
Not all RP cases are genetic

Rhodopsin

A photopigment found in rod discs
Each eye has 120 million rods, each with 1,000 discs, and each disc has 10,000 molecules of rhodopsin
The large number of rhodopsin molecules allows the eye to capture light and increases sensitivity at nighttime
Weber's fraction (ΔI/I) = 0.14, representing the minimum change in stimuli needed to alter vision

Cones

About 6-7 million cones per eye
Remain constant with age
Color sensitive
Good acuity (20/20)
Adapt rapidly
Weber's fraction (ΔI/I) = 0.015, representing better contrast sensitivity than rods

Cone Photopigments

3 photopigments in the eye: erythrolabe (red), chlorolabe (green), and cyanolabe (blue)
Erythrolabe: peaks around 557 nm, most present (64%), and concentrated in the fovea centralis
Chlorolabe: peaks around 530 nm, about 32% of all cones, and concentrated in the fovea centralis
Cyanolabe: peaks around 426 nm, about 5-10% of all cones, and concentrated outside the fovea centralis

Dark Adaptation

Sensitivity increases by 5 log units in about 35 minutes (100,000 more sensitive)
First arm of the curve (cone section) plateaus after 10 minutes
First arm of the curve breaks after 12 minutes (rod-cone break)
The break represents the time when rods become more sensitive than cones
Second arm of the curve plateaus after 35 minutes
Represents the scotopic threshold for the specific wavelength

Other Retinal Disorders

Congenital Stationary Night Blindness (CSNB): a genetic disorder affecting rod function, primarily X-linked recessive
Traumatic Retinopathy: mimics RP, caused by traumatic injuries
Syphilis: can cause retinal damage
Rubella: can cause abortions, eruptions in skin, and can be mistaken with RP
Thioridazine Retinopathy: an anti-psychotic medication causing crystalline deposits in the retina, confused with RP

Argus II: Bionic Eye

A device that gives black and white detection, outlines, borderlines, and patterns to RP patients
Artificial retina; helps RP patients

Medications Affecting the Eye

Acne medication (Isotretinoin): can cause dry eyes and sudden decrease in night vision
Antihistamines: can cause angle-closure glaucoma in some people
Antimalarial drugs (Chloroquine, Hydroxychloroquine): can cause retinal toxicity, leading to vision loss
Corticosteroids (Prednisone): increase the risk of cataract and glaucoma (steroid responders)
Erectile dysfunction drugs (Sildenafil, Tadalafil, Vardenafil): can cause blurred vision, sensitivity to light, and seeing a blue tinge
Ethambutol (Myambutol): an antibiotic used to treat TB, associated with optic neuropathy problems
Tamoxifen (Nolvadex): an antiestrogen drug used to treat breast cancer, can lead to crystalline retinopathy
Topiramate (Topamax): an anticonvulsant used to treat seizure disorders, associated with angle-closure glaucoma
Vitamin A deficiency: required for synthesis of rhodopsin, can occur due to dietary restrictions or diseases

Description

This quiz covers the basics of human vision, including the effects of UV light, the structure of the retina, and the differences between day and night vision. Learn about the roles of cones and rods in our visual system.