Vision

Refraction

•  diopters-reciprocal of focal distance (m) •  cornea is 0.024 m, 42 diopters •  Emmetropia-normal, •  Hyperopia-far-sighted, need convex lens, •  Myopia-near-sighted, need concave lens, •  involves abnormal elongation of the eye •  visual angle, acuity - Snellen eye chart -

20/20 is seeing letter 5 min (1/60 degree)

Accomodation

•  loss of accomodation with age Presbyopia

•  Benjamin Franklin developed bifocals •  If ciliary muscle is relaxed, ligaments

are tight and lens is stretched flat •  If ciliary muscle contracts, ligaments

have slack and lens relaxes to greater bulge for near vision.

Other disorders

•  Glaucoma - pressure aqueous humor, ganglion cells die, drops or surgery

•  Floaters in vitreous especially in myopia •  Diabetic retinopathy blood vessels overgrow, laser

decreases angiogenesis •  Cataract - intraocular lens, made of polymethyl

methacrylate, fire (and since about 1988, these have been doped with UV blockers)

Genetic blindness

•  Retinitis pigmentosa is tragic, lose rod vision (tunnel vision [ring scotoma] because rods are in mid-periphery). eventually cones

•  autosomal & X-linked, dominant & recessive •  Vs stationary (not progressive) blindnesses •  molecules of transduction cascade & other rod and

cone molecules. •  web site (site) •  Age related macular degeneration

Pupil

•  atropine, a muscarinic antagonist, dilates •  parasympathetic nervous system constricts •  parasympathetic = cranio-sacral, •  occulomotor nerve (#III) •  the sympathetic n.s. dilates (in dim light), •  nerve from the superior cervical ganglion •  (of the thoraco-lumbar system) •  A bright light in one eye causes the other pupil to

constrict too.

sensitivity

•  Rod can see one quantum •  Calculation involves Planck’s constant •  Auditory sensitivity is also amazing

counts

•  Photoreceptors- 125 million receptors •  20/1 rods to cones •  (converge on 1 million ganglion cells) •  sensitivity of rods (converge) •  acuity for cones (one on one)

summary

•  rods off-fovea, cones on-fovea •  Rod, peripheral vision, dim black and white,

sensitive - "scotopic"Very sensitive - 1 photon •  Cone, fovea, color, acuity - "photopic” •  RPE: (1) melanin that blocks light reflection •  (2) metabolism to provide 11-cis retinal

(chromophore of visual pigment, rhodopsin) •  (3) phagocytosis and recycling of shed rod tips

Color Vision

•  Young -Helmholtz trichromatic theory •  3 kinds of cone 420 530 560 •  3 cone opsins humans & OW monkeys •  green and yellow (middle and long

wavelength) cone opsin near each other on X

•  (blue cone opsin is on human chromosome 7, rod on chromosome 3)

more

•  evolutionary bottleneck hypothesis •  Red or green color blindness - on X, males. •  number of copies in humans variable •  cross-over accidents can make chimeric genes •  Female "carriers" should actually be mosaics •  Mary Lyon X-inactivation hypothesis •  superfamily of G-protein-coupled receptors (7

transmembrane domain receptors)