Lecture: Physiology of Vision
I. Overview of Light and Optics
A. Wavelength and Colors of Visible Radiation
1. electromagnetic radiation
gamma rays X-rays
UV light VISIBLE LIGHT Infrared Radio Wave
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short
medium
long
(10-5 nm)
(380-750 nm)
(102 nm)
2. wave-photon duality - light travels in wave-like fashion with "single packets" of energy called
photons
3. visible spectrum - different colors of light have different wavelengths
Violet
Blue
Green
Yellow
Orange
Red
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380 nm
480 nm
550 nm
630 nm
680 nm
730 nm
4. color of an object - the color of an object is determined by which wavelengths are
REFLECTED back to the retina (not absorbed by the object)
a. white - all wavelengths reflected by object
b. black - all wavelengths absorbed by object
B. Refraction of Light and Convex Lenses
1. light refraction - light will bend when it passes from one medium (air) into another (lens)
e.g. pencil in glass of water
2. convex lens - (thicker at center, tapered at edge) causes light to bend so that it comes together
at a focal point
a. real image - image at focal point of convex lens ---> inverted & reversed
3. focusing light on the retina
a. cornea - constant (unchanging) refraction
b. lens - can change refraction and focal length; ciliary muscles change convexity of the lens
4. Focusing for Distance Vision
a. far point of vision - distance beyond which lens will not change its shape (about 20 feet)
(flattest point of the lens)
b. emmetropic eye - normal, healthy eye
5. Focusing for Close Vision
Less than 6 feet, several adjustments are made:
a. accommodation of lens - lens shape becomes more convex, light rays bend more sharply,
shorter focal length for the closer object (ciliary muscles for lens)
i. near point of vision - shortest distance for focusing (maximum convexity of lens);
about 8-10 inches; gets worse with age
ii. presbyopia - poor close vision in elderly; inelasticity of the lens
b. accommodation of pupils - constriction of pupils; better focus, less divergent rays
(constrictor muscles of iris)
c. convergence of eyes - eyes rotate medially to keep image on center of the retina
(medial rectus muscles of eyeballs)
C. Vision Problems Related to Refraction
1. myopia ("nearsighted") - distant objects are blurred; distant objects are focused in front of the retina,rather
than directly on it
a. eyeball too long; lens too strong
b. concave lens can correct light before eye
2. hyperopia ("farsightedness") - close objects are blurred; close objects are focused beyond the
retina, rather than directly on it
a. eyeball too short; poor refraction of a lens
b. convex lens can correct light before eye
3. astigmatism - blurry images at all distances; unequal curves on lens and/or cornea, creating
discontinuous image on the retina
II. Anatomy, Biochemistry, & Physiology of Photoreceptors
A. Functional Anatomy of Photoreceptors
1. General Structure of Rods and Cones
"pigmented base"
of retina
"neural layer"
outer segment (pigmented discs)
connecting stalk
inner segment (mitochondria)
outer fiber
cell body (nucleus)
inner fiber
synaptic ending
bipolar cell
ganglion cell (axons carried to brain by optic nerve)
a. outer segment - contain membrane-bound discs with pigments that absorb and react to light
i. rods - pigment discs stacked like pennies all the way to the base, membranes are
DISTINCT from the plasma membrane
1. sensitive to dim light (night vision)
2. respond to ALL wavelengths (colors)
3. only "grey" information to the brain
4. 100 rods per ganglion cell to brain
5. widely spread throughout the retina
6. not good for visual acuity
ii. cones - pigment discs taper off toward the base, membranes are CONTINUOUS with the
plasma membrane
1. require bright light for stimulation
2. different cones have different pigments specific for certain wavelengths (colors)
3. can convey color information to brain
4. 1-3 cones per ganglion cell to brain
5. primarily concentrated in fovea (center)
6. essential for visual acuity
B. Biochemistry of Visual Pigments
1. opsin - transmembrane protein in the membrane of pigmented discs of rods and cones
2. retinal - light absorbing molecule that changes shape when struck by a photon of light
a. vitamin A - precursor to retinal (eat your carrots!!!!!!!!!!!!!)
b. 11-cis isomer of retinal - non-activated form of retinal, prior to absorption of photon
energy; has a "kinked" double bond
c. all trans isomer of retinal - activated form of retinal, after struck by photon of light; double
bond straightens out
C. Excitation of Rods
1. rhodopsin - visual pigment in rods; in membranes of pigmented discs of outer segment
2. bleaching of pigment - breakdown of rhodopsin after the absorption of light
11-cis retinal ----------> rhodopsin --------> all-trans retinal
+ scotopsin
light + scotopsin
3. all-trans retinal - causes HYPERPOLARIZATION of rod
a. Na+ channels (open in dark) are closed
b. rod is hyperpolarized (increased negativity)
c. Ca++ channels in synapse close
d. less neurotransmitter released by the rod
D. Excitation of Cones
1. photopsins - 3 distinct pigments in cones are sensitive to 3 different parts of visible spectrum
a. blue cones - maximum sensitivity at 455 nm
b. green cones - maximum sensitivity at 530 nm
c. red cones - maximum sensitivity at 625 nm
2. different colors - differential activation of each of the three different cones
3. color blindness - inherit gene for one of the photopsin proteins that is deficient (mainly male);
most common are red and green mutations
E. Light and Dark Adaptation of Rhodopsin
1. light adaptation - very dark ----> very bright
a. rhodopsin in rods is quickly bleached out
b. sensitivity to shallow light disappears
c. rods are inhibited by other retinal cells
d. cones are activated to take over (5 mins.)
e. consensual pupil reflex - constriction
2. dark adaptation - very bright ----> very dark
a. cones are gradually cease to be stimulated
b. "bleached out" rods can produce rhodopsin
c. rods eventually take over in the dim light
d. pupillary dilation - pupils increase size
3. nyctalopia (night blindness) - deficiency in function of rods during dim-light situations
a. vitamin A deficiency is general cause
III. The Visual Pathway: Photoreceptors to Occipital Cortex
RETINA
photoreceptors (rods & cones) ->
bipolar cells ->
ganglion cells (axons = optic nerve) ->
AXON PATH
optic nerves (from each eye retina)
optic chiasma (medial fibers cross over)
optic tracts (opposite visual field)
THALAMUS
lateral geniculate body of thalamus ->
AXON PATH
optic radiation (fibers to cortex)
CEREBRAL CORTEX
occipital lobe - primary visual cortex
other brain areas that receive visual information:
1. superior colliculi - for control of extrinsic eye muscles
2. pretectal nuclei - mediate pupillary light reflexes
3. suprachiasmatic nucleus of hypothalamus - circadian rhythm
IV. Binocular Vision and Depth Perception
A. binocular vision - two eyes have overlapping regions of the visual field, so that the same point is
from slightly different angles
1. depth perception - a result of binocular vision in which person can perceive relative distances
based on information gathered in both eyes
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