L6: Vision I Flashcards
define refraction
the bending of light waves at an angulated surface of a transparent material
light striking a perpendicular surface will ?
continue thru w/o bending
what does the degree of refraction depend on ?
- ratio of two refractive indices of the 2 medias
2. degree of angulation between the interface and entering waves
define refractive index
the ratio of the velocity of light in air to the velocity of light traveling in the substance
RI of air = 1.0
define refractive power
a measure of how much a lens bends light waves
measured in diopters
define diopter
1 diopter = 1 meter / focal length of lens
define focal point
the point thru which all parallel rays of light will pass
after passing thru the lens
define focal length
the distance from lens center to focal point
function of iris
controls the amount of light entering the eye
from darkness to light conditions
total refractive power of the eye
59 diopters
refractive indices of parts of the eye: cornea, aqueous humor, lens, vitreous humor
cornea = 1.38 ah = 1.33 lens = 1.4 vh = 1.34
define emmetropia
a person with ideal/perfect vision
parallel light rays from distant objects come into sharp focus on the retina when ciliary muscles completely relax
all distant objects can be seen clearly
define hyperopia
farsightedness
being able to seen far objects clearly but closer objects are not clear
cause of hyperopia
usually due to an eyeball that is too short
or
occasionally a lens system that is too weak
define myopia
nearsightedness
cannot see distant objects but closer things are clear
when ciliary muscle is completely relaxed, light rays coming from distant objects are focused in from of the retina
cause of myopia
usually due to too long of an eyeball
compared to the focusing power of the eye
define visual acuity
a measure of the resolving power of the eye
average diameter of a cone
and
max visual acuity for 2-pt sources of light
average d for cone = 1.5 micrometer
thus
max va for 2-pt source is = 1.5 to 2 micrometers
depth of focus of the lens ______ as the pupillary diameter ______.
increases
decreases
negative relationship
ions moved to create aqueous humor
ions drawn into space in ciliary apparatus created by 2 layers of epithelial cells
Na active trans in
Cl in
bicarbonate in
how is aqueous humor
as the ions are drawn into the space
water follows via osmosis
thus creating an aqueous solution that is passed into the anterior chamber
flow of fluid in the eye
fluid flows from:
ant. chamber
canal of schlemm
aqueous veins in sclera
what is used to measure intraocular pressure?
tonometer
normal p = 15 mmHg
how is a tonometer used?
eye is numbed and a stain applied
a tool is used to just touch the cornea and can measure the pressure w/in the eye
cheaper way but less accurate is the air pistol where they put a puff of air into the eye
what is glaucoma?
why is it so dangerous?
increased fluid pressure in your eye
this increased p can damage the optic nerve and lead to blindness
the fluid drains too slow, so pressure slowly builds but it is painless so it can happen over years w/o notice
photosensitive pigments in rods and cones
rods = rhodopsin
cones = 3 dif. color pigments
list the functional parts of photoreceptors
outer segment
inner segment
nucleus
synaptic body
outer segment
site of light sensitive photochemicals
inner segment
contains organelles
especially mito
synaptic body
connects w/ horizontal and bipolar cells
how does light affect rhodopsin?
rhodopsin + light = converts 11cis-retinal to all trans-retinal
requires isomerase and metabolic nrg to return to normal
metarhodopsin II
an intermediate between the transforming of retinal
it excites electrical changes in rods
vitamin A is also called?
all-trans-retinol
an alternate pathway for all-trans-retinal to return to 11-cis-retinal
results of an excited rod
–increased negativity of intrarod memb. potential (hyperpolarization)
decomposition of rhodopsin ?
decreases rod memb. conductance for Na ions in the outer segment
resulting in hyperpolarization of entire rod memb.
describe outer segment membrane of rods in dark
leaky to Na ions that flow thru cGMP-gated channels
this slow leak into cell neutralizes the negativity of the entire cell
thus reduced electronegativity in dark conditions
-40mV instead of normal -70 to -80 mV
inner segment of rods
continuously pumps Na out and K in
K leaks out via non-gated channels
a negative potential is created inside cell
when rhodopsin is exposed to light?
it begins to decompose
the retinal portion is activated and stims transducin (G-protein)
active transducin
activates cGMP phosphodiesterase
which catalyzes cGMP to 5’GMP
reduction in cGMP causes?
closure of Na channels
thus rod becomes hyperpolarized
when rhodopsin is activated, __________ channels close for each ?
several hundred channels close
for each rhodopsin molecule activated
cones are similar to rods except that the protein portions (_____) of the photochemicals are ?
opsins
are dif. from the scotopsin in the rod photochemicals
colors cones are sensitive to?
blue
green
red
overlapping of light absorption spectrum results in humans ability to see wide number of colors
wavelength of colors
blue = 390 - 530
green = 440 - 620
red = 480 - 690
scotopsin + 11-cis-retinal = ?
rhodopsin
rhodopsin + light = ?
scotopsin + all-trans-retinal
now that it is converted it will separate from scotopsin
alternative pathway using Vit. A
all-trans retinal –> all-trans-retinol
all-trans-retinol –> 11-cis retinol —> 11-cis retinal
