Chapter 2 - study notes

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19 Dec 2010
-light is form of electromagnetic radiation energy produced by vibrations of electrically
charged material, 2 types of light: waves=fluctuation that travels through a medium by
transferring energy from one particle or pt to another w/o causing any permanent
displacement of medium ; photons = quantum of visible light or other form of
electromagnetic radiation demonstrating both particles and wave properties. Light made up
of waves when it moves around and made up of photons when it is absorbed
-full spectrum of electromagnetic radiation is very wide, visible light only small portion of
spectrum, from 400-700nm, violet to red
-light can be absorbed = to take up, refracted = to alter as water does to light entering it
from the air, to measure degree of refraction in a lense or eye, reflected = to redirect,
scattered = to disperse, transmitted = to convey smt from one place to another
-light hits cornea (1), it is transparent thus most light photons are transmitted rather than
reflected or absorbed, transparent cuz made of no blood vessels which absorbs light,
arrangement of fibers, high transparent sensory nerve endings, which forces eyes to close
and produce tears if cornea is scratched, preserving itz transparency
-aqueous humor (2) = fluid derived from blood, fills space right behind cornea, supplying
oxygen and nutrients to and removing waste from cornea and crystalline lens.
-(3)pupil = controls amount of light that reaches retina via papillary light reflex, iris
-when coming from dark room to bright, pupil constricts and photic sneeze reflex results
from crossed wires in brain
- (4)crystalline lens. Lense like cornea has no blood supply so transparent, it enables
changing focus shape of lense controlled by ciliary muscle
-(5) vitreous humor = light is refracted for the 4th and final time, only not refracted in
pupil, longest part of hte journey through eyeball, 80% of internal volume of eye, gel-like,
-floaters = small bits of biodebris that drift around in vitreous
-(6) retina = only a bit o f light reach here. Half reaches corner, role of retina is to detect
light and tell brain about aspects of light that area related to objects in world. Retina is
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where seeing really begins. it contains rods and cones, receives image from lens and send it
to brain through optic nerve
-image on retina is reversed right to left and is upside down. Hold in retina where optic
nerve leaves eyeball is the optic disc where there is no photoreceptors ie blind spot
-cornea is curved has highest refractive surface in eye but aqueous and vitreous humors are
fixed structures and thus ca not refract much. lense does the focusing whose refractive
power can be alter by changing itz shape accommodation =change in focus done through
contraction of ciliary muscle which is attached to lens via zonules of Zinn fibers. When
ciliary musclerelax, onules are stretched and lens is flat, eye then focuses on distant objects.
When ciliar muscle contracts it focus on closer stuff, the len bulge, the bulger ie fatter the
lens the more power it has
-accommodation enables power of lens to vary by 15 diopters, ability to accommodate
declines with age, 8yr, we lose about 1 diopter of accommodation every 5yrs up to age 30
-presbyopia = old sight, loss of near vision cuz of insufficient accommodation, can see near
stuff, the lens becomes sclerotic, hard, and capsule that encircles lense that enables it to
change shape loses its elasticity. Invention of bifocals lenses that have one power at top,
allowing to see distant objects, and a diff power at bottom, seeing close objects
-emmetropia = normal, no refractive error, the refractive power of the eyes 4 components
is perfectly matched to length of eyeball . Myopia = nearsighted, cant see far stuff, eye is
too long.trt w/ neg lenses which diverge rays of starlight b4 they enter eye, image falls IN
FRONT Of retina Hyperopia is far sighted, cant see near stuff, eye is too short, image
falls BEHIND retina. Trt accommodation or use + lenses which converge rays of light b4
enter eye
-opacities of lens, lost of transparency = cataracts caused by irregularity of crystallines
lens, cataract interfere with vision cuz they absorb and scatter more light than normal lens,
trt is replaced with plastic or silicone implant of the opacified lens
-human eye is 24mm on average but w.e the size as long as refractive power match it is
good. Most BB are hyperopia cuz optical part is more developed than the eyeball
-when cornea is not spherical =astigmatism vertical lines might be focused slightly in front
of the retina while horizontal lines are focused slightly behind it, this is caused by unequal
curving of one or more of the refractive surfaces of eye, usually cornea
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Purpose of camera is to take pix but for human visual system is to interpret image
Light energy is transduced (conversion from one form of energy to another ie light to
electricity) to neural energy that can be interpreted by brain
Fundus= back layer of retina seen via ophthalmoscope, optic disc is where the arteries and
veins that feed retina enter eye and where axons of ganglion cells leave eye via optic nerve,
only place in the body where one can see the arteries and veins directly, important window
on well-being of body’s vascular system, it not provide a detail view of retina, we need
-light entering retina passes through: ganglion cell layer, inner/intermediate nuclear layer
(ie the amacrine cells, horizontal cells, bipolar cells) then photoreceptor layer then pigment
epithelium, lastly sclera . then it comes back out from photoreceptors to intermediate layer
to frontmost layer of retina made up of ganglion cells whose axons pass through optic nerve
to brain.
-transduction of light energy into neural energy beings in backmost layer of retina ie the
- retina contains 5 types of neurons: photoreceptors, bipolar, horizontal, amacrine and
ganglion cells
-retinas have 2 types of photoreceptors: rods and cones =duplex retinas. Both consist of an
outer segment, Inner segment , and synaptic terminal. inner segment make visual
pigments that are stored in outer segment. Visual pigment consist of protein called opsin,
that determines which wave length of light they absorb and a chromophore which
captures light photons
-each photoreceptor has only 4 types of visual pigments. Pigment rhodopsin found in rods,
each cone has one of the other 3 pigments which respond to long, medium, short
-recently found 3rd type of photoreceptor that adjust our biological rhythms to match the day
and night of the external world
-photoactivation = activation by light. Light goes to outer segment of rod and is absorbed
by molecule of rhodopsin, it transfers its energy to chromophore portion of visual pigment
molecule that then closes channels in cell membrane that normally allow ions to flow into
rod outer segment. Closing channels alters balance of electrical current /b/ inside and
outside of rod outer segment, making inside of cell more neg charged = hyperpolarization
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