What is refraction?
The bending of light waves at an angulated surface of transparent material -note that might striking a perpendicular surface continues on through without bending
What does the degree of every fraction, amount of bending, depend on?
The ratio of the two reflective indicates of the two transparent media and the degree of angulation between the interface in the entering wavefront of the light waves
What is the reflective index?
It is the ratio of the velocity of light and air to the velocity of light traveling in the substance ratio index of air equals 1.0
What is the refractive power
It is a measure of how much a lens bends light Waze. It measured in diopters
One diopter equals
1 m divided by the focal length of a lens
What is a focal point?
The point through which all parallel rays of light will pass after passing through each part of the lens
What is the focal length?
The distance from the center of the lens to the focal point
Compare the eye to a camera
How does the iris of the iPhone's and in a similar manner to the iris diaphragm of a camera?
It controls the amount of light entering the eye from darkness to light conditions
the depth of focus of the lens to increases
the pupillary diameter decreases
What is emmetropia?
A normal eye with regard to depth of focus, parallel light rays from distant objects are in sharp focus on the retina when the ciliary muscle is completely relaxed, all distant objects can be seen clearly
What is hyperopia?
farsightedness usually due to an eyeball that is too short or occasionally a lens system that is too weak all distant objects can be seen clearly
What is myopia?
nearsightedness when ciliary muscle is completely relaxed, light rays coming from distant objects are focused in front of the retina usually due to too long of an eyeball
What kinds of lenses would be used to correct hyperopia and myopia?
What is visual acuity?
A measure of the resolving power of the eye
What is the avg diameter of a cone in the retina?
What is teh maximum visual acuity for two point sources of light?
1.5 to 2 mm
What will light form a distant point source normally create?
a spot with a diameter of about 11 um and a center that is brighter than the periphery
Where is aqueous humor formed?
The ciliary processes
What does teh ciliary apparatus consists of?
two opposing layers of epithelial cells with a space btw them. Na ions are actively transported inot this space and are accompanied by Cl and bicarbonate ions
Where do Cl and bicarbonate ions draw water via osmosis into?
The space and the aqueous solution is then passes into the anterior chamber
How does the flow of fluid occur?
Fluid flows from the anterior chamber into the canal of Schlemm and from there into aqueous veins in the sclera
What is used to measure the intraocular pressure?
a tonometer -normal pressure is 15 mm Hg
A group of eye conditions that leads to damage to the optic nerve
Photosensitive pegments in rods:
photosensitive pigments in cones:
three different color pigments
What are these photosensitive pigments?
transmembrane conjugated proteins
site of light sensitive photochemicals
contains organelles esp mitochondria
connects with horizontal and bipolar cells
What are the major functional segments of the rod and the cone?
outer segment inner segment nucleus synaptic body
scotopsin + 11-cis retinal
Rhodopsin + light --->
scotopsin +all-trans retinal
What is 11 cis retinal converted to?
all trans retinal and dissociates from scotopsin
What is metarhodopsin II?
an intermediate in this pathway that excites electrical changes in the rods
scotopsin or all trans retinal
all trans retinal to
11 cis retinal via isomerase or all trans retinol (vit A)
11 cis retinal to
11 cis retinol to
11 cis retinal
all trans retinol vitamin A isomerase to
11 cis retinol -requires isomerase and metabolic energy
What does excitation of the rod cause?
Increased negativity of the intrarod membrane potential =hyperpolarization
What does decomposition of rhodopsin cause
decreases rod membrane conductance for Na ions in the outer segment
What does decomposition of rhodopsin result in?
hyperpolarization of entire rod membrane
What does the inner segment of the rod do?
continually pumps Na from inside rod to outside and K in opposite direction
How does K lead out of rod cell?
via non gated Channels
What kind of potential is created in the Rod cell
Outer segment of rod in the dark
-membrane is leaky to Na ions that flow through cGMP-gated channels -Na ions leak into cell and neutralize much of negativity on the inside of the entire cell -under dark conditions, there is reduced electronegativity inside the membrane (-40 mV rather than the expected -70 to -80 mV
When rhodopsin is exposed to light?
it begins to decompose
The retinal portion of the rhodopsin complex is .... in the light?
activated and stimulated transducin (G protein)
What does transducin activate in the light?
What does cGMP phosphodiesterase catalyze in the light?
cGMP ---> 5'GMP
What does reduction in cGMP cause in the light?
closure of Na channels
What happens to photoreceptors bc of closure of Na channels in the light?
What happens when rhodopsin is activated in the light?
several hundred channels close for each rhodopsin molecule activated
How are cones different from rods?
the protein portions (opsins) of the photo-chemicals are different from the scotopsin in the rod photo-chemicals
What is the same in rods and cones?
What are cones sensitive to?
blue, green and red
What are the 5 layers of the retina?
photoreceptors horizontal cells bipolar cells amacrine cells ganglion cells
Photorecetpors include, transmit and synape?
include rods and cones transmit signals to outer plexiform layer synapse with bipolar cells and horzontal cells
plexiform layer is what?
layer of synaptic connections
What do the horizontal cells transmit signals from?
rods and cones to bipolar cells
What is another place horizontal cells transmit signals too?
outer plexiform layer
Horizontal cells output is?
always inhibitory (lateral inhibition)
Bipolar cells transmit signals from?
rods, cones, and horizontal cells
bipolar cells transmit signals to?
inner plexiform layer
bipolar cells synapse
with amacrine an ganglion cells
Amacrine cells transmit signals?
-directly from bipolar to ganglion cells -within inner plexiform layer from axons of bipolar cells to dendrites of ganglion cells or to other amacrine cells
ganglion cells transmit signals from?
retinal to brain
ganglion axons make up?
What is special about ganglion axons?
-only retina cells that transmit action signals -others use electronic conduction which allows graded conduction of signal strength
interplexiform cells transmit?
from inner plexiform layer to outer plexiform layer (retrograde)
interplexiform cells send
inhibitory signals -lateral inhibition
The foveal region has three neurons in direct pathway?
cones bipolar cells ganglion cells
What do pure rod vision consist of, four neurons?
rods bipolar cells amacrine cells ganglion cells
What is the neurotransmitters from rods and cones ---> bipolar cells
what are the neurotransmitters for amacrine cells
eight or more kinds of neurotransmitters including GABA, glycine, dopamine, acetylcholine, indolamine (all inhibitory)
Amacrine cell that is a direct pathway for
amacrine cells that response strong to?
onset of continuing visual signal but fades rapidly
Some amacrine cells respond strongly at the offset of visual signals, but?
the response fades rapidly
Some amacrine cells respond when a light is?
turned off or on
some amacrine cells response to movement of a ?
spot across the retina in a specific direction
Most amacrine cells are interneurons that?
help analyze visual signals before they leave retina
What is the avg number of rods and cones on each ganglion cell?
60 rods and 2 cones converge on each ganglion cell and the optic nerve fiber from the ganglion cells
As ganglion cells approach the fovea, what happens?
fewer rods and cones converge on each optic fiber and rods and cones become more slender -increases visual acuity in central retinal -in central fovea there are only slender cones and no rods
What is the peripheral retina more sensitive to?
weak light -as many as 200 rods converge on a single optic fiber in the more peripheral regions of the retina
What are the types of ganglion cells?
W X Y
What are W ganglion cells?
-make up about 40% of all ganglion cells - small -transmit signals 8 m/sec - receive most of their excitation from rods transmitted by way of small bipolar cells and amacrine cells -they have broad fields in the peripheral retina bc their dendrites spread widely in the inner plexiform layer
What are X ganglion cells?
-Make up about 55% of all ganglion cells -medium diameter -transmit signal 14 m/sex -they have small fields ----signals represent discrete retinal locations
What does every X ganglion cell receive an input form?
at least on cone cell; therefore, probably responsible for all color vision
What are Y ganglion cells?
-make up about 5% of all ganglion cells -lg diameter ( up to 35 um) -transmit signals 50 msec or faster -response to rapid chagnes in visual image
What cells presumably apprise the CNS almost instantaneously when a new visual event occurs anywhere in the field without great accuracy with respect to location of field?
Y ganglion cells
The dorsal lateral geniculate nucleus relays info from optic tract to visual cortex by?
Way of optic radiation (geniculocalcarine tract) -high degree of spatial fidelity from retina to the visual cortex
What has 50% decussation in optic chiasm?
Dorsal lateral geniculate nucleus
What are the six nuclear layers in lateral geniculate nucleus?
II, III, V receive signals from lateral half of ipsilateral retina I, IV, VI receive signals from medial half of opposite retina
What are layers I and II of the dorsal lateral geniculate nucleus?
magnocellular layers contain lg neurons receive input almost entirely from lg Y type ganglion cells provides rapidly conduction pathway to visual cortex transmits only black and white point to point transmission is poor
What are layers III through VI of dorsal geniculate nucleus?
parvocellular layers contain small to medium size neurons receive input almost entirely from lg X type ganglion cells provides moderate conduction pathway to visual cortex transmit color accurate pt to pt transmission
Lateal geniculate nucleus controls how much of the signal is?
allowed to pass to the cortex
source of gating control of the dorsal lateral geniculate nucleus
corticofugal fibers from primary visual cortex reticular areas of the mesencephalon -both of these sources are inhibitory and help highlight visual information that is allowed to pass
The primary visual cortex is also called?
the striate cortex
Where is the primary visual cortex located?
Where do signals from macular area terminate for the primary visual cortex?
terminate near the occipital pole
Where do signals from the more peripheral retina terminate for the primary visual cortex?
at or in concentric half circles anterior to the ole but still along the calcarine fissure
How many distinct layers does the primary visual cortex have?
Where does geniculocalcarine fibers terminate?
mainly in layer IV of the primary visual cortex
Layer IV is organized into what for the primary visual cortex?
signals from Y ganglion cells terminate in layer IV c,alpha signals from X ganglion cells terminate in layer IV c beta and IVa
The visual cortex is organized into several million?
vertical columns of neurons diameter = 30 to 50 um =1000 neurons per column
Where are special column like areas called color blobs located?
among the columns of the secondary visual areas -receive lateral signals from adjacent visual columns and are activated specifically by color signals
The signals from the two separate eyes enter?
alternating stripes of columns in layer IV
What does the cortical area decipher?
whether the respective areas of the two visual images from the two separate eyes are in register w each other
How is the deciphered information from the cortical areas used?
to adjust the directional gaze of the separate eyes -this is also required for stereopsis
When does the lens assume an almost spherical shape?
when the lens is in a relaxed state with no tension on its capsule
What do the suspensory ligaments attached radially around the lens create?
a tension that causes the lens to remain relatively flat under normal eye conditions
What doe meridional fibers of ciliary muscles do?
contract and release tension on lens
What CN controsl both sets of ciliary muscles?
What are accommodations to the eye that come with age?
Lens becomes larger and thicker w age lens becomes less elastic w age power of accommodation decreases to less than 2 diopters by the age of 45 to 50 it decreases to 0 by the age of 70 presbyopia