vision I and II - jullet Flashcards
What is the compound eye better at? What about the refractive eye?
Compound: sensitivity to movement and wider range of wavelengths. Refractive: Resolving power (acuity)
What type of eyes are found in insects? Vertebraes?
Insects: compound eye. Vertebraes: refractive eye.
What are the 3 layers of the eye? What do they consist of?
OUTER: sclera + cornea. CHOROID: pigment epithelium + blood vessels. RETINA: photoreceptor cells.
What is the major refractive element of the eye? What is the adjustable refractable element of the eye?
Major: cornea; Adjustable: lens
What fluid is filled in the anterior chamber?
aqueous humor
What type of cells produce aqueous humor?
ciliary epithelium
What is the flow of aqueous humor?
Produced by CILIARY EPITHELIUM, circulates out into the anterior chamber, where it is cleared out into the venous circulation via trabecular meshwork and then out of the Canal of Schlemm.
What is the basis of glaucoma?
blocked trabecular network or canal of Schlemm; results in fluid buildup and increased pressure in the eye
The pupil, or aperture of the eye, controls two these two things:
1) amount of light into the eye, 2) depth of field
What happens to the light and the depth of field when the pupil constricts?
LIGHT: goes through the center of the lens, where there is less distortion. DEPTH OF FIELD: increases.
What is accomodation?
change in the refractive power of the lens due to its change in shape.
What is the shape of the lens controlled by?
degree of tension in the suspensory ligaments, which is controlled by contraction of the ciliary muscles
What type of innervation does the ciliary muscles receive (which affects the shape of the lens)?
parasympathetic nervous system
What happens when the ciliary muscles contract?
it decreases the tension on the lens and the lens become more spherical (for near vision; most refractive power)
What happens when the ciliary muscles are relaxed?
the lens is stretched out so it’s even flatter and thinner (least refractive power)
The lens have the most refractive power when…
it is spherical (accomodation, for near vision; when the ciliary muscles are contracted state)
The lens have the least refractive power when…
it is stretched out so it’s flatter and thinner (when the ciliary muscles are in a relaxed state)
Why does the amount of accomodation decrease with age?
the lens become less elastic with age.
What is emmetropia?
normal eyesight
What is myopia? How is it corrected?
Near-sightedness. Caused by an elongated eyeball or curved cornea, which results in te focal plane being in FRONT of the retina. Use concave lens.
What is hyperopia?
Far-sightnedness. Caused by a flattened eyeball, which results in the focal plane being BEHIND the retina. Use convex lens.
What are cataracts?
opacities in the lens that interfere with vision and can result in loss of parts of the visual field.
What happens when light hits the retina? (two fates)
it is 1) absorbed by the photoreceptors or 2) passes and through and hits the pigmented epithelium lining the back of the eye.
What is the purpose of the pigmented epithelium lining the back of the eye? (2)
1) contains melanin, which absorbs any light that is not captured by the retina and prevents it from being reflected from the back of the eye back into the retina and distorting the image. 2) role in recycling the photosensitive visual pigments and the phagocytosis of photoreceptor tips
Why is the fovea the region of greatest visual acuity? (2)
1) highest density of cone photoreceptors, which have better resolution than rods. 2) non-neuronal cells of the retina are pulled to the side, resulting in a more direct path of light to the photoreceptor cells (less distortion, increased acuity)
What is the optic disk?
blind spot; where the optic nerve leaves the eyeball and there are no photoreceptors.
What is tapetum?
reflective layer on pigmented epithelium that reflects light that wasn’t been absorbed in the first pass through the photoreceptive layer of the retina, so that there is a chance for the photons to be absorbed in the second pass; increases sensitivity and enhances night vision, but because the light was reflected/scattered, it decreases acuity.
How does the tapetum affect vision?
it increases sensitivity and enhances night vision, but because the light was reflected/scattered, it decreases acuity.
The tapetum is normally present in which animals?
nocturnal animals (deer, cows, cats)
What is retinal detachment?
separation of the retina from the pigment epithelium (and blood supply) in the choroid
What is the basis of retinal detachments?
during embryonic development, the end of the optic stalk invaginates to form the optic cup. The outer layer becomes the pigmented epithelium while the inner layer becomes the neural retina. The junction between the two layers is a structural weak point and can be pulled apart.
What germ layer is the lens formed from?
ectoderm (lens vesicle)
What germ layer is the retina fromed from?
neuroectoderm
What is macular degeneration? What are the two forms of MD?
loss of pigment epithelial cells, followed by photoreceptors. WET: tissue degradation and abnormal angiogenesis. DRY: deposition of drusen (protein + lipid) onto the retina
What are the 5 major types of neurons in the retina?
1) photoreceptor cells (rods/cones), 2) bipolar cells, 3) horizontal cells, 4) amacrine cells, 5) ganglion cells
Which cells are the actual cells that fire APs in the retina?
ganglion cells
What are the structural features of photoreceptors?
SYNAPTIC TERMINAL (contains glutamate-containing vesicles). INNER segment (nucleus + synthetic machinery). OUTER segment (microtubules + disks containing photopigments)
How is the outer segment of rods different than that of cones?
RODS: the disks are free-floating disks (that was formed via pinching of the membrane). CONES: the disks remain connected to the plasma membrane.
What is the purpose of the vertical stacked disks in the outer segment of photoreceptors?
it increases the probability that light coming down through the outer segment will be detected
T/F Photoreceptors divide to replace/replenish old photoreceptors.
False. Photoreceptors do NOT divide, but they constantly renew their outer segments by producing more disks at a rate of 3 disks/hour. The old disks are phagocytosed by pigment epithelial cells.
Where are the photoreceptor disks removed?
at the tip of the photoreceptor cells; eliminated by pigment epithelium cells.
Compare the rods and cones in terms of sensitivity and what its specialized for.
RODS: high sensitivity, specialized for night vision. CONES: lower sensitivity, specialized for day vision.
Why do rods have higher sensitivity, but lower acuity than cones? (3)
RODS: 1) more photopigment, 2) longer, 3) several rods converge on a given bipolar cell in the retina, which increases the sensitivity, but loses resolution (acuity)
Which photoreceptor has better visual acuity?
CONES have higher acuity, because they have a 1:1 cone to bipolar cell ratio. (whereas rods have lower acuity because several rods transmit info to one bipolar cell)
Which photoreceptor mediates day vision? Which photoreceptor mediates night vision?
DAY: cones, NIGHT: rods
Which photoreceptor has better temporal resolution?
CONES have better temporal resolution because they can respond faster and can detect flicker frequencies of a certain frequency
Which photoreceptor has better spatial resolution?
Cones, because 1) they a concentrated area of cones in the fovea, and 2) their conical shape makes them more sensitive to direct axial rays of light.
How do photoreceptors respond to light?
HYPERpolarize
What is the dark current?
the dark current is current that keeps the cell slightly depolarized in the dark (unstimulated condition); occurs because a small % of the ion channels are opened due to the presence of cGMP. At the relatively depolarized membrane potential, VG Ca channels are open and allow for Ca to enter the synaptic terminal, which induces GLUTAMATE release from the photoreceptor cell
What happens when light hits the photoreceptors?
The cation-selective channels in the outer segment that are open in the dark now closes, and with fewer + ions flowing into the cell, and less of this depolariing stimulus, the cell hyperpolarizes, which cause the VG Ca channels to close. Less Ca comes in, resulting in LESS glutamate release in response to light.
What happens when a photoreceptor absorbs a photon of light?
it activates rhodopsin, which activates a series of second messenger systems that ultimately LOWERS the levels of intracellular cGMP. When this happens, more of the cation channels responsible for the “dark current close”. Less (+) current flows into the cell, and the cell hyperpolarizes.
How is the response to light terminated? (2)
via 1) Ca-mediated feedback of cGMP metabolism and 2) recycling of opsin (retinal recycling).
What is retinal recycling?
a mechanism used to terminate the response to light: when the retinal binding protein (RBP) transports trans-retinol to pigment epithelial cells, where it is converted to retinol (vitamin A) and evenutally 11-cis retinal, which is recycled to the photoreceptor and used to synthesize more rhodopsin.
Vitamin A deficiency leads to:
deficits in night vision
What is calcium feedback?
a mechanism used to terminate the response to light: normally Ca enters the cell as part of the dark current. When cGMP channels close, the Na/Ca exchanger keeps working so intracellular Ca levels decrease, resulting in the DECREASE in the 1) Ca-inhibition of rhodopsin kinase, which can then phosphorylate rhodopsin and inactivate it 2) Ca-inhibition of guanylate cyclase, which synthesizes cGMP. Both work to increase cGMP levels
Ca2+ has an inhibitory effect on which two enzymes in the photo-conduction pathway?
Rhodopsin kinase + guanylate cyclase - both enzymes work to increase cGMP levels, but are inactivated by Ca2+
REVIEW PATHWAY FOR THE MOLECULAR BASIS OF PHOTORECEPTOR FUNCTION (PAGE 36)
REVIEW PATHWAY FOR THE MOLECULAR BASIS OF PHOTORECEPTOR FUNCTION (PAGE 36)
What is the response to a sustained level of light?
all of the cation channels are shut down and the cell hyperpolarizes. The Na/Ca exchanger causes a decrease in Ca concentration and this releases the inhibition of guanylate cyclase, resulting in an increase in cGMP synthesis. Some of the channels are now open again, the cell depolarizes, and the cell is able to respond to changes in light. A higer level of illumination and drive to decrease cGMP has now been matched with an increased rate of synthesis by guanylate cyclase, and there is a new “set point” or balance that has been reached as the guanylate cyclase catches up.
What determines the optimal wavelength at which a photopigment will absorb light?
it is determined by the opsin it contains
Humans have how many types of cone photopigments?
3 different opsins found in cones - all have different but overlapping absorption spectra
Humans have how many types of rod photopigments?
all rods have the same opsin and the same absorption spectra.
Which two opsin genes are located on the X chromosome?
Red and Green - which may result why males experience color-blindness more than females.
Which opsin gene is located on the 3rd chromosome?
ROD opsin
Which opsin gene is located on the 7th chromosome?
BLUE opsin
Why is colorblindness more prevalent in males than females?
because two opsin genes (red and green) are located on the X chromosome, and genetic abnormalities can result in color blindness(x-linked)
How does colorblindess occur?
Usually through unequal homologous recombination events during cell division that results in abnormal combinations of opsin genes (hybrd, loss, or duplication)
What is Protanopia?
colorblind in RED (using green and blue opsins)
What is a Deuteranopia?
colorblind in GREEN (using red and blue opsins)
What is Tritanopia?
colorblind in BLUE (using red and green opsins)
What are M cells?
magnocellular cells are ganglion cells in the retina that has a LARGE receptive field and are associated with MOVEMENT detection
What are P cells?
parvocellular cells are ganglion cells in the retina that has a SMALLER receptive field and are associated with COLOR VISION
What are the output cells of the retina?
ganglion cells - will fire APs when sufficiently depolarized
What is the receptive field of the ganglion cell?
a specific area of the retina that the ganglion cell monitors for light activity. Stimulation of photoreceptor cells in that area will cause either an increase or decrease in the firing rate of the ganglion cell.
The response of the ganglion cell to illumination of the center of its receptive field is opposite to the illumination of its periphery. What is an on-center ganglion cell?
ON-CENTER: Illumnation of the center of cell INCREASES the cells activity. Illumination of the periphery causes the activity to DECREASES.
The response of the ganglion cell to illumination of the center of its receptive field is opposite to the illumination of its periphery. What is an OFF-center ganglion cell?
OFF-CENTER: Illumnation of the center of cell DECREASES the cells activity. Illumination of the periphery causes the activity to INCREASE
What are W cells?
ganglion cells that respond to the overall illumination of their receptive field.
What type of photopigment does W cells have?
Melanopsin, which stimulates G-protein coupled pathways; most sensitive to blue light (480nm)
Which type of ganglion cells are most active at twilight?
W cells
What ganglion cells are important for circadian rhythms and pupillary reflexes?
W cells
What type of ganglion cells have the center-surround organization? Which one do not?
CENTER-SURROUND: M cells, P cells. DO NOT HAVE THIS: W cells
What is the difference between an on-center and and off-center ganglion cell?
the response to the bipolar cell to glutamate. ON-CENTER cell will respond to glutamate by increasing K channel activity, resulting in hyperpolarization. OFF-CENTER cell will respond to glutmate by opening cation channels, resulting in depolarization.the response to the bipolar cell to glutamate. ON-CENTER cell will respond to glutamate by increasing K channel activity, resulting in hyperpolarization. OFF-CENTER cell will respond to glutmate by opening cation channels, resulting in depolarization.
What type of receptors do bipolar cells from ON-CENTER ganglion cells have? What stimulates these cells and what happens?
ON-CENTER ganglion cell have bipolar cells with METATROBIC GPCR receptors, which when stimulated by glutamate released by the photoreceptor cell, leads to K CHANNEL ACTIVITY and HYPERPOLARIZATION
What type of receptors do bipolar cells from OFF-CENTER ganglion cells have? What stimulates these cells and what happens?
OFF-CENTER ganglion cells hav bipolar cells with NMDA/AMPA ionotropic receptors, which respond to glutamate by opening of cation channels and allowing cations to flow into the cell (depolarization)
How are bipolar cells, photoreceptor cells, ganglion cells, amacrine cells, and horizontal cells functionally organized?
photoreceptor cells are linked to bipolar cells, which are linked to ganglion cells. Amacrine cells synpase onto bipolar cells and ganglion cells. Horizontal cells synapse onto ganglion cells
What is the purpose of the ganglion cells?
to help enhance the contrast between differences in illumination (to provide information about the boundaries or edges about a stimulus.
What is retinitis pigmentosa?
photoreceptor degeneration - with an eventual loss of vision.
What is the bionic eye?
camera that digitizes information to a processor which sends the signal to the optic nerve with the hope that the nervous system can “learn” to interpret the signals
How does an artifiical retina work?
retinal element (photosensitive elements, “ganglion cells”) are incorporated into a microchip, which is surgically implanted into a eye.
What are the projection of the retinal ganglion cell axons to the brain?
1) hypothalamus, 2) pretectum, 3) superior colliculus, 4) lateral geniculate nucleus
The retinal ganglion cell projects to the hypothalamus (suprachiasmatic nucleus, SCN). What does this pathway regulate? What type of ganglion cells predominate here.
regulation of circadian rhythm. W ganglion cells predominate here.
The retinal ganglion cell projects to the pretectum. What does this pathway regulate?
reflex control of pupil and lens
What are the neural pathway of the pupillary reflex?
The optic nerve projects back to the pre-tectal area, which projects to the Edinger-Westphal Nucleus. From there, parasympathetic fibers in CN III project to the ciliary ganglion, which synapses with a post-ganglionic cell that control the smooth muscles of the pupillary sphincter.
The retinal ganglion cell projects to the superior colliculus. What does this pathway regulate?
orientating movements of head and eyes; this region coordinates visual, somatic, and auditory information in a point-to-point manner.
The retinal ganglion cell projects to the lateral geniculate nucleus. What does this pathway regulate?
it projections to the regions of the cortex involved in vision; projections to the lateral geniculate from the retina is highly ordered in point-to-point projection, or topographic representation of the retina on the lateral geniculate.
In the pupillary reflex, what is the difference between the direct response and the consensual response?
DIRECT: shining a light in one pupil will cause it to constrict. CONSENSUAL: the pupil in the other eye also constricts. This reflex is mediated by W type ganglion cells.
What type of cell mediates the pupillary reflex?
W cells ganglion cells.
What pathways mediate the pupillary reflex?
this reflex arises due to pretectal projections to one side projecting to both Edinger-Westphal nuclei.
What are the layers of the lateral geniculate?
There are 6 layers. Layers 1, 2 convey info. about movement. Layers 3-6 convey info. about color vision.
What are the parvocellular layers of the lateral geniculate? What is the pathway involved in?
outermost 4 layers of the lateral geniculate; receive input from P type ganglion cells (smaller receptive fields). This pathway is involved in color vision and fine discrimination of shape.
What are the magnocellular layers of the lateral geniculate? What is the pathway involved in?
inner 2 layers of the laeral geniculate; receive input from M type ganglion cells (larger receptive fields). This pathway is involved in motion detection, depth, contrast; not color or fine detail.
The lateral geniculate receives input from …?
retina and parts of the cortex (thought to serve as feedback mechanisms regulating the input of information to the visual system).
