Flashcards in 2: Vision Deck (34):
Describe the structure of the eye and the process of refraction of light by listing each of the layers of the eye light passes through on its way through to the back of the eye.
1. Light enters through the cornea, the MAJOR refractive element in the eye (~40/60 diopters of total refractive power of eye)
2. Through anterior chamber full of aqueous humor
3. Through the pupil, a hole of regulated diameter surrounded by the iris, which regulates how much eye is coming into the eye
4. Through the lens, the ADJUSTABLE refractive element in the eye
5. Through vitreous humor.
6. To the retina: neuronal layer, then pigmented layer
Describe the organization of the retina and the different cell types that are present. (6)
Ganglion cells: output neuronal cells
Amacrine cells: form synapses between neighboring bipolar cells and ganglion cells
Bipolar cells: photoreceptor cells to ganglion cells
Horizontal cells: form synapses between photoreceptors
Photoreceptor cells: rods and cones
Muller cells: function in maintaining K concentration
Describe the sensitivity of the rods and cones to light, including distribution, sensitivity, temporal resolution, acuity, and chromatic/achromatic.
--Distributed in periphery
--High sensitivity, specialized for night vision
-----Normally saturated during the day
--Low temporal resolution: can distinguish up to 12 Hz
--More sensitive to scattered light
--Low acuity: more rods converge onto a single bipolar cell
--Concentrated in fovea
--Lower sensitivity, specialized for day vision
--High temporal resolution: can distinguish up to 55 Hz
--More sensitive to direct axial rays
--High acuity: one cone -> one bipolar cell
--Chromatic: contain 3 different types of cones, each with a different pigment
Discuss the mechanisms for sensitivity to different wavelengths of light: what happens to photoreceptor cells when they are in the dark or stimulated with light?
Photoreceptor cells HYPERpolarize in response to light
--GRADED response: stronger stimulus -> large hyperpolarization
--Do NOT generate action potentials
DARK: "the dark current"
1. Elevated cGMP concentration
2. More cGMP-gated cation channels open -> Na/Ca enter cell
3. Cell slightly depolarized -> release more NT (glutamate)
1. Lower cGMP concentration
2. More cGMP-gated channels closed -> NO Na/Ca enter cell
3. Cell HYPERpolarizes -> less NT release
Discuss the mechanisms for color blindness.
Genetic abnormalities in the opsin genes
Lack red or green opsin: can't see red or green (can't compare the two)
Lack blue opsin: can't see blue or yellow (can't compare the two)
Describe the different types of retinal ganglion cells in terms of their size and sensitivity to visual modalities (magnocellular v. parvocellular; movement v. color detection).
Magnocellular: larger receptive field; involved in motion detection
--Projects to layers 1 and 2 of LGN
Parvocellular: smaller receptive field; involved in color vision
--Projects to layers 3-6 of LGN
Describe the different types of retinal ganglion cells in terms of their center surround organization (on-center v off-center).
--Light hits center -> increased activity
--Light hits surround -> decreased activity
--Light hits center -> decreased activity
--Light hits surround -> increased activity
BOTH: no change in activity if entire field is in dark or light, or if exactly half of center and surround are in light/dark
Explain W-type ganglion cells.
Ganglion cells which do NOT have a center-surround receptive field
Contain melanopsin, a photopigment, and are PHOTOSENSITIVE
Respond by increasing intracellular Ca to bluish light
Important in circadian rhythms
Diagram the projections of the retinal ganglion cells and explain the role of each projection. (4)
Retinal ganglion cells -> optic nerve -> optic chiasm -> optic tract -> 4 main regions:
1. Hypothalamus: suprachiasmatic nucleus (SCN) in particular
--Important in circadian rhythms
--Projections from W-type ganglion cells
2. Pretectal area -> BOTH Edinger-Westphal nuclei -> ciliary ganglion -> ciliary muscle in iris
--Projections from W-type ganglion cells
--Reflex control of pupil and lens
3. Superior colliculus:
--Aligns visual, auditory, and somatosensory maps
--Orienting movements of head and eyes
4. Lateral geniculate nucleus of thalamus -> primary visual cortex in occipital lobe
--~80% of axons
--Each LGN receives input from both eyes
--Point to point projection -> mapping image from retina on LGN -> cortex
Where is aqueous humor produced? Where does it drain out of?
Produced by ciliary epithelium
Drains through trabecular meshwork out through Canal of Schlemm into venous system
What causes glaucoma?
Blockage of trabecular meshwork or Canal of Schlemm -> buildup of pressure in eye
What is accommodation? How does it occur?
The change in the refractive power of the lens due to change in its shape
Ciliary muscles contract -> suspensory ligament relaxed -> lens rounded out for near vision
Ciliary muscles relax -> suspensory ligament taut -> lens flattened for distant vision
What happens to a person's accommodation ability with age?
Decreases. Lens becomes less elastic.
What are cataracts?
Opacities in the lens of the eye that interfere with vision
Often develop with age
Results from crystallin breakdown
What is the optic disk?
A blind spot, where all the axons from the retinal ganglion cells leave out in the optic nerve
What is the fovea?
A location where retinal cells are displaced to result in a more direct path of light to the photoreceptor cells
Also has a higher density of cones
What is the purpose of the pigmented layer of the retina?
Absorbs photons that get past the neuronal layer of retina
How does retinal detachment occur?
Jarring or a blow to the eye can lead to the neural retina getting pulled away from the pigment epithelium
Because of the way it develops, the junction between the two is not a strong mechanical junction
What is macular degeneration? What is drusen?
Most common cause of vision loss in the elderly
Loss of pigment epithelial cells, then photoreceptors
Drusen: yellow or white extracellular accumulations of proteins and lipids, seen in macular degeneration
Why do photoreceptors have disks?
Increase surface area - have photosensitive pigment in their membrane
In what direction are the disks of photoreceptors synthesized and shed?
Made from cell, then move out
Once they reach the end, they are phagocytosed
What determines the optimal wavelength at which a photopigment will absorb light?
The OPSIN it contains
Humans are trichromats: three different types of cone photopigments
Have four opsin genes: rod, blue, green, and red
Describe the exact mechanism of how a photon of light results in closure of cGMP-gated ion channels.
Photons of light interact with RHODOPSIN in disk membrane: opsin + 11-cis-retinal
--Retinal sits inside opsin, a TM protein
--Photon of light switches 11-cis retinal -> all TRANS retinal (activated)
--Activated opsin activates TRANSDUCIN, a G-protein
--Transducin activated PDE -> breaks down cGMP -> GMP
--Decreased cGMP concentration -> closure of cGMP-gated ion channels
Describe the two ways (rapid and slow) in which the response to light is terminated.
1. Ca-mediated feedback of cGMP metabolism - rapid
--Normally, Ca that enters in dark current is pumped out by a Na/Ca exchanger, and Ca entering inhibits guanylate cyclase (GC) and rhodopsin kinase
--Na/Ca exchanger keeps working when cGMP channels close -> Ca levels decrease
--Decreased Ca decreases inhibition of rhodopsin kinase, which can then phosphorylate/inactivate rhodopsin
--Also decreases inhibition of GC, which makes cGMP
--cGMP returns to normal
2. Recycling photobleached opsin (trans-retinal) - slow
--Retinal binding protein (RBP) transports trans-retinol -> pigment epithelial cells, where it is converted to retinol -> 11-cis retinal
--11-cis retinal -> photoreceptors, makes more rhodopsin
What does vitamin A deficiency lead to?
Visual deficits, particularly night blindness due to reduced levels of rhodopsin in rods
Describe the structure of the photoreceptor cells (rods and cones).
(ordered from bipolar cells to pigmented epithelium)
1. Synaptic terminal
2. Inner segment: nucleus, mitochondria
3. Outer segment: large number of disks connected to inner segment by a cilium
--Rods: tons of flat round disks (rod-shaped)
--Cones: looks like the point of a crayon (cone-shaped)
What is myopia? What is hyperopia?
MYOPIA: elongated eyeball/curved cornea -> focal plane in front of retina -> can only see things that are nearby
HYPEROPIA: focal plane falls behind retina because eyeball is flattened, or refractive system too weak
What is a receptive field of a ganglion cell?
The area in which if light hits the retina, it changes the activity of that particular ganglion cell
--Typically circular, with center-surround setup
--Can increase or decrease activity depending on where light hits
--Center: either on or off
--Surround: opposite change from light hitting center
How does an on-center/off-surround organization of retinal ganglion cells arise? Include what happens with NT release, bipolar cells, and ganglion cells.
From synaptic interactions!
1. Light hits photoreceptor in center
2. Photoreceptor hyperpolarizes, releases less inhibitory NT
3. Bipolar cell no longer inhibited -> depolarizes, releases more NT
4. NT causes ganglion cell to increase its firing rate
1. Light hits photoreceptor in surround
2. Photoreceptor hyperpolarizes, releases less excitatory NT on horizontal cell
3. Horizontal cells are hyperpolarized, stop inhibiting center photoreceptors
4. Center photoreceptors, now not inhibited, depolarizes, releases more NT
5. Bipolar cell is hyperpolarized, releases less NT
6. Decreased NT causes ganglion cell to DECREASE its firing rate
What determines whether a receptive field will be on-center vs. off-center?
Determined by which glutamate receptor is expressed on bipolar cell
--On-center: metabotropic mGluR6 receptors, respond with HYPERPOLARIZATION (so less NT release from receptor cell upon its hyperpolarization by light -> DEPOLARIZATION/ACTIVATION of bipolar cell)
--Off-center: ionotropic NMDA/AMPA receptors, respond with DEPOLARIZATION (so less NT release from receptor cell upon its hyperpolarization in light -> HYPERPOLARIZATION/INACTIVATION of bipolar cell)
NT release from bipolar cell upon depolarization ACTIVATES ganglion cell
How do the functional properties of ganglion cells help enhance the contrast between differences in illumination?
Helps tell you where the edge between dark and light is
Ganglion cell receptor fields overlap -> very sharp change in activity where there is a change in illumination
--Contrast: see no change in response if all ganglion cells are in light or all are in dark
What are five avenues of repair and replacement in the eye to treat the loss of vision?
1. Gene therapy: prevent choroid degeneration; mediate delivery of photoreceptor proteins
2. Stem cell/precursor cell therapy: replace lost photoreceptors with precursor cells that can integrate into the retina
3. Develop photosensitive cells using photoswitchable models: make non-photoreceptor cells in the retina that are sensitive to changes in illumination
4. Artificial lenses: treat cataracts
5. Artificial retinas: develop a small chip/grid that has photosensitive spots
Why do you get a consensual pupillary response to light in one eye?
Constriction in both eyes from light in one (consensual pupillary response) because you get bilateral projection from both E-W nuclei