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72 Cards in this Set
- Front
- Back
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What are the 3 basic steps to the transformation of an image?
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1) Transduction of image by photoreceptors
2) Transmission to BP cells via excitatory chem synapses 3) Transmission to GC by excitatory chem synapses |
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What are the five layers in the neural tissue of the retina?
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1) Outer nuclear layer
2) Outer plexiform layer 3) Inner nuclear layer 4) Inner plexiform layer 5) Ganglion cell layer |
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What is contained in the outer nuclear layer?
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Cell bodies of photoreceptors.
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What is contained in the outer plexiform layer?
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Cone and rod terminals
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What is contained in the Inner Nuclear layer?
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Horizontal cell soma, Mueller cell soma, Bipolar cell soma, Amacrine cell soma
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What is contained in the Inner Plexus Layer? What are the two sublayers?
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Terminals of BP cells. Dendrites of GC and AII cells.
OFF and ON |
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What is the path of a rod during twilight? ON.
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Rod =(chem, excite)> BP =(gap)> AII => cone-BP-ON terminal =(chem, excite)> cone-BP-ON
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What is contained in the outer segment of the photoreceptor?
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9000 stacked discs. Photo pigment rhodopsin in membrane.
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What is contained in the inner segment of the photoreceptor?
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Mitochondria and pumps
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What cones are involved with color? At what wavelength is the peak?
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S cone - peak 450 nm (blue)
M cone - peak 550 nm (green) L cone - peak 570 nm (red) |
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Describe a cone terminal including an invagination area.
What type of receptor is used to connect the terminal to the connecting neurites? |
Candelabra-like dendritic arbor.
Triad in each invagination area: Hz cell/BPs(2)/Hz cell. Ribbon. |
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Describe a rod terminal
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It is a dyad that makes synapses to two neurites, e.g. BP and an AII.
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Where do ON cells arborize?
Where do OFF cells arborize? |
Lower part of the inner plexiform layer.
Upper part of the IPL |
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How does a rod control an OFF cell?
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rod-bp => AII => BP terminals => cone-OFF-dendrites. Inhibitory signal sent from AII cell.
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Where are the arbors of an AII cell?
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ON layer of IPL.
"Lobular appendages" at OFF layers. |
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AII cells use large gap junctions to connect to ____
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Axon terminals of cone-ON-BP cells.
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What types of primary connections synapse into the dendritic arbor of the GC?
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Excitatory synapses from BP cells and inhibitory synapses from AII cells.
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What is the circuit for eye movements?
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GC => nucleus of optic tract
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What is the circuit for reseting the circadian rhythems?
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GC => suprachiasmatic nucleus
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What is the circuit which helps to orient the head and eyes?
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GC => super colliculus
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What are the first two stages of the circuit for vision?
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GC => lateral genate nucleus => striate cortex
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What are two types of ganglion cells along with there alternate names?
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Large (beta, P) and midget (alpha, M)
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What is the morphology and discharge behavior of large cells?
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Wider, sparser dendritic tree. Looks like a parasol.
Transient response. |
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Where do "M" cells project?
Where do "P" cells project? |
geniculate magnocellular layer
lateral geniculate parvocellular layer |
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What is the response of ON-midget cell and OFF-midget cell to increasing light?
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ON-midget: freq of firing increases
OFF-midget: freq of firing decreases or stops |
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What is the electrical response of ON and OFF large cells?
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ON: Increased firing rate blip followed after a while by negative going blip.
OFF: Negative blip followed after a while by a couple of positive blips. |
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What is one of the basic functions of the horizontal cell? How is this accomplished?
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Averages the light signal and allows this value to be subtracted from the light signal.
Hz cells are electrically coupled, gather their info from receptors and feedforward onto BP cells and negatively to PR terminals. |
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What is one of the principle functions of type AII amacrine cells?
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Part of rod's starlight pathway. Feeds forward a signal from rod BP to BP of cone or input of GC cell.
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Why are ribbon synapses important?
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Because of their long aspect ratio, they have a larger active zone. 5 to 10 fold more vesicles are docked compared to a conventional synapse. The peak sustained release rate is larger than the std synapse.
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Where are ribbon synapses used?
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PR to BP
BP to GC Not amacrine or Hz synapses |
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What is the structure of a rod PR terminal?
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One ribbon points between two Hz cells toward two rod bipolar dendrites.
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What type of receptors are on the BP cells in OFF circuits? In ON circuits? What is the action in each case?
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OFF: Ionotrophic, glutamate, AMPA or kainate, opens a cation channel.
ON: metabotrophic, glutamate, MGluR6, closes a cation channel. |
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Where are the 3 places that gap junctions are used in the OPL?
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1) Each cone to its immediate neighbors
2) Each rod connects to 2 neighboring cones 3) Wide-field Hz-Hz cells and narrow field Hz-Hz cells. |
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Give the sequence of events when light shines on an ON cone during the day
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Increase light => Hyperpolarization of receptor => decrease glutamate released by PR => opening of MGluR6 receptors on BP cell => increased glutamate release by BP cell => depolarization of GC => increased firing rate.
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What is the value of having ON and OFF cells?
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1) Doubles the dynamic range of light sensitivity: On-Avg signal, Avg-OFF signal gives 2x the signals.
2) Helps in looking for contrast. |
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What is the use of dopamine within the retina? What type of cell generates it?
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Dopamine reduces the sensitivity of all cells within its diffusion radius?? It also regulates horizontal cell coupling.
There are dopamengeric amacrine cells that are paracrine in nature. |
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Dopamengeric amacrine cells have their dendritic arbor in the OFF layer of the IPL. Yet they are activated by light. How can this be?
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A theory is that melanopsin GCs are activated directly by light. It releases GABA which results in the dopamine A cells releasing dopamine.
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How does the surround work?
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Light on the annulus hyperpolarizes the cones surrounding the center cones. Hyperpolarization => reduced excitation of Hz cells => reduced GABA release => depolarization of PR. This is the antagonism to its light response.
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Draw out the firing rate diagram for full lighting of the center, full lighting of the annulus, full lighting of both center and annulus. Do this for OFF and ON cells.
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1) center-ON: increased firing rate
2) center-OFF: decreased firing rate 3) annulus-ON: increased firing rate 4) annulus-OFF: decreased firing rate 5) both: firing continues with no changes. |
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What are four ways to improve the S/N ratio?
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1) Coding methods
2) "Surround" mechanism reduces redundancy??? 3) Parallel circuits 4) Ribbon synapses |
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What are four ways that coding improves the S/N ratio?
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1) Cones are connected to neighboring cones via gap junctions so light in several will be correlated and light in only one will be uncorrelated for which the FF signal is strongly attenuated
2) Further pooling: convergence of cones into BP cells and BP cells into GC. 3) Optical blur??? 4) Domed distribution of BP synapes across GC dendritic field??? |
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How does the "surround" mech reduce redundancy?
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The surrounding cones are connected to Hz cells which in turn are connected to each other via gap junctions.
The Hz cells find the avg field intensity and effectly subtract this from signal so only the difference or contrast is propagated forward. |
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Compare large and midget cells for color contrast, luminance contrast, spatial frequency, temporal frequency.
Which type would be best for color? For detail? For motion? For low light? |
Feature L cells Midget cells
Color contrast No Yes Lum. contrast Higher Lower Spatial freq Lower Higher Temporal freq Higher Lower Color: midget, detail: midget, motion: large, low light: large |
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What are the circuits for twilight and for starlight?
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twilight: Rod => cone-PR-terminal => cone circuit.
starlight: Rod => AII cell => GC |
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What are some of the aspects that allow seeing in starlight?
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1) Only 0 or 1 response measured from rods.
2) High level of convergence. 20 to 120 rods converge on 1 rod BP cell. 3) Rod BP cells use about 30 ribbon synapses to AII cell. |
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Give the sequence of events when light hits the photoreceptor for increasing light intensity.
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activate rhodopsin -> activate G-protein (transducin) -> activate phosphodiesterase -> breakdown cGMP -> close Na+/Ca++ channels => hyperpolarize membrane.
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What are the four specializations of the primate fovea?
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1) depression resulting from lateral displacement of neurons allwoing light to strke the foveal PR directly.
2) Only cone PRs (high acuity vision) 3) One cone to one bipolar to one ganglion cell (high acuity vision) 4) Blood vessels avoid fovea |
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What are the 4 components of the structure of photoreceptors?
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Outer segment, inner segment, cell body, terminal
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What are the RPE? How are they integrated? What is their function?
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Outer segment is interdigitated amoung processes of retinal pigment epithelial (RPE) cells. RPE are cuboidal cells bound together by tight junctions that provide nutrients to photoreceptors, absorby stray photons not cuaght by PRs.
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What are the two components of rhodopsin?
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chromophore (11-cis-retinal)plus protein (opsin)
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How many rhodopsin molecules are contained in the disc membrane?
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80,000
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Discuss the changes in the visual pigment when light is absorbed. How fast do these steps occur?
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Photon is absorbed by visual pigment
Energy imparted in 3 fs. 11-cis retinal -> all-trans retinal (tail of molecule is traightened Photoisomerization which takes about 200 fs. |
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How much of the photon's energy is required for photoisomerization?
What happens if low photon absorption? |
All the photon's energy is needed.
The chromophore is always taking and throwing off small amounts of energy via molecular collsions. This could be confused with light if it were not the fact that all of the photon's energy is needed to flip 11-cis to all-trans. |
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What activates rhodopsin and how long does it take?
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Chromophore isomerization alters the shape of opsin. It takes about 0.25 to 0.50 msec.
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In the PR, how many channels are open? What gates them? What ions pass through the channels?
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10,000
cGMP Ca++, Na+ |
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What is the relationships of G-protein to GDP? How many G-proteins does one activated rhodopsin affect? How quickly does this happen? What percentage of G-protein is affected?
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G-protein is bound to GDP in the dark.
When the G-protein is activated by rhodopsin, GTP replaces GDP and G-protein is activated. One activated rhodopsin bumps 700 G-proteins in about 100 msec or 8% of G-protein on disc surface. |
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How many cGMP binding sites does each cGMP gated channel have? How many are needed to activate the channel?
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4 binding sites.
3 cGMP need to bind to open channel. |
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What is the algebraic relationship between the fraction of open channels and the concentration of cGMP? If cGMP is reduced by 3%, what percentage of channels are closed?
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# open channels proportional to cube of cGMP concentration.
1% change in cGMP results in 3% change in # open channels, i.e. 3% change in current. |
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What is the bottom line regarding the ampification of the activation of a single activated rhodopsin?
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A single activated rhodopsin reduces cGMP conentration by 1400 molecules or about 0.7% so inward current decreases about 2%.
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What are the two consequences of the PR syanptic deactivation?
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1) reduction of inward current - hyperpolarization leads to closing of voltage dependent Ca channels in terminal and lowers Ca concentration.
2) Lower Ca decreases synaptic vesicles binding to terminal membrane and thus less transmitter (glut) release. |
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What is the result of light on horizontal cells?
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Hz cells respond to light by hyperpolarizing and feedback onto receptors producing a depolarization of the receptor (sign reversal).
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What is the result of light in center on the on-center and off-center response?
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1) Light in center of bipolar cell and ganglion cell receptive field produces depolarization if on-center.
2) Light in center that causes a hyperpolarization is off-center. . |
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What is the result of illumination of the concentric response regarding the center of the receptive field?
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Illumination of the concentric surround decreases response to illumination at the center of the receptive field.
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What cells are involved with the center response versus the surround response?
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The center response reflects direct receptor-bipolar synaptic interaction.
The surround reponse is mediated by Hz cell activity |
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What condition do we have if tau = T?
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Entrainment
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What is tau?
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The natural circadium period
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What is the phase response curve to light?
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The amount of delay, advance or no change that will be seen in the start of the freerun period when an animal in the dark is exposed to a pulse of light, e.g. 1 hour. It is dependent upon when in the circadian cycle that the light pulse occurs.
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What is the tau response curve to light?
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???
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What two things can be observed for freerun in constant conditions?
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species-specific tau in DD
change in tau in LL (Aschoff's Rule) |
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What is Ascoff's Rule?
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Tau will tend to increase when the animal is kept in LL.
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What are 3 features of circadian rhythms?
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entrainment tau = T
freerun in constant conditions temperature compensation |
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What is the phase angle dependence on tau in DD?
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???
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