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39 Cards in this Set
- Front
- Back
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What gives the BRB its selective permeability?
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- tight junctions
- no fluid transport between cells |
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(T/F) RPE is multilayered.
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F - it is a monolayer rich in melanin
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Give the functions of the RPE?
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- BBB and BRB
- phagocytosis of outer segment (regeneration of photoreceptors) - storage of vitamin A - retina adhesion - light scatter |
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What are some differences between rods and cones?
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rods: night vision, have more pigment than cones (more sensitive), low visual acuity, none in fovea, slow response to light, membrane-enclosed discs (not attached to membrane), 130 *10^6, rhodopsin pigment (only one type), achromatic vision
cones: day vision, sensitive only to direct light, high visual acuity and better spatial resolution, concentrated in fovea (but no blue), fast response to light, discs attached to outer membrane, 6 *10^6, three types of pigment, color vision. |
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Describe the structure of the photopigment rhodopsin
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opsin molecule - 7 alpha helices with ~350 AAs
chromophore (derivative of Vitamin A: 11-cis-retinal, inactive) bound to 7th helix at a LYS residue |
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What gives chromatic sensitivity to pigment?
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AA sequence
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which pigment AA sequences are most similar?
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Red-Green
Blue-Rhodopsin |
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Which pigment defect is most common? In which gender?
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Red-Green
Male |
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What are the steps of phototransduction???? :)
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1. +hv absorption of photon by opsin
2. isomerization of 11-cis-retinal to all trans retinal also causing opsin-chromophore dissociation 3. all trans retinal transfered to all trans retinol and then transported out of photoreceptor for recycling 4. activated opsin reacts with G-protein (transducin)*** [this is the amplification step!!! activated opsin can activate ~800 transducin molecules] 5. transducin dissociates into alpha, beta/gamma subunits 6. alpha subunit of transducin bind to the gamma unit of phosphodiesterase - this changes its conformation and frees a catalytic site which converts cGMP to inactive 5'GMP 7. decrease in the amount of cGMP cannot keep Na+ channels open and therefore Na+ entry to the cell is decreased and the inside becomes more negative and the membrane is hyperpolarized 8. deactivation of rhodopsin is dependent on Ca2+ mechanisms |
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What is the definition of phototransduction?
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conversion of photon into electrical signal ---> brain as vision
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what is the visual cycle?
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recycling of all trans retinal back to initial 11-cis retinal
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what are the steps in the visual cycle?
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*** process is in the RPE ***
1. transporter molecules bind to all trans retinol to bring it to RPE (phagocytosis) 2. re-isomerized into 11-cis retinol, esterified or hydrolyzed back to 11-cis retinal 3. without RPE there will be no recycling of retinal therefore no vision |
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what is the cilium?
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zone where discs are produced
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in which photoreceptor are discs continuously produced?
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rods
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cone pedicle has large or small terminal?
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large - up to 50 synapses - some invaginating and some flat
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difference between invaginating synapses and flat synapses?
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invaginating - pre-synaptic ribbon + 2 horizontal cell dendrites and dendrite of metabotropic bipolar cell
flat - no pre-synaptic ribbon - only bipolar cells with ionotropic receptors make contact |
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what type of terminal do rods have? cones?
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rods - small terminal - one invaginating synapse with 2 HCs and one metabotropic BP cell dendrite
cones - invaginating and flat synapses |
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Photoreceptors (depolarize/hyperpolarize) in response to light.
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hyperpolarize
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What other types of retinal cells are there in the retina (other than rod and cone)?
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HC
BP Amacrine Ganglion |
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What type of action do HCs constitute and why? How are they connected to one another?
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inhibitory - from GABA neurotransmitter
connected by gap junctions |
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What is the function of HCs?
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- adjust photoreceptor sensitivity
- role in color vision |
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What are two types of bipolar (BP) cells?
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hyperpolarizing (ionotropic) and depolarizing (metabotropic)
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Where are hyperpolarizing BP cells ?
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At flat synapses (cones only)
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Which pathway (ON/OFF) are hyperpolarizing (ionotropic) cells important for?
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OFF pathway
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where are depolarizing (metabotropic) cells located? To which pathway are they important?
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invaginating synapses
ON pathway |
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What do amacrine cells do
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action potentials in retina
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Glutamate receptors on BP cells are (metabotropic/ionotropic).
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metabotropic - Glu release in dark
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two types of HCs?
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HI (rods and cones)
HII (cones only) |
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describe - Cone BP cells
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hyperpolarizing AND depolarizing BP cells in cones.
hyper: sign conserving, ionotropic Glu receptor, flat synapse, OFF system dep: sign inverting, metabotropic Glu receptor, invaginating synapse, ON system |
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describe rod BP cells
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depolarizing only
invaginated synapse metabotropic Glu receptor |
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what is the function on Ganglion cells?
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- retinal output -
- all converge to optic nerve head - spiking activity - midget (small - central retinal area) or parasol (large - periphery) |
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what are the main pathways of Ganglion cells?
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parvocellular - form detection
magnocellular - movement detection |
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describe - basic retinal circuit (cones)
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cones:
invaginating synapse --> BP cells --> metabotropic receptor --> ON pathway flat synapse --> BP cells --> ionotropic receptor --> OFF pathway |
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describe - basic rod circuit
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depolarizing (sign inverting) --> invaginating synapse --> metabotropic --> NO output on Ganglion cells
scotopic condition - rod to rod BP mesopic - rod to cone (gap junction) |
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describe circuits in different light conditions
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photopic - cone to cone BP
mesopic - rod to cone BP (gap junction) scotopic - rod to rod BP |
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define: receptive field
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area of visual world where light stimulus can elicit a response from a cell
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what are Mach Bands?
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visual system works by detecting "edges" and amplifying contrast - illusionary bands of dark and light which are not present in the actual image are perceived by the eye - partial illumination of RF
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what is the Herman grid concept?
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dark circles appear at intersection of white lines
- results from partial excitation of periphery of RF |
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what is another example of lateral inhibition (other than Herman grid)?
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simultaneous contrast - gray image will look lighter when presented on a dark background
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