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66 Cards in this Set
- Front
- Back
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2 Things involved in Phototransduction:
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1. Photopigments
2. Excitatory cascade |
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What color do rods absorb most?
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Blue-green
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What color do cones absorb most?
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there are 3 different types!
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3 Cones - what does each absorb?
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Short - absorbs blue
Middle - absorbs green Long - absorbs red |
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pneumonic for cones:
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sml the bgr
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What are visual pigments?
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G-protein coupled receptors activated by light instead of a ligand.
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5 Steps in Phototransduction:
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1. Light activates Rhodopsin
2. Rhodopsin activates 700 Transducins - gproteins 3. transducin activates cGMP PDE 4. PDE brks down cGMP -> GMP 5. Na/Ca channels close |
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Effects of closing Na/Ca channels in the photoreceptor cell:
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Cell hyperpolarizes due to loss of sodium influx; calcium loss causes decreased PDE activity and increased GC activity so actually reforms cGMP
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5 Inherited visual defects and degenerations associated with molecules involved in transduction:
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1. Color blindness
2. Blue cone monochromacy 3. Retinitis pigmentosa 4. Congenital stationary night blindness 5. Cone, cone-rod, and macular degeneration |
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Macular degeneration affects:
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Visual acuity - the macula contains the fovea - zone of highest visual acuity.
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Who gets color blindness?
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8-10% of males
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What causes blue cone monochromacy?
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Mutations in both L and M (long and middle) cones - leaves only S cones (short) which are normally only 5% of our cones.
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What is the problem in Retinitis pigmentosa?
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Loss of vision in the periphery
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3 molecules associated with Retinitis pigmentosa:
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1. Rhodopsin
2. PDE 3. GMP-gated ion channels (also arrestin) |
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What is Congenital stationary night blindness?
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The inability to see in dim light.
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3 molecules associated with CSNB:
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-Rhodopsin
-Pde -Transducin (also rhodopsin kinase) |
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What is defective in cone and macular degeneration?
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GCAP1
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How many rods are in each eye? For what?
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~100 million - for vision in very dim light.
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How many cones are in each eye? For what?
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~5 million - for daytime vision
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Majorly important difference between rods and cones:
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Rods saturate
Cones don't |
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What is saturation?
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As the amt of light that reaches the retina is increased, the rods' response increases until it reaches its maximum response.
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How much light does it take to max out the rod's response?
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VERY dim light
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So what state are rods in at normal room light?
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maxed out - saturated
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So what does this explain?
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Why we use cones for day vision and rods only for dim light vision - that's the only time they're ever active.
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How much of our receptors are we typically using for vision?
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5% - that's the percentage of cones - rods are 95%
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What is the response of photoreceptors to light?
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Hyperpolarization
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We have 100 million rods maximally stimulated (saturated) all during the daytime; why don't we see snowblind then?
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Because the rods are all seeing the same thing; when this is the case no signal reaches conscious perception. Same thing as when you shut your eyelids and see the backs of them - there's not NOTHING there, but you just don't register it.
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How are rods and cones mostly structurally different?
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Rods have discs that are freefloating within their outer segments; cones have discs that remain connected to the PM.
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What is the receptive field of a neuron?
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The retinal area of photoreceptors that when stimulated influences the activity of that neuron.
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What are HORIZONTAL CELLS?
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the bane of my existence; Inhibitory neurons that interconnect cones with cones and rods with rods
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How do horizontal cells work?
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When a photoreceptor is excited (depolarized) it excites its horizontal cells; these inhibit neighboring photoreceptors (hyperpolarizes them)
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What is the sum action of horizontal cells?
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Mutual inhibition - each photoreceptor inhibits neighbors, and each photoreceptor is inhibited by neighbors.
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What happens when a cone is depolarized?
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It releases the typically excitatory NT glutamate
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What is the nature of photoreceptor membrane potential changes?
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They hyperpolarize, depolarize, and release NT all in a GRADED manner. The more depolarized they are, the more NT they release.
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What do photoreceptors NOT do?
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Produce action potentials
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What is released by depolarized
-Photoreceptors? -Horizontal cells? |
Photoreceptors release Glutamate
Horizontal cells release GABA |
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Glutamate is
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typically excitatory
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GABA is
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Inhibitory
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Effect of GABA:
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hyperpolarization of the target
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Nature of GABA's effect:
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Graded - causes graded hyperpolarizations depending on how much of it is released.
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How do all photoreceptors behave in response to light?
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They hyperpolarize; thus release less neurotransmitter.
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So what does light do to photoreceptors?
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Turns them OFF.
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How would an extremely tiny spot of light illuminating a single cone effect the cone?
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It would turn it OFF; hyperpolarize it.
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How would a tiny ring of light that was dark in the center so that a single cone was in the dark but the cones around it were illuminated effect the center cone?
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It would turn it ON; depolarize it. The cones surrounding it would be turned OFF because they are in the light.
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2 reasons why the center cone turns on:
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1. Cones depolarize in the dark
2. Light falling on cones hyperpolarizes them; no longer depolarized so can't excite horizontal cells; stop inhibiting the center cone. |
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What are all cones?
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Off-center - light centered on them turns them off.
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What turns a photoreceptor ON?
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Light falling on photoreceptors in the region surrounding the photoreceptor we're talking about.
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What are all bipolar cells?
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One of two types:
1. On center or 2. Off center |
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2 things to know about ONcenter bipolar cells:
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1. Invaginating contacts with photoreceptor axons
2. Light centered on the photoreceptor (receptive field) turns the cone OFF but this bipolar cell ON. |
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2 things to know about OFF center bipolar cells:
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1. Flat contacts w/ photoreceptor
2. Light centered on the receptive field turns these off just the same as it turns the cone/rod off. |
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What do we call the response of ONCENTER bipolars to light centered on the receptive field?
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Sign reversing
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What is the effect of GLUTAMATE (typically excitatory) when the cone is DEPOLARIZED (in dark) on the on-center bipolar?
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It turns it off - this is an abnormal case where glutamate is inhibitory.
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What is the effect of glutamate released from a cone when it is in dark on its OFFcenter bipolar cell?
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glutamate excites the offcenter bipolar cell and depolarizes it - excitatory.
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If oncenter bipolars are sign reversing, what are offcenter bipolars?
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Sign conserving - when the cone is depolarized/on, these will be on.. and vice versa.
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2 types of cells that bipolars talk to:
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1. Amacrine
2. Ganglion |
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2 functions of amacrine cells:
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1. Provide lateral connections
2. Many produce transient depolarizing responses |
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What is unique about ganglion cells compared to all other retinal neurons up to this point?
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THEY PRODUCE ACTION POTENTIALS!!
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2 Types of Ganglion cells:
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-on center
-off center |
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2 types of responses that either on or off center ganglion cells can have:
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1. Sustained (for seeing form)
2. Transient (for seing motion) |
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2 anatomical types of ganglion cells:
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1. Parasol ganglion cells
2. Midget ganglion cells |
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What type of behavior is exhibited by Parasol and midget cells? what percentage of ganglion cells does each compose?
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Parasol - M cell behavior (about 10%)
Midget - P cell behavior (about 90%) |
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Which cell is larger; parasol or midget?
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Parasol
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What type of receptive field and response to parasol cells have?
WHAT DO THEY SEE? WHAT IS THEIR RESPONSE? |
Large receptive fields
Transient response -SEE MOTION - CONTRAST RESPONSE |
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What type of receptive fields and response do Midget cells have?
WHAT DO THEY SEE? |
-Small receptive fields
-Sustained responses -SEE COLOR - HIGH ACUITY |
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Where do parasol ganglion cells project to?
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M - magnocellular layers of the LGN
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Where do midget ganglion cells project to?
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P - parvocellular layers of LGN
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