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66 Cards in this Set

  • Front
  • Back
2 Things involved in Phototransduction:
1. Photopigments
2. Excitatory cascade
What color do rods absorb most?
What color do cones absorb most?
there are 3 different types!
3 Cones - what does each absorb?
Short - absorbs blue
Middle - absorbs green
Long - absorbs red
pneumonic for cones:
sml the bgr
What are visual pigments?
G-protein coupled receptors activated by light instead of a ligand.
5 Steps in Phototransduction:
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
Effects of closing Na/Ca channels in the photoreceptor cell:
Cell hyperpolarizes due to loss of sodium influx; calcium loss causes decreased PDE activity and increased GC activity so actually reforms cGMP
5 Inherited visual defects and degenerations associated with molecules involved in transduction:
1. Color blindness
2. Blue cone monochromacy
3. Retinitis pigmentosa
4. Congenital stationary night blindness
5. Cone, cone-rod, and macular degeneration
Macular degeneration affects:
Visual acuity - the macula contains the fovea - zone of highest visual acuity.
Who gets color blindness?
8-10% of males
What causes blue cone monochromacy?
Mutations in both L and M (long and middle) cones - leaves only S cones (short) which are normally only 5% of our cones.
What is the problem in Retinitis pigmentosa?
Loss of vision in the periphery
3 molecules associated with Retinitis pigmentosa:
1. Rhodopsin
2. PDE
3. GMP-gated ion channels
(also arrestin)
What is Congenital stationary night blindness?
The inability to see in dim light.
3 molecules associated with CSNB:
(also rhodopsin kinase)
What is defective in cone and macular degeneration?
How many rods are in each eye? For what?
~100 million - for vision in very dim light.
How many cones are in each eye? For what?
~5 million - for daytime vision
Majorly important difference between rods and cones:
Rods saturate
Cones don't
What is saturation?
As the amt of light that reaches the retina is increased, the rods' response increases until it reaches its maximum response.
How much light does it take to max out the rod's response?
VERY dim light
So what state are rods in at normal room light?
maxed out - saturated
So what does this explain?
Why we use cones for day vision and rods only for dim light vision - that's the only time they're ever active.
How much of our receptors are we typically using for vision?
5% - that's the percentage of cones - rods are 95%
What is the response of photoreceptors to light?
We have 100 million rods maximally stimulated (saturated) all during the daytime; why don't we see snowblind then?
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.
How are rods and cones mostly structurally different?
Rods have discs that are freefloating within their outer segments; cones have discs that remain connected to the PM.
What is the receptive field of a neuron?
The retinal area of photoreceptors that when stimulated influences the activity of that neuron.
the bane of my existence; Inhibitory neurons that interconnect cones with cones and rods with rods
How do horizontal cells work?
When a photoreceptor is excited (depolarized) it excites its horizontal cells; these inhibit neighboring photoreceptors (hyperpolarizes them)
What is the sum action of horizontal cells?
Mutual inhibition - each photoreceptor inhibits neighbors, and each photoreceptor is inhibited by neighbors.
What happens when a cone is depolarized?
It releases the typically excitatory NT glutamate
What is the nature of photoreceptor membrane potential changes?
They hyperpolarize, depolarize, and release NT all in a GRADED manner. The more depolarized they are, the more NT they release.
What do photoreceptors NOT do?
Produce action potentials
What is released by depolarized
-Horizontal cells?
Photoreceptors release Glutamate
Horizontal cells release GABA
Glutamate is
typically excitatory
Effect of GABA:
hyperpolarization of the target
Nature of GABA's effect:
Graded - causes graded hyperpolarizations depending on how much of it is released.
How do all photoreceptors behave in response to light?
They hyperpolarize; thus release less neurotransmitter.
So what does light do to photoreceptors?
Turns them OFF.
How would an extremely tiny spot of light illuminating a single cone effect the cone?
It would turn it OFF; hyperpolarize it.
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?
It would turn it ON; depolarize it. The cones surrounding it would be turned OFF because they are in the light.
2 reasons why the center cone turns on:
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.
What are all cones?
Off-center - light centered on them turns them off.
What turns a photoreceptor ON?
Light falling on photoreceptors in the region surrounding the photoreceptor we're talking about.
What are all bipolar cells?
One of two types:
1. On center
2. Off center
2 things to know about ONcenter bipolar cells:
1. Invaginating contacts with photoreceptor axons
2. Light centered on the photoreceptor (receptive field) turns the cone OFF but this bipolar cell ON.
2 things to know about OFF center bipolar cells:
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.
What do we call the response of ONCENTER bipolars to light centered on the receptive field?
Sign reversing
What is the effect of GLUTAMATE (typically excitatory) when the cone is DEPOLARIZED (in dark) on the on-center bipolar?
It turns it off - this is an abnormal case where glutamate is inhibitory.
What is the effect of glutamate released from a cone when it is in dark on its OFFcenter bipolar cell?
glutamate excites the offcenter bipolar cell and depolarizes it - excitatory.
If oncenter bipolars are sign reversing, what are offcenter bipolars?
Sign conserving - when the cone is depolarized/on, these will be on.. and vice versa.
2 types of cells that bipolars talk to:
1. Amacrine
2. Ganglion
2 functions of amacrine cells:
1. Provide lateral connections
2. Many produce transient depolarizing responses
What is unique about ganglion cells compared to all other retinal neurons up to this point?
2 Types of Ganglion cells:
-on center
-off center
2 types of responses that either on or off center ganglion cells can have:
1. Sustained (for seeing form)
2. Transient (for seing motion)
2 anatomical types of ganglion cells:
1. Parasol ganglion cells
2. Midget ganglion cells
What type of behavior is exhibited by Parasol and midget cells? what percentage of ganglion cells does each compose?
Parasol - M cell behavior (about 10%)
Midget - P cell behavior (about 90%)
Which cell is larger; parasol or midget?
What type of receptive field and response to parasol cells have?
Large receptive fields
Transient response
What type of receptive fields and response do Midget cells have?
-Small receptive fields
-Sustained responses
Where do parasol ganglion cells project to?
M - magnocellular layers of the LGN
Where do midget ganglion cells project to?
P - parvocellular layers of LGN