Flashcards in Chapter 12: Retina Deck (40):
What is the retina composed of?
The retina is composed of two laminar structures that are derived from the invagination of the embryonic outer cup that folds an ectodermal layer into apex-to-apex contact with itself (thus creating a subretinal space)
- Outer RPE and Inner Neurosensory retina are created
What cell types are located in the Retina?
- Photoreceptors (Rods/Cones)
-Bipolar Cells (Rod-On, Cone-On, and Cone-Off)
- interneurons (horizontal and amacrine cells)
- Ganglion cells and their axons
- Supporting cells (astroglia, oligodendroglia, schwanna cells, microglia, vascular endothelium, pericytes)
What is phototransduction?
process of cathcing light and converting its energy into a neural response
occurs within the outer segment of photoreceptors
Why do rods have a higher sensitivity to light then cones?
They have a higher light sensitivity than cones b/c they contain more membrane than cones
What is the Rod Outer Segment composed of?
composed primarily of plasma-membrane organized into flattened sacs, each containing 1x10^6 rhodopsin molecules (membrane bound)
There are approx. 1000 sacs per rod, each containing the protein machinery to capture and amplify light energy
What is the purpose of rhodopsin in the outer segment of photoreceptors?
Rhodopsin captures the lights energy, and responds to a single quantum of light.
What type of light is rhodopsin best at absorbing and why? What type of light is it insensitive to, and why is this significant?
Best at absorbing green light, Lamdba= 510 nm
also able to absord blue and yellow light
Insensitive to red light due to longer wavelength. Significant b/c explains why red light doesnt bleach out photorecepters at night, adn therefore why its best to use a red flashlight in the dark
What happens to rhodopsin after absorbing a quantum of light?
The opsin molecule undergoes structural changes and becomes Metarhodopsin II, which controls the inflow of cations into the rod outer segment via cGMP-gated cationic channel on the outer membrane (opens channel)
Rhodopsin also actives transducin, which helps to amplify this reaction
How do rod sacs differ from cone sacs?
Cone sacs are connected to the outer membrane, rod sacs are not.
What unique proteins are found on the membrane of the outer segments of the photoreceptors?
Peripherin and Rod Outer Regment Protein 1
Fx= play a role in development and maintenance of sacs curvature (also found in cones)
What is the ABC protein? where is it found? What is its function?
ATP binding casette protein, is unique to Rods, and functions as a transporter of all-trans-retinol
How do the outer segments produce energy?
Via glycolsysis. (b/c mitochondria are all contained within the inner segments of the photoreceptors)
How does rod phototransduction compare to cone phototransduction?
Compared to rod phototransduction, cone phototransduction is insensitive, but is faster and able to adapt to ambient levels of illumination.
The greater the ambient illumination, the faster and more temporally accurate the response
Why do higher levels of illumination decrease sensitivity?
B/c they bleach away photopigments
Describe the process of cone phototransduction
Light activates cone Opsins-> enzymatic cascade-> hydrolysis of cGMP-> closure of cGMP-gated cation channels
What kind of feedback do the photoreceptors have?
Neurally-mediated negative feedback (horizontal cells of the inner nuclear layer synapse antagonistically bac onto cones and release GABA, which inhibits the cones)
What happens when light hyperpolarizes a cone?
Cone hyperpolarized--> hyperpolarizes neighboring horizontal cells--> inhibits horizontal cells-> disinhibition of cones
Increases fusion flicker frequency in cones (around 100 Hz) compared to rods (30 Hz)
How do primates have trivariant color vision?
Through the presence of a 3 cone mechansim Red-Green color vision is enabled
L-type cones and M-type cones: contribute to both achromatic and chromatic contast, and are more numerous than S-type cones in the human retina
Why do most visual defects involve red-green color discrimination?
b/c they are located tandem on the x chromosome, related to unequal crossing of L- and M- cone opsin genes, and creating a different spectral absorption function
What is the most common AD mutation of rhodopsin that causes ADRP?
P23H, causes an abnormal folding of a protein that accumulates in RER
accounts for 10% of RP cases in US
What do defects in the L- and M- cone opsins cause?
Blue cone monochromatism, which occurs in males 2/2 location on X chromosome.
What do defects in all 3 cone opsin genes cause?
Achromotopsia (= Rod Monochromatism)
What do defects in L- or M- cone opsins cause?
Red-Green color deficiencies, almost exclusively occurs in males 2/2 location on X Chromosome.
What cell types are located in the Inner Nuclear Layer of the Retina?
Neurons: Horizontal, Amacrine, Bipolar
Glial Cell: Muller Cells
How many types of bipolar cells exist?
Rods and cones have separate bipolar cells, and cones have two types of bipolar cells--On-type and Off-type
However, S cones only have one type of bipolar cell (similar to Rods)
How do bipolar neurons interact with photoreceptors?
On bipolar cells are inhibited/Off are excited by release of glutamate
What are horizontal cells?
antagonistic interneurons that inhibit photoreceptors by releasing GABA when depolarized
Dendrites synapse on cones (one class modulates L- and M-types, another S-types)
Provide negative feedback onto rods/cones (photoreceptors release glutamate, and horizontal cells release GABA back onto the photoreceptors)
What are amacrine cells?
interneurons that mediate antagonistic interactions among on-bipolar, off-bipolar, and ganglion cells
How many types of retinal ganglion cells exist?
Two types: On type and Off type
On Type: center cells excited
Off Type: center cells inhibited by light in the center of their receptive field
What are the main sub-types of retinal ganglion cells?
Tonic cells (driven by L- and M-cones), Tonc cells (driven by s-cones), and Phasic cells
What are the tonic cells driven by L- and M-type cones?
include small cells concentrated in fovea, are responsible for high acuity vision
Project to parvocellular layers of the LGN and mediate high spatial resolution and color vision
What are the tonic cells driven by S-cones?
Designed to detect successive color contractq
What are the phasic cells?
Project to the magnocellular layer of the LGN and mediate movement detection
What are Muller cells?
glial cells that play a supportive role to neural tissue extending from inner segments of photoreceptors to the ILM
Also buffer ionic concentration in extracellular space, seal off the subretinal space by forming the ELM, and play a role in cone vitamin A metabolism
What are the functions of the macroglia/microglia?
Function is to provide physical support, respond to retinal injury, regulate ionic/chemical composition of ECM, participate in blood-retina barrier, form myelination for ON, guide neuronal migration during development, and exchange metabolites with neurons
What are pericytes?
Cells that surround retinal vessels and play a role in autoregulation, and structurally support endothelium/suppress proliferation
Are an early target in DM
What happens if pericytes are injured, such as in diabetes?
Damage to pericytes leads to increased vessel permeability and microaneurysm formation
What is the difference between the responses of cones and rods to light hyperpolarization?
Cones respond rapidly, turn off while light is still on, and overshoot the dark potential
Rods have a prolonged response that turns off slowly
What affect does the dark have on cones? Rods?>
Cones- depolarized by darkness
Rods- little effect from darkness