Day 7 (3): Physiology of the Retina and the Vitreous Flashcards
What are the two primary functions of the retina?
- Phototransduction
- transformation of LIGHT energy into CHEMICAL signals then into ELECTRICAL impulses that can travel to the brain
- hyperpolarization of photoreceptors by light in a steady potential or slow graded response - Translation
- conversion of ELECTRICAL impulses (of light and dark patterns) into NEURAL signals
Four types of layers of the retina.
- Nuclear Layers: nucleus of photoreceptors, bipolar cells and ganglion cells
- Plexiform Layers: axons and dendrites interdigitate in synapses
- Photoreceptor Layer: inner and outer segments of photoreceptors
- Nerve Fiber Layer: axons of ganglion cells coalescing to form the optic nerve
What is the retinal neuronal chain?
- 3-neuron chain that transforms light into neural signals
- 2 pathways:
1. Vertical Pathway - communication between cells from DIFFERENT layers
2. Horizontal Pathway - communication between cells from the SAME layer
- cells are contained in 3 nuclear layers:
1. Outer Nuclear Layer: photoreceptor nuclei - Rods
- Cones
2. Inner Nuclear Layer - Bipolar cell nuclei
- Horizontal cell nuclei
- Amacrine cell nuclei
3. Ganglion Cell Layer: ganglion cell nuclei
Differentiate Rods vs Cones
- Each eye can be said actually to contain not one retina but rather two retinas superimposed on each other.
1. Rods layers: sensitive to low levels of light (dusk and dawn)
2. Cones layer: color and sensitive to broad daylight - REMEMBER: retina is not designed to record ABSOLUTE intensity of light but rather the DIFFERENCES IN THE INTENSITY of the light striking it at different points
Rods
- 120 million in number
- scotopic (low light) vision
- more pigments = higher sensitivity to light = need small amounts of light in low light situations to activate
- low visual acuity
- slow response to light
- ONE type of photosensitive pigment = ACHROMATIC vision
- ABSENT in fovea
Cones
- 6 million
- photopic (bright light) vision
- less pigments = lower sensitivity to light = need more light to detect images
- high visual acuity
- fast response to light
- quicker rate of signal amplification
- THREE types of photosensitive pigments for different wavelengths of light = COLOR vision
- concentrated in the FOVEA
What are the parts and components of the photoreceptors?
- Outer Segment (DARK Band) = Photoreceptor Layer
- comprised of disks or modified cilia containing opsin-retinal complex
- closer to the choroid and sclera - Inner Segment = Photoreceptor Layer
- contains cytoplasm
- Ellipsoid Zone (LIGHT Band): mitochondria
- Myoid Zone (DARK Band): other organelles
——————–ELM (LIGHT Band)—————— - Nucleus (DARK Band) = Outer Nuclear Layer
- cell body/nucleus of photoreceptors - Axons (LIGHT Band) = Outer Plexiform Layer
- synapses of photoreceptors with bipolar cells
Compounds:
1. Retinal: chromophore
- aldehyde form of Vitamin A attached to Opsin
- Dark: 11-cis
- Light: All-trans
- Opsin:
- protein that absorbs photons of light
- Rhodopsin: Rods
- Photopsin/Iodopsin: Cones; reacts differently to different wavelengths of light (color discrimination)
What are the FOUR HYPERreflective outer retinal bands seen in the SS-OCT?
+ Outer Nuclear Layer (DARK)
1. External Limiting Membrane (LIGHT): apical processes of Muller Cells
+ Myoid Zone of Inner Segment (DARK)
2. Ellipsoid Zone of Inner Segment (LIGHT): densely packed with mitochondria causing increased backscattering of light and high refractive index
+ Outer Segment (DARK)
3. Interdigitation Zone (LIGHT): outer segment tips interfacing with the RPE
4. RPE/Bruch’s Membrane Complex (LIGHT)
What happens to the retina in the DARK?
- No photon from light thus retinal in 11-cis form
- Opsin is NOT converted into metarhodopsin II
- Transducin and subsequently cGMP phosphodiesterase are NOT activated
- Resulting HIGH concentrations of cGMP keep cGMP-gated Na+ channels OPEN
- Na+ enters (DARK CURRENT)
- DEpolarization of photoreceptors (~ -30 mV)
- Voltage-gated Ca2+ channels OPEN
- Ca2+ enters
- Glutamate is RELEASED from vesicles at synaptic clefts
- INHIBITION of bipolar cells
What happens to the retina with LIGHT?
PHOTOTRANSDUCTION:
- Photon from light is absorbed by 11-cis retinal bound to opsin
- Isomerization: 11-cis retinal becomes all-trans retinal
- Conformational change of opsin to metarhodopsin II
- Metarhodopsin II activates transducin
- Activated transducin α-subunit activates cGMP phosphodiesterase
- cGMP PDE hydrolyzes cGMP to 5’-GMP
- DECREASE cGMP concentration CLOSES cGMP gated Na+ channels
- HYPERpolarization of photoreceptors (~ -70 mV)
- Voltage-gated Ca2+ channels CLOSE
- Decrease in Ca2+ concentration STOPS release of glutamate
- ACTIVATION of bipolar cells
What happens during the recovery phase or dark adaptation when the light is turned off?
Stimulus: Low Ca2+ concentrations
Effects:
1. Ca-recoverin-RK complex dissociates into Ca2+, RECOVERIN and RHODOPSIN KINASE (RK).
- free RK phosphorylates metarhodopsin II to decrease binding affinity for transducin
- Arrestin completely deactivates the phosphorylated-metarhodopsin II
- Corresponds to S2 component of dark adaptation
- Ca2+ dissociates from GTPase-Activating Protein (GAP)
- free GAP deactivates transducin and in effect, PDE
- STOPS hydrolysis of cGMP to GMP
- Deactivation of transducin: RATE LIMITING STEP in the deactivation of the cascade - Ca2+ dissociates from Guanylate Cyclase Activating Protein (GCAP).
- free GCAP activates Guanylate Cyclase to convert GTP to cGMP
- INCREASES levels of cGMP - Ca2+/Calmodulin complex within the cGMP-gated Na+ channels becomes more sensitive to low cGMP levels
- keeps cGMP-gated Na+ channels open even at low cGMP levels - All-trans retinAl is transported back to RPE and reduced to All-trans retinOl (precursor of 11-cis retinal)
- will be transported back to photoreceptors
End Result:
INCREASING levels of cGMP re-opens the cGMP-gated Na+ channels, restoring the dark current and release of glutamate
What is a receptive field?
- An area in the field of vision “covered” by each neuron in the retina where the presence of an appropriate stimulus will modify the activity of this neuron
- Become increasingly complex as the stimuli ascends from the photoreceptor cells to the visual cortex
Photoreceptors
- limited to the tiny spot of light that corresponds to its precise location in the retina
Bipolar Cells
- circular
- a ray of light that strikes the center of the field has the opposite effect from one that strikes the area surrounding it (“surround”)
Ganglion Cells
- circular, similar to bipolar cells with a center-surround antagonism
- unlike bipolar cells, they do not respond by depolarizing or hyperpolarizing, but rather by increasing or decreasing the frequency with which they discharge action potentials
Visual Cortex
- rectangular
What are the two types of bipolar cells?
- Distinguished by the way they respond to light on the centers of their receptive fields
- Display Center-Surround Antagonism: the response to the stimulation of the center of the receptive field is always inhibited by the stimulation of the surround
- Synapses with photoreceptors at the OUTER Plexiform Layer
ON-center Bipolar Cell
- active when lights are ON
- metabotropic receptor: HYPERpolarized by glutamate
- inhibited by glutamate; excited if no glutamate (lights ON)
- light on the CENTER = excitatory –> DEpolarized
- light on the SURROUND = inhibitory –> HYPERpolarized
OFF-center Bipolar Cell
- active when lights are OFF
- ionotropic receptor: DEpolarized by glutamate
- excited by glutamate (lights OFF); inhibited if no glutamate
- light on the CENTER = inhibitory –> HYPERpolarized
- light on the SURROUND = excitatory –> DEpolarized
How are bipolar cells and ganglion cells connected?
- Synapses at the INNER Plexiform Layer
- Glutamate is released from synaptic clefts of bipolar cells if DEpolarized
- The more glutamate released by bipolar cells, the more action potentials propagated by ganglion cells
Connections:
ON-center Bipolar Cells = ON-center Ganglion Cells
OFF-center Bipolar Cells = OFF-center Ganglion Cells
What are the lateral connections between photoreceptors, bipolar cells and ganglion cells?
Purpose: ADAPTATION and AMPLIFICATION of signals
Horizontal cells
- receive inputs from photoreceptors and transmit it to SURROUNDING bipolar cells
- in the OUTER plexiform layer
- important for CONTRAST sensitivity
- integrates and regulates inputs from MULTIPLE photoreceptors
- negative feedback to other photoreceptors
- DEpolarized by glutamate released from SURROUND photoreceptors when lights are OFF causing release of GLYCINE which further amplifies inhibition of glutamate release on the CENTER photoreceptors
Amacrine cells
- receive inputs from bipolar cells and transmit it to
SURROUNDING ganglion cells
- in the INNER plexiform layer
- important for MOVEMENT sensitivity
- enhances center-surround antagonism in GC receptive fields
- connects ROD bipolar cells to CONE bipolar cells to allow GC to respond to the entire range of light levels
Summary of events in photoreceptors, bipolar cells and ganglion cells when light is turned on and off.
Lights ON
- Photoreceptor HYPERpolarized
- Glutamate release from photoreceptor INHIBITED
+ ON-center BC (hates glutamate) DEpolarized
+ Glutamate release from ON-center BC STIMULATED
+ ON-center GC FIRES action potential
- OFF-center BC (loves glutamate) HYPERpolarized
- Glutamate release from OFF-center BC INHIBITED
- OFF-center GC DOES NOT FIRE action potential
Lights OFF
- Photoreceptor DEpolarized
- Glutamate release from photoreceptor STIMULATED
- ON-center BC (hates glutamate) HYPERpolarized
- Glutamate release from ON-center BC INHIBITED
- ON-center GC DOES NOT FIRE action potential
+ OFF-center BC (loves glutamate) DEpolarized
+ Glutamate release from OFF-center BC STIMULATED
+ OFF-center GC FIRES action potential
+ Horizontal cells DEpolarized and release GLYCINE which amplifies glutamate release inhibition in SURROUND photoreceptors
What are Muller cells?
- Glial cells of the retina
- Structural support and stability of the retina
1. uptake of neurotransmitters
2. removal of debris
3. regulation of K levels
4. glycogen storage
5. electrical insulation
6. mechanical support - Forms the following layers:
1. External Limiting Membrane
+ apical/outer foot processes forming junctional complexes with inner segment of photoreceptors
+ between inner segment of PRL and ONL
2. Internal Limiting Membrane
+ basal/inner foot processes
+ between NFL and vitreous; separates neurosensory retina from vitreous
What are Retinal Pigment Epithelium?
Purpose: Photoreceptor maintenance and health
- Storage and conversion of Vitamin A for rhodopsin synthesis
- Regeneration of bleached (used) photopigments
- Production of GAGs that envelope photoreceptors
- Phagocytosis of shed lamellar discs
- (+) Melanosomes: absorption and screening of scattered light
- Retinal adhesion
- Fluid transport from the subretinal space
- Forms the OUTER Blood Retina Barrier: (+) tight junctions/zonula occludens
What is the subretinal space?
Potential space in between the neurosensory retina and retinal pigment epithelium
Discuss the Vitamin A cycle happening in the retinal pigment epithelium.
- Beta-Carotene
- precursor of Vitamin A
- not synthesized in the body; obtained from diet
—TRANSPORT and PROCESSING in LIVER—
- All-trans retinOL
- stored form in the liver
- binds to serum Retinol Binding Protein (RBP)
- carried to the choriocapillaries and transported into the RPE
–ISOMERIZATION (by RPE65 protein in RPE)–
- 11-cis retinOl
————————–OXIDATION———————-
- 11-cis retinAl
- binds to Interstitial Binding Protein and transported to the outer segment of photoreceptor layer
- binds to OPSIN in the photoreceptor layer
-ISOMERIZATION (by interaction with photons)-
- All-trans retinAl
- induces conformational change in opsin forming RHODOPSIN
————————REDUCTION———————-
- All-trans retinOl
- formed by using up (bleaching) of photopigments after phototransduction is completed
- recycled form
—————TRANSPORT back to RPE————-
Note:
Cis to Trans form: isomerization
RetinAl to RetinOl: reduction
RetinOl to RetinAl: oxidation
Discuss the phagocytic function of the RPE.
- Happens in the interdigitation zone between the RPE and the photoreceptor tips/outer segment of the photoreceptor layer
- Old discs located in the outer segment of the photoreceptors are ingested by the microvilli of the RPE and digested
Purpose:
1. Clear dysfunctional discs and debris
2. Recycle components
Pathology: Age-Related Macular Degeneration
- Dysfunction of phagocytic capability causes accumulation of debris which can cause stress to the RPE
- (+) DRUSEN formation
Discuss the pigment screening capability of the RPE.
Melanosomes
- intracellular organelles found in the microvilli at the apex of RPE surrounding the outer segment of photoreceptors
- minimize scattering of light in between photoreceptors by absorption
What are the forces that maintain normal retinal adhesion?
Passive forces: NOT energy-requiring
1. Interdigitation of the outer segments of photoreceptors with apical microvilli of RPE and interphotoreceptor matrix
2. Tight junctions between the RPE
- forms the OUTER Blood Retina Barrier
3. Passive transretinal fluid gradient
4. Osmotic pressure from the proteins in the choriocapillaries
- drives fluid from the subretinal space to the choroid
Active forces: energy-requiring
Na-K pump and HCO3 transport system
- keeps subretinal space dehydrated
- maintains ionic composition of subretinal space
Pathology: separation of the neurosensory retina from the RPE will affect the phototransduction cascade
What is the vitreous?
Properties:
- clear semisolid hydrogel
- provides structural support and stability
- optically clear: allows transmission of light
- pathway for nutrient delivery to and clearance of wastes from the internal eye
- ~ 80% of globe
- refractive index: 1.336
- volume: 4 mL
- weight: 4 grams
- pH: 7.5 (slightly alkaline)
Composition:
- 98-99% water
- 1 - 2% collagen and GAGs
Describe the structure of the vitreous.
- Spherical with an anterior central depression
Zones:
+ Vitreous Cortex/Vitreous Membrane: 2%
- layer of collagen separating the vitreous from the internal eye
- condensed and fibrillar
- (+) cells: hyalocytes, fibrocytes
- metabolic center of the vitreous: production of hyaluronic acid
- 2 parts:
1. Anterior Hyaloid Membrane
2. Posterior Hyaloid Membrane
+ Vitreous Medulla: 98%
- gelatinous or liquid depending on age, refraction
- lower amount of fibrils
- NO cells
- first to liquefy in vitreous SYNERESIS or liquefaction
What are the boundaries of the vitreous?
Anterior Hyaloid Membrane
- thin layer running from the pars plana to the posterior lens separating the anterior vitreous from the lens
- in contact with ciliary processes, lens zonules and posterior lens capsule
- greater density of collagen fibrils
- (+) Patellar Fossa: anterior central depression in contact with the lens
- (+) Weiger’s Ligament/Egger’s Line: attachment of the anterior cortical vitreous to the posterior lens capsule
Posterior Hyaloid Membrane
- separates the posterior vitreous from the retina
- in contact with the ILM layer of the retina
- a FALSE anatomical membrane
- accentuated if (+) pre-retinal hemorrhage or (+) vitreous contraction
Where are the vitreous attachments located?
- Vitreous Base
- strongest attachment of the vitreous to the retina
- 2 - 6 mm wide area straddling the ora serrata
- anterior margin:
+ 1.5 - 2.0 mm anterior to the ora or
+ 5.0 mm posterior to the limbus
- posterior margin:
+ 1.8 mm posterior to the ora temporally
+ 3.0 mm posterior to the ora nasally - Optic Nerve Head/Optic Disc Margin
- 2nd firmest area of attachment of the vitreous
- due to vitreous fibrils intertwining with the ILM and epipapillary membrane
- (+) Weiss Ring: ring of fibrous astrocytes and collagen seen when vitreous completely detaches from the optic disc in cases of Complete Posterior Vitreous Detachment - Fovea
- Major retinal blood vessels
What are the functions of the vitreous?
- Structural support
- Shock absorber
- Metabolic repository
- Scavenging capability
- Optical clarity
- Molecular sieve
What components of the vitreous is responsible for its structural stability?
Collagen-Hyaluronate Network
- responsible for the stability, structure, volume and transparency of the vitreous
Components:
1. Collagen
- crosslinked into rods to form fibrils
- plasticity: ability of a material to KEEP its deformed shape when the external force is removed
- equal distribution of forces within the eye
- highest concentration: vitreous cortex and base
- lowest concentration: medullary vitreous
- Vitrosin: fibrous collagen found only in vitreous humor important for adhesion and attachment
- Hyaluronic Acid
- polysaccharide enmeshed in the collagen network
- viscoelasticity: ability of a material to RETURN to its original shape after the external force is removed
- prevents separation into solid and liquid phases
- prevents collapse of the gel structure
- highest concentration: posterior vitreous cortex
- turnover rate: 0.45 ug/day
What component of the vitreous is responsible for shock absorption?
Hyaluronic Acid
- polysaccharide enmeshed in the collagen network
- viscoelasticity: ability of a material to RETURN to its original shape after deformation
- better shock absorption and force dissipation with HIGHER concentrations of HA
- protects internal eye during eye movement, physical activity, minor trauma, friction and vibration
What component of the vitreous acts as a molecular sieve and chemical barrier?
Hyaluronic Acid
- polysaccharide enmeshed in the collagen network
- negatively-charged compound that inhibits movement of water and positively-charged molecules
- major resistance to transvitreal flow of water: for every 1% increase in HA concentration, resistance increases by 100-fold
- barrier to inflammatory proteins and cells
- limits inflammatory response by inhibiting:
1. lymphocyte stimulation
2. macrophage phagocytosis and prostaglandin synthesis
What mechanisms are responsible for the flow current in the internal eye?
- Travels across the retina, between aqueous and vitreous and between vitreous and lens
- Mechanisms:
1. Simple diffusion
2. Hydrostatic pressure: from blood flow
3. Osmotic pressure: from proteins and solute
4. Convection currents: from compartment temperature differences
5. Active transport of solutes between compartments
What is the nutritional and metabolic function of the vitreous?
Nutritional: source of nutrients for the internal eye
1. Ions, Oxygen
2. Glucose, Galactose, Mannose, Fructose
3. Amino Acids
Drainage: metabolic wastes of the internal eye
1. Lactic Acid
2. Carbon Dioxide
What component of the vitreous has scavenger abilities?
Ascorbic Acid
- antioxidant that acts as a scavenger of radical oxygen species and metabolic wastes from the lens and retina
What are the different potential spaces in the vitreous and retina?
Preretinal/Subhyaloid Space
- between posterior vitreous cortex and ILM of the retina
Subretinal Space
- between the photoreceptor layer and the RPE
What properties of the vitreous are responsible for its optical clarity?
Optical clarity
- allows efficient transmission of light with minimal scattering
- refractive index: 1.336 (similar to tear film and aqueous)
- allows 90% of visible light to pass (380 - 750 nm wavelength)
Reasons:
1. High water content
2. Thin and uniform size of collagen fibrils
3. Bulky HA molecules keep collagen fibrils far apart allowing light to pass through and minimizing light scatter
4. Majority of vitreous (medullary) is essentially free of cells and macromolecules
- HA sieve acting as a physical and chemical barrier
- if HA concentrations decrease, exudates increase
How does vitreous degenerative changes progress?
- Collapse of the Collagen-Hyaluronic Acid network
- due to depolymerization & precipitation of HA
- etiology: aging, trauma, inflammation, infection, myopia, hemorrhage, heat, iatrogenic - Liquefaction/Syneresis: formation of fluid-filled spaces or lacunae which coalesce
- Matting of collagen fibrils forming prominent strands
- (+) FLOATERS
- loss of optical clarity from increased scattering of light - Vitreous contraction and shrinkage
- Thinning of vitreous cortex and eventual rupture
- Intralacunar fluid enters subhyaloid/preretinal space further separating the vitreous from the retina
Endpoint: Complete Posterior Vitreous Detachment
What are the sequelae of posterior vitreous detachment?
Sequelae:
1. Vitreous Hemorrhage/Preretinal Hemorrhage
- vitreous detachment causing traction on the retinal vessels
- Macular Hole
- vitreous detachment from its firm attachment to the fovea - Weiss Ring
- optic nerve head/optic disk margin: last area to detach in PVD due to its firm attachment with the vitreous - Retinal Tear –> Rhegmatogenous Retinal Detachment
- the vitreous detachment proceeds anteriorly eventually reaching the ora serrata
- vitreous is firmly attached to the retina at the vitreous base in the ora serrata
- detachment of the vitreous pulls on the NS retina causing a tear
- liquefied vitreous in the subhyaloid/PREretinal space enters the SUBretinal space in between the NS retina and RPE causing retinal detachment
Note:
Rhegmatogenous Retinal Detachment (RRD)
- most common type of retinal detachment
- retinal detachment associated WITH a retinal tear
