Retina Physiology

Physiology of the Retina

• The retina is a highly specialized neural tissue.
• It coverts light into a neural signal.
• The retina lines the inner surface of the globe.
• It is separated from the sclera by the choroid.
• Ocular media focuses light onto retinal photoreceptors like rods and cones.
• Light changes photoreceptor cell photopigment, altering cell membrane potential.
• This leads to a neural signal conveyed by retinal interneurons.
• This signal reaches the brain via the optic nerve.

Layers of the Neural Retina (from outermost to innermost)

• Photoreceptor layer (Ph) contains the inner and outer photoreceptor cell segments.
• Outer limiting membrane (OLM) consists of zonulae adherens between Müller cells and photoreceptor inner segments.
• Outer nuclear layer (ONL) contains the photoreceptor cell bodies.
• Henle's fiber layer (HL) contains photoreceptor axons.
• Outer plexiform layer (OPL) features synapses between photoreceptor, bipolar, and horizontal cells.
• Inner nuclear layer (INL) contains horizontal, bipolar, and amacrine cell bodies, as well as Müller cell nuclei.
• Inner plexiform layer (IPL) features synapses between bipolar, amacrine, and ganglion cells.
• Ganglion cell layer (GCL) contains ganglion cells and displaced amacrine cells.
• Nerve fiber layer (NFL) contains ganglion cell axons traveling towards the optic nerve.
• Internal limiting membrane (ILM) is formed by Müller cell endfeet.

Organization of the Neural Retina

• The neural retina has ten layers
• Retinal neurons are photoreceptor, horizontal, bipolar, interplexiform, amacrine, and ganglion cells.
• Cones pass neural signals to bipolar cells and then to ganglion cells.
• Rods send signals to rod bipolar cells, then indirectly to ganglion cells via amacrine cells.
• Ganglion cell axons travel through the retinal nerve fiber layer.
• They pass uninterrupted through the optic nerve to central nervous system targets.
• Retinal neurons are supported by radial glial cells called Muller cells.

Macula Lutea

• Located centrally, it has structural features important for vision.
• Antioxidant carotenoid pigments lutein and zeaxanthin give it a yellow hue.

The Fovea Centralis

• This is a small depression at the central macula.
• It provides highest visual acuity.
• Depression results from centrifugal displacement of inner retinal cells.
• Vessels are absent from the central fovea (foveal avascular zone) reducing light scattering.
• The central fovea depends on the choroid for O2 and metabolic supply.
• The fovea has no rods or blue-sensitive cones.
• Foveal cones are elongated and densely packed.

The Parafovea

• The fovea is surrounded by the parafovea, which is in turn surrounded by the perifovea.
• Cone density decreases with increasing eccentricity, while rod density rises, peaking at about 200 off fixation.
• The parafovea contains the thickest GCL, IPL, and INL layers in the retina.
• It processes signals from foveal and parafoveal photoreceptors.
• The Henle's fiber layer is formed by photoreceptor axons and is especially prominent in the parafovea.

Photoreceptor Cells

• Photoreceptor cells have an outer segment, inner segment, and cell body.
• Rods and cones are two classes with different functions, morphology, and distribution.
• Photoreceptor cells use phototransduction to convert light into neural signals.
• It happens in the outer segment disk membranes where rhodopsin and phototransduction enzymes are located.
• Membrane lipid structure and composition are important.
• They maintain optimal enzyme position for phototransduction.

Inner Segment

• Rich in mitochondria and sub-cellular organelles.
• Contains cationic channels and Na+/K + ATPase pumps.
• These restore resting membrane potential after light-induced hyperpolarization.
• Synthesizes components for OS renewal.
• Includes bilayered lipid membranes and molecules necessary for phototransduction.
• Connected to the OS via a thin connecting cilium.

Cell Body

• Cell nucleus is contained within.
• It sends a synaptic projection apically (spherule in rods or pedicle in cones) into the OPL.

Synaptic Terminals of Rod Spherule

• The rod spherule is sphere shaped.
• It contains mitochondria, glutamate containing vesicles, and synaptic ribbons near the presynaptic membrane.
• The glutamate-containing vesicles fuse to either side of the ribbon before release.
• The rod spherule forms a synaptic triad with ON bipolar and H1 horizontal cells axons.

Synaptic Terminals of Cone Pedicle

• Similar structure to the rod spherule but larger.
• It contains several triads with more ribbons and synaptic targets.
• Forms complexes with ON and OFF bipolar cells and H1 and H2 horizontal cells.

Inter-photoreceptor junctions

• Gap junctions exist between photoreceptor cell terminals.
• This permits electrical coupling.
• Rod-rod and cone-cone coupling are stronger than rod-cone coupling.
• Rod-rod coupling decreases visual resolution but improves photoreceptor gain.
• Rod-rod coupling is important for rod function under dark-adapted conditions.

Membrane Potential - The Dark Current

• In the dark, light-sensitive membrane cation nucleotide-gated (CNG) channels are kept open by cyclic guanosine monophosphate (cGMP).
• These maintain a resting dark current: Na+ and Ca2+ enter the OS, while K+ leaks out from the IS.
• This depolarizes the membrane to near -40 mV.
• This leads to glutamate release, an excitatory neurotransmitter from the synaptic terminal.

Membrane Response to Light

• Light induces phototransduction, which closes the CNG channels, disrupts the dark current, and hyperpolarizes the membrane.
• Ca 2+ channels closure occurs at the perisynaptic region.
• A reduced intracellular Ca 2+ level leads to glutamate release decrease.
• Membrane hyperpolarization and reduced glutamate release are proportional to light intensity (graded).
• Maximal hyperpolarization is -60 mV.

Differences Between Cone and Rod Membrane Response to Light

• The speed of current is greater in cones, thus less influenced by light intensity than in rods.
• The restoration of the baseline membrane potential after light stimulation is faster in cones than rods.
• Rods need fewer photons to achieve full membrane hyperpolarization (-50 to -60 mV).
• Rods are more sensitive to light than cones and can detect single photons.

Influences on Membrane Potential

• Background light attenuates the response to a flash, a process known as photoadaptation.
• Inner segment K+ and Ca2+ inward conductances return membrane potential to its resting value after hyperpolarization.
• This maintains a dynamic range of neurotransmitter release.
• Cell-cell coupling via gap junctions and synaptic negative feedback via horizontal cell inhibition allow neighboring photoreceptors to influence one another.

Phototransduction

• Biochemical events within the photoreceptors lead from photon capture to membrane hyperpolarization and slow glutamate release.
• It is fast, happening within milliseconds of a light signal.
• The process is similar in rods and cone, however, cones differ in spectral sensitivities with faster biochemical responses than rods.
• It includes an activation and a recovery phase of a light response.

Activation Phase

• When light hits a photoreceptive pigment within the rods.
• The pigment, rhodopsin, consists of large proteins called opsin and 11 cis-retinal (a derivative of vitamin A).
• A photon of light causes the chromophore to isomerize into 11-transretinal.
• This change activates rhodopsin into metarhodopsin II, which is a freely moving molecule in the lipid bilayer.
• Metarhodopsin II activates a regulatory protein called transducin, which is fuelled by guanosine triphosphate (GTP).
• This leads to activation of c GMP-PDE.
• cGMP breaks, allowing ion channels to close.
• Influx of positive ions is prevented.
• Hyperpolarization of the cell occurs.
• Neurotransmitter release is stopped.
• Photoisomerization of rhodopsin results in light converting Rhodopsin (opsin + 11-cis retinal) into all-trans retinal and Metarodospin II
• Activation guanine nucleotide binding protein involves MII (Gt + GDP) converting to Gt+GTP (Activated protein)
• Increased activity of PDE 6 Enzyme (PDE *)
• Breakage of cGMP from CNG channel connection.
• This stop in flux of Na+ and Ca+2 (Hyperpolarization).
• Inhibition of neurotransmitter release occurs (Dis-inhibition of bipolar cells).
• Impulses propagate passively in the form of electrical signals.

Recovery Phase - Restoring Function in the Dark

• Molecules used to receive photon are regained.
• CNG channels in the outer segment are open because c GMP is bound to them.
• Positively charged ions enter the photoreceptor cells, bringing potential to -40 mV.
• The photoreceptor has a potential of -40mV, while in other nerve cells it is usually -70 mV
• Rhodopsin kinase and Arrestin binding
• Meta rhodopsin II converts to all-trans retinal and all trans retinol.
• 11- cis retinal in RPE binds opsin and 11-cis retinal, and rhodopsin is formed.

Summary - The Visual Cycle

• Photoisomerization of 11-cis retinal to metharohodopsin II in photoreceptors is the first step in the vision cycle.
• Continuous function of photoreceptors is require and this function requires that M II be converted back into 11-cis retinal via the visual cycle
• Newly generated 11-cis retinal is transported back into the sub-retinal space.
• This enables photoreceptors to regenerate photosensitive visual pigments.
• Dark adaptation time is how long it takes for photoreceptor cells to regain sensitivity after being bleached by a photon.
• Dark adaptation time varies between cones and rods.

Phototransduction and Disease: Clinical Correlation

• Retinal degeneration, night blindness, and color blindness can result from defects in photo transduction.