Retinal Physiology PDF

Retinal Physiology Karen Gil MD, MHSN RPE cells in Retinal Physiology 1. RPE is fundamental in the health of the neural retina and the choriocapillaris – Zonula occludens joining the RPE cell Are part of the blood retinal barrier – – efficient isolation of the inner retina from systemic influences a...

Retinal Physiology Karen Gil MD, MHSN RPE cells in Retinal Physiology 1. RPE is fundamental in the health of the neural retina and the choriocapillaris – Zonula occludens joining the RPE cell Are part of the blood retinal barrier – – efficient isolation of the inner retina from systemic influences at the choroidal side This is of importance for the immune privilege of the eye and for a highly selective transport between the blood and the subretinal space Electively control movement of nutrient and metabolite from the choriocapillaris into the retina and removal of waste products Diagram of the major functions of the RPE (Strauss, 2005) RPE Cells in Retinal Physiology Water is eliminated from subretinal space by active transport by the RPE Ion movement occurs by – – Na+/K+ ATPase pumps Na+/K+/Cl- and Na+/2HCO3contransporters – Cl-/2HCO3- exchangers – gated and ungated ion channels RPE Cells in Retinal Physiology A proton-lactate water contransporter moves a significant amount of lactate -product of anaerobic metabolismacross the RPE Water passage occurs through aquaporins and Cl- and K+ RPE Cells in Retinal Physiology Glucose transporters located in both apical and basal membrane maintains steady supply of glucose to the active photoreceptors Two families of glucose transporter have been identified: – the facilitated-diffusion glucose transporter family (GLUT family) Mainly use in the retina – Na+-dependent glucose transporter one (SGLT family) RPE Cells in Retinal Physiology 2. RPE cells phagocytose fragments from the continual shedding of the photoreceptor outer segment discs – Undigested material accumulates as lipofuscin – 2000 discs daily yellow-brown pigment granules composed of lipidcontaining residues of lysosomal digestion A2E substance identified in lipofuscin contributes to RPE cell death RPE Cells in Retinal Physiology 3. RPE metabolizes and stores vitamin A – Important for the biochemical process in the rod disc renewal system – Also called all-transretinol – Helps in night vision – Precursor of rhodopsin RPE Cells in Retinal Physiology 4. RPE Cells contribute to the formation of the IPM (interphotoreceptor matrix) between the RPE layer and the photoreceptors Levels of organization of hyaluronan (HA), which forms the basic IPM scaffold, drawn by David Schumick, Medical Illustrator at The Cleveland Clinic Foundation. Left panel shows the antiparallel alignment of linear HA molecules forming the basic matrix scaffold structure (adapted from Scott et alCenter panel depicts the continuous three-dimensional scaffold complex (not to scale) in the extracellular compartment adapted from electron microscope images of the IPM. Right panel depicts the interaction of the scaffold (not to scale) with HA-binding motifs on cells that border the IPM (CD44 on apical microvillae of Müller cell and RHAMM on apical RPE processes) and secreted molecules within the IPM (SPACR, SPACRCAN; Pigment Epithelium-Derived Factor [PEDF], and IRBP). RPE Cells in Retinal Physiology 5. RPE produces growth factors that drive certain cellular processes – Secrets vascular endothelial growth factor (VEGF) – which maintain choriocapillaris function Overproduction of VEGF results in neovascularization RPE secrets an antiangiogenic factor pigmented epithelial derived factor (PEDF) – Maintain balance contributing to healthy function RPE Cells in Retinal Physiology 6. Pigment granules within the RPE cells absorb light – reduce excess light scatter (as in a camera, improves the quality of the optical system of the eye by absorbing scattered light) * * * Relationship between the RPE and the photoreceptors is reciprocal When either layer dysfunctions the other is affected Retinal degenerative diseases and dystrophies often cause changes in the RPE that are clinically visible Diagram of the major functions of the RPE (Strauss, 2005) ROP- degeneration of Photoreceptors (early stage rods then cones) Scotopic Vision Dim light – rod predominate Rods are extremely sensitive in poorly light condition – scotopic vision Light-sensitive retina allows detection of an objet at low levels of illumination Ability to recognize fine details is poor Objects seen in shades of gray Photopic vision Cone activity dominate retina is responsive to a broader range of light wavelengths Bright illumination is necessary for sharp visual acuity and color discrimination Cones are designates depending on the wavelength that they absorb – Red 588 nm – Green 531 nm – Blue 420 nm Composition of Visual Pigments Cones and rods pigments have the same basic structure – Opsin membrane protein Forms a long helix that loops back and forth across the membrane bilayer seven times Determines the wavelength absorption – Chromophore – Molecule that absorbs the photon 11-cis retinal (derivate of vitamin A) Composition of Visual Pigments Cones – three photopigments (differ in aa composition) L-cone cells is red sensitive M-cone cells is green sensitive S-cone cells is blue sensitive – utilized bright light and color Rod – rhodopsin pigment – Located in the disc membrane – dim vision Structural rhodopsin Formation of Visual Pigments Damage of the photoreceptors in the outer segment due to light absorption induce a constant replacement Rod outer segments are shed in the morning and are removed by RPE phagocytosis Cone outer segments are shed and renewed in the evening Vitamin A – – Oxidized in the RPE to give retinal access to the RPE via diffusion through the large fenestrations in the choriocapillaris Physiology of the Neural Retina Retinal Synapses Information transmission between retinal neurons occurs by ion channel activity at – Gap junctions Electrical synapse that allow current pass directly between cells Rapid rate of signal transmission No chemical mediator necessary Found between – photoreceptor and photoreceptor – Photoreceptors and horizontal cell – Horizontal cells and horizontal cells – Bipolar axon and amacrine cells – Neurotransmitter release in chemical synapses Transmitter binds to specific sites on the postsynaptic membrane – Eliciting in an excitatory or inhibitory change in that neuron Retinal Neurotransmitters and Neuromodulators Neurotransmitters – Glutamate Excitatory neurotransmitter Release by all photoreceptors and bipolar cells and most ganglion cells – GABA and Glycine Inhibitory neurotransmitters Released by most amacrine cells and horizontal cells Neuromodulators – Chemicals that alter neuron transmission Dopamine – ex. Change the conductance of gap junctions between horizontal cells Phototransduction Phototransduction – – Process by which a photon of light is changed to an electrical signal In the photoreceptors The absorption of light (outer segment) by rhodopsin triggers phototransduction and start the process of vision A series of biochemical changes follow and cell hyperpolarizes – Start an electrical current flow through the retina – Signal passes to bipolar horizontal cells – Some organization and processing occurs – Then the signal is transferred to amacrine and ganglion cells – Ganglion cell is activated and its axon carries the message to the brain Phototransduction Photoreceptor maintain a slight negative electrical charge of about -40 mV in the dark Caused by active transport of cation such as (from within to outside the cell) – Sodium – Calcium Cyclic GMP-gated channels in the outer segment membrane are responsible for the light-induced changes in the electrical activity of photoreceptors. (Neuroscience, Purves et al., 2001) Phototransduction Na/K ATPase pump on the inner segment plasma membrane uses ATP to pump Na+ out of the inner segment while moving K+ inside Na+ then re-enters the photoreceptor via Na channels in the outer segment Dark current – the flow of sodium and other cations into and out the cell (while in the dark) Phototransduction Dark – Photoreceptors are depolarized and constantly release glutamate to bipolar cells – cGMP keeps Na channels open to allow depolarization Dissociation of rhodopsin (by light stimulation) triggers the activation of the G protein transducin – Leading to a cascade that leads to a decrease in the concentration of cGMP cGMP is converts into 5’GMP (no cyclic) Protein responsible for keeping Na+ channels open in the outer segment Details of phototransduction in rod photoreceptors. (A) The molecular structure of rhodopsin, the pigment in rods. Rhodopsin is a G-protein coupled receptor consisting of opsin (a seven transmembrane domain protein) and 11-cis-retinal (a covalently bound chromophore). (B) The second messenger cascade of phototransduction. 1. Light stimulation of rhodopsin in the receptor disks leads to the activation of a G-protein (transducin). 2. The GTP-bound alpha subunit of transducin activates a phosphodiesterase (PDE). 3. The activated phosphodiesterase hydrolyzes cGMP into GMP, reducing its concentration in the outer segment and leading to the closure of sodium channels in the outer segment membrane. (Neuroscience, Purves et al., 2001) Cyclic guanosine monophosphate (cGMP) guanosine triphosphate (GTP) Phototransduction Closure of sodium channels – Key event during phototransduction – Prevent Na+ from entering and the negative charge increases to about -75mV Causes hyperpolarization of the cell membrane Triggering a decrease in glutamate release to bipolar cells Once the level of cGMP is restored – Ion channels open – once again the cell becomes depolarized and release glutamate Phototransduction https://www.youtube.com/watch?v=JIPE3in2EcQ Phototransduction In a photoreceptor (rod) light absorption causes the transformation of 11-cis retinal to all-trans retinal which is moved into the cytoplasm form the disc lumen And is reduced to all trans retinol transported to the RPE by an specific carrier proteins within interphotoreceptor matrix (IPM) RPE contains enzymes that convert all-trans retinol to 11-cis retinol Finally oxidize back to 11-cis retinal and transporting (IPM) into to the photoreceptors for incorporation into disc outer segments ABCR: ATP binding Cassette transporter atRDH: all-trans retinol deshydrogenase IRBP: Intersticial retinal binding protein CRBP: Cellular retinol binding protein CRALBP: Cellular retinaldehyde binding protein LRAT: Lecithin retinol acethyltransferase RPE65: retinal pigment epithelium specific protein 65 11cRDH: 11 cis retinol dehydrogenase Information Processing Once the photoreceptor is activated and the message begins its circuit through the retina Organization and processing will take place Control and relay of photoreceptor message Retinal neurons are designed as ON and OFF cells by the light condition when the cell is depolarized – Depolarized with light OFF – OFF cell – Depolarized with light ON – ON cell https://www.youtube.com/watch?v=L8kC3zU1pqM Receptive Fields Area in the visual field or the area of the retina that with stimulation elicits a response in a retinal neuron Arranged in – Center-surround pattern When light activates cells in the center of the field When light falls on the surround an antagonist response occurs (inhibiting the response) – Circular receptive field ON-center/OFF surround OFF-center/ON surround When light falls on the annular region the message from the center is inhibited Retinal Cells Bipolar Cells Have center-surround receptive fields (spatial antagonism) Cone cells can hyperpolarize or depolarize bipolar cells Cone bipolar cells come in two types depending in the response – ON-center hyperpolarizing – ON-center depolarizing Both respond with graded potentials Retinal Cells Ganglion Cell on off Hyperpolarization of photorecpetors Glutamate inhibition Hyperpolarization of bipolar cells Depolarization of bipolar cells Inhibit glutamate release Post inhibitory nerve impulse rebound Decrease firing rate https://www.youtube.com/watch?v=AuLR0kzfwBU Estimulation of Glutamate release Nerve impulse Increase firing rate Retinal Cells Horizontal Cells Receive input from a large number of photoreceptors Contribute to spatial summation They also have graded potentials and hyperpolarized in response to light Horizontal cells impact the surround responses of bipolar cells by either providing inhibitory feedback to photoreceptor cells – which then impact the bipolar – or by directly synapsing with the bipolar cell (feed-forward synapse) Lateral inhibition Summary diagram of the synaptic organization of the primate retina. R, rod; C, cone, FMB, flat midget bipolar cell; IMB, invaginating midget bipolar cell; H, horizontal cell; IDB, invaginating diffuse bipolar cell; RB, rod bipolar cell; I, interplexiform cell; A, amacrine cell; G, ganglion cell; MG, midget ganglion c Retinal Cells Amacrine Cell Receptive field have center-surround organization and respond with action potentials AII amacrine cells – are the most common – Play a huge role in the rod pathway, relaying signals from rod bipolar cells to center-hyperpolarizing (OFF center) rod ganglion cell dendrites Retinal Cells Ganglion Cell Have center-surround receptive fields of two types – ON-center/OFF-surround – OFF-center/ON-surround They respond with action potentials Midget ganglion cells are small cells whose single dendrite (in fovea)synapses with one midget bipolar cell – allowing visual information to be carried from a single cone Receptive Fields Light and Dark Adaptation Visual system highly specialized for and analysis of patterns of light Dark adaptation can take 30 min for the retina to adapt fully going from bright sunlight to complete dark Light adaptation takes 5 to 10 min from complete dark to bright light Adaptation is regulated by Na which influence the concentration of cGMP (controls gated ion channels in the photoreceptor membrane) Circadian Rhythm Wake/sleep or rhythm 24 hours period Directed by pineal melatonin secretion influenced by the hypothalamus (suprachiasmatic nucleus) 3000 ganglion cells that contain melanopsin across the retina, their axons project to the surpachiasmatic nucleus helping synchronizing the rhythm Retinal Metabolism Retinal tissue requires energy Primary source – glucose metabolism Glucose moves out from the blood into retinal tissue via facilitated diffusion Glucose transporters are located on apical and basal membranes of RPE and endothelium of capillaries Retina can switch form glycolisis to oxidative metabolism High rate of anaerobic glycolisis normally Oxygen utilization in photoreceptors is 3-4 times higher than other CNS neurons, so blood flow in choriocapillaris is higher