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Retinal anatomy and physiology 2016

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TABLE I. FACTS AND FIGURES CONCERNING THE HUMAN RETINA Modified from the Table on the Webvision website ( http://webvision.med.utah.edu ) http://webvision.med.utah.edu/book/part-xiii-facts-and-figures-concerning-the-human-retina/ 1. Size of the human retina: 32 mm from ora to ora along the horizontal meridian. Area is 1094 mm2. Average eye is 22 mm from anterior to posterior poles, 72% of the inside of the globe is retina 2. Size of optic nerve head or disc: 1.85 x 1.75 mm 3. Degrees and distance in micrometers: One degree of visual angle is equal to 288 µm on the retina without correction for shrinkage. One mm on the retina =~3.5 degrees of visual angle. 4. Foveal position: 11.8 deg or 3.4 mm temporal to the optic disk edge 5. Cross diameter of the macula: 3 mm of intense pigmentation, surrounded by 1 mm wide zone of less pigmentation 6. Cross diameter of the central fovea from foveal rim to foveal rim: 1.2-1.5 mm 7. Cross diameter of central rod free area: 400-750 µm (~250 µm2) 8. Vertical thickness of the fovea from ILM to ELM: Foveal pit - 150 µm, and foveal rim - 400 µm 9. Central region of fovea where there are no cone pedicles: 200 – 300 µm 10. Length of foveal cone axons (Henle fibres): 150-300 µm 11.Age when fovea is fully developed: Not before 4 years of age 12. Highest density of cones at center of the fovea (50 x 50 µm): 96,900-281,000/mm2 - mean 161,900/mm2 13. Total number of cones in fovea: ~200,000 (17,500 cones/deg2). Rod free area is ~ 1 deg2

14. Total number of cones in the retina: 6,400,000 (~5% of the photoreceptors 15. Total number of rods in the retina: 110,000,000 to 125,000,000 16. Rod distribution: Peak density 18o (5mm) from the center of the fovea forms a ring with 160,000 rods/mm2. Average density is 80-100,000 rods/mm2. Central 200-375 µm (radius) is rod free 17. Number of axons in the optic nerve: 1,200,000 18. Number of cones/retinal pigment epithelial cell (RPE): 30 cones/RPE cell in fovea 19. Number of rods/retinal epithelial cell (RPE): At peak (4-5 mm from foveal center) -28 rods/RPE cell. In periphery - 22 rods/RPE cell 20. Photoreceptor protein found in retinal ganglion cells that project to the pretectum and the supra chiasmatic nucleus: Melanopsin

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OPTO 5335: Ocular anatomy and physiology Dr. Frishman (2112; lfrishman@uh.edu) RETINA I. MORPHOLOGY AND FUNCTION OF RETINAL ELEMENTS; SPECIALIZED CIRCUITRY I. Functions of the retina A. Transduction of light energy to neural signals B. Adaptation to variations in retinal illumination to provide high contrast sensitivity over a very large range (>10 Log units of mean illumination)

C. Regional specializations that affect acuity and sensitivity- e.g. fovea, rod/cone distribution, coverage factors

D. Specialized circuitry for specific visual information and capabilities: responding to light

increments (ON pathways) or decrements (OFF pathways), seeing under light- or dark- adapted conditions, color vision, spatial and temporal resolution

II. Organization of the retina (vertical-Fig. 1) Neural Retina A. Three cell body layers of the neural retina 1. Outer nuclear layer (ONL), which contains cell bodies of photoreceptors 2. Inner nuclear layer (INL), which contains cell bodies, of horizontal cells, bipolar cells, amacrine cells, and interplexiform cells, as well as retinal glial cells (Müller cells) 3. Retinal ganglion cell layer (GCL), which contains retinal ganglion cells whose axons form the optic nerve, and displaced amacrine cells (formerly thought to be axonless, i.e. amacrine) B. Two plexiform layers where cells make synaptic contacts 1. Outer plexiform layer (OPL) where photoreceptors contact horizontal and bipolar cells 2. Inner plexiform layer (IPL) where bipolar cells contact ganglion cells, amacrine cells, and interplexiform cells C. Nerve fiber layer (NFL) or optic fiber layer (OFL) that is composed of ganglion cell axons D. Inner limiting "membrane" - Inner, or proximal, processes of Müller cells covered by basal lamina (basement membrane) E. Outer limiting "membrane" - Outer or distal processes of the Müller cells joined to adjacent photoreceptors by means of belt desmosomes RPE F. Retinal pigment epithelium (RPE) - outermost retina, monolayer of epithelial cells G. Verhoeff's "membrane - tight junctions and adjacent belt desmosomes between RPE cells H. Bruch's "membrane" - three layers between RPE and the choriocapillaris (1) basement membrane of RPE, (2) collagen layer, (3) elastic layer Choroid I. Choriocapillaris - vascular bed of the choroid J. Stroma of Choroid K. Suprachoroidea

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Fig. 1. Vertical organization of the retina: Layers from outer to inner retina

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III. Lateral organization of the retina (Fig. 2) A. Macular region: foveola, fovea, parafovea, perifovea (Fig. 2A) 1. Perifovea - radius of about 9 deg. reaches to outer edge of perifovea (ganglion cells reduced to a single layer) 2. Macula lutea - ill-defined yellow region (Xanthophyll) inside macula 3. Parafovea - radius of 4.2 deg. has highest density of nerve cells in retina 4. Fovea - radius of 2.5 deg., thin retina (Fig. 2C) 5. Fovea slope or clivis 6. Foveola (foveal pit) - radius about 0.6 deg, only photoreceptors, and Müller cells. avascular region - rod free, blue (short wavelength) cone free. (Conversion factor: 288 - 300 µM = approx. 1 deg. on the retina) B. Optic nerve head - about 17 deg. from center of foveola C. Peripheral retina, mainly rods, convergence of many photoreceptors on postreceptoral circuits D. Ora Serrata - 21 mm from center of optic disk E. Rod and cone distribution (Fig. 2B) F. Retinal thickness – thickest at the top of the foveal slopes Fig. 2A – lateral organization of the macula

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Figure 2B – rod and cone density along the horizontal meridian Fig. 2C – human fovea

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IV. Embryology (Fig. 3A and 3B)

A. The retina, like the CNS, derives from the neural tube and in that sense it is anatomically part of the brain. 1. The neural tube evaginates at gestational wk. 3 to form two optic vesicles. 2. Optic vesicles invaginate at wk. 4 to form optic cups with two walls (neuroectodermal layers). a. The inner wall of optic cup is the "neural" or "sensory" retina, initially one cell thick. It becomes multilayered with neural retinal cells in its central extent (pars neutralis), and peripherally, the inner layer of the ciliary body (pars ciliaris) and the iris, (pars iridica). b. The outer wall of optic cup is the RPE, and peripherally, the outer pars ciliaris and iridica c. The ventricle between inner and outer walls collapses

d. Neural retina is attached to RPE only at optic nerve head, and ora serrata e. Order of cell genesis: Ganglion cells, horizontal cells, cones, amacrine cells, Müller

cells, bipolar cells, rods f. The final arrangement of neural retinal cells and their processes is completed during the 6th month of gestation g. Central retina continues to develop after birth (at least 4 years; FIG. 3B), and RPE cells increase in density for 2 years

Fig. 3A – Development of the eye.

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Fig. 3B Pre- and postnatal development of foveal cones

V. Cells types and synaptic organization of the neural retina in humans and macaque monkeys

A. Photoreceptors - rods and cones: Outer segment (OS), inner segment (IS), cell body (in ONL), special endings in OPL (Rods - spherules; cones - pedicles); neurotransmitter is glutamate (Fig. 4) Fig. 4 Photoreceptors – scanning EM of outer segments. These cells hyperpolarize in response to light onset.

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Fig. 4a Photoreceptors –scanning EM of outer segments Photoreceptor structure Rod and cone terminals Cone pedicle Rod Spherule

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B. Neurons in the inner nuclear layer (outer to inner = distal to proximal) 1. Horizontal cells - types HI, HII, HIII: neurotransmitter may be GABA (Fig. 5), but these

cells also communicate with photoreceptors via other means. H-cells hyperpolarize in response to light onset.

2. Bipolar cells - 1 type of rod bipolar cell, and at least 6 types of cone bipolar cells: midget invaginating, midget flat; diffuse invaginating, diffuse flat; blue cone bipolar; bistratified; neurotransmitter is glutamate (Fig. 6). Fig. 5 Horizontal cells: H1 type contacts cones with cell body and rods with axon terminal

Fig. 6 Bipolar cells

Parallel pathways are set up at the first synapse in the retina between photoreceptors and bipolar cells in the OPL.

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Organizing principles: there are parallel pathways through the retina for handling different types of visual information: rod vs cone signals, ON vs OFF signals, midget vs diffuse (large vs small areas of visual field “seen” by one neuron). These pathways are set up by various types of bipolar cells. “ON” cells depolarize in response to light onset, “OFF” cells hyperpolarize in response to light onset.

ON

On cone bipolar cell

Rod bipolar cell

OFF coneBC

ON vs OFF pathways through the retina: OPL and IPL: 1. ON bipolar cells form invaginating synapses in photoreceptor terminals in the OPL; bipolar cell axons terminate in the B lamina of the IPL, where they contact ON ganglion cells and amacrine cells 2. OFF bipolar cells form flat synapses (Basal junctions) in photoreceptors terminals in the OPL(Cone only); bipolar cell axons terminate in the A lamina of the IPL where they contact OFF ganglion cells and amacrine cells.

Classical figure – first characterization of First report of On and Off Pathways, Science 1978

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3. Amacrine cells: more than 24 varieties including AII (A2) which is obligatory in the rod bipolar to ganglion cell circuit; many neurotransmitters are used by amacrine cells, including glycine (AII and others), GABA (A17 and others), dopamine (A18), acetylcholine (starburst) (Fig. 7). On or off, or On-Off. Some cells produce action potentials. 4. Interplexiform cells: feeds back input from IPL, to OPL; neurotransmitter in humans is dopamine. Fig. 7 – Amacrine cells C. Retinal ganglion cells (Fig. 8) 1. 70% Midget (small cell bodies, small dendritic field) a. 95% of all ganglion cells in central retina, 45% in peripheral retina (Fig. 8) Signals from L- and M- cones (and rods in extrafoveal cells) 2. 10% Parasol (large cell bodies, large dendritic field) Signals from L- amd M- cones (and rods in extrafoveal cells)

3. 2-8% small bistratified (medium sized cell body, dendritic field size similar to that of parasol cells) Signals from S-cones (center) and L & M cones (surround)

3. 10% other (including “giant” ganglion cells) and cells that have melanopsin (light receptors) that contribute to pupil control and circadian rhythms. Retinal ganglion cells produce action potentials, either at light onset (“On”cells) or at light offset (Off cells). Fig. 8. Human retinal ganglion cells

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D. Glial cells in the retina:

1. Müller cells - retinal glia whose end processes from inner and outer limiting membrane 2. Astroglia - associated with retinal ganglion cell fibers 3. Micro glia - roving phagocytes 4. Retinal pigment epithelial ( RPE ) cells act like glia as well as being epithelial cells

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E. Synaptic organization of the OPL: EM analysis (ribbon synapses, basal junctions, gap junctions, triads) (Fig. 9A shows a triad)

1. Ribbon synapses: Ribbon is electron dense material - a conduit for synaptic vesicles to the release site (photoreceptors to bipolar and horizontal cells). On bipolar cells are invaginating.

2. Basal junctions: photoreceptor to flat (Off) cone bipolar cells in mammals - no vesicles apparent.

3. Gap junctions -electrical synapses, flow of ions and small molecules - coupling of cells can spread signals from one cell into another, an increase receptive field size

F. Synaptic organization of the IPL: EM analysis (Ribbon synapses and conventional synapses, dyads) (Fig. 9B shows a dyad)

1. Ribbon synapses: as in C1, but no invaginating synapses: a conduit for release of neurotransmitter from bipolar cells to amacrine and ganglion cells.

2. Conventional synapses - 20 nm gap, in retina mostly inhibitory (found only from amacrine cell contacts)

3. Gap junctions

VI. Electrical potentials that occur in retinal neurons

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A. Graded potentials (receptor potentials in photoreceptors, only graded potentials occur in bipolar cells, horizontal cells, some amacrine cells B. Action potentials (ganglion cells; some amacrine cells) The receptive field of a visual neuron (from Oyster, The Human Eye) Fig. 10

VII. Physiology of the cell types in the retina (Fig. 10 - intracellular recordings) - response type (on

or off, sustained or transient), and receptive field characteristics A. Photoreceptor: receptor potential - sustained hyperpolarization in response to light; small receptive fields B. Horizontal cells: sustained hyperpolarization in response to light; large receptive fields - spatial convergence of photoreceptor input. These cells provide surrounds for bipolar cells. C. Bipolar cells: rod - depolarization (On), and cone - depolarization (On response) or hyperpolarization (Off response) to light. Receptive fields may be concentrically organized with spatially antagonistic centers and surrounds. D. Amacrine cells: sustained or transient hyperpolarization or depolarization in response to light, and some cells produce action potentials. Receptive fields may be small or large, and often concentrically organized with centers and surrounds.

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E. Retinal ganglion cells (Fig. 11, 12 and Table 1): Produce action potentials (spikes) On center, Off surround Off center, On surroung

VIII. Neurotransmitters present in the retina A. Excitatory neurotransmitter- the major retinal transmitter is GLUTAMATE (an excitatory amino acid released by photoreceptors, bipolar cells, and ganglion cells). Specificity is in the receptors Metabotropic glutamate receptors 1. mGlur6 (APB or AP4) receptor - found on ON bipolar cells Ionotropic glutamate receptors (named for their exogenous agonists) 1. Kainate (KA) receptor - found on OFF bipolar cells, horizontal, some amacrine and ganglion cells 2. AMPA receptor - found on, horizontal, OFF bipolar, amacrine and ganglion cells

3. n-methy-D-aspartate (NMDA) receptor - found on amacrine and ganglion cells B. Inhibitory neurotransmitter - GABA and glycine (released by amacrine cells; and horizontal cells). Receptors: GABA (A, B, C) receptors found on bipolar, amacrine and ganglion cells, glycine receptors on bipolar, amacrine and ganglion cells. C. Other neurotransmitters and neuromodulators found in amacrine cells; acetylcholine, dopamine; indoleamines, neuropeptides, NO

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AII amacrine cell releases Glycine A17 Amacrine cell releases GABA IX. Specialized parallel circuitry in the retina (rod vs cone circuits, ON vs OFF circuits, magno-projecting vs parvo-projecting, and blue cone circuit) A. Rod vs cone pathways through the retina (Fig. 13) 1. Rod: There is only one type of rod bipolar cell in the rod pathway. A special amacrine cell (AII) relays the rod signal to ganglion cells via the ON cone bipolar cells (Gap junctions), and the OFF cone bipolar cells (chemical synapse).

2. Cone: Cones contact ON and OFF cone bipolar cells, and these bipolar cells contact ON and OFF retinal ganglion cells.

a. midget pathways: Individual L (red) and M (green ) cones in fovea** and parafovea contact individual midget bipolar cells which pass the signal to midget ganglion cells (also called “P=Parvo-projecting cells.” cells have center-surround receptive fields that show center-surround spectral opponency.

** private lines through the retina for foveal cones via the midget system - Setting the limits of spatial resolution via the foveal cone mosaic in central

retina, and the midget ganglion cells in peripheral retina b. S (blue) cones contact blue cone bipolar cells (ON) which pass the signal to small

bistratified (SBS) ganglion cells. The OFF surround is conveyed to the SBS ganglion cell via a OFF diffuse bipolar cells.

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c. Diffuse pathways: Several L and M cones contact ON and OFF diffuse bipolar cells which pass the signal to parasol ganglion cells (also called “M=Magno-projecting cells). These ganglion cells have center-surround receptive fields, but not color opponency. Thus they code luminance, not color.

B. Parallel projections of different ganglion cell types in monkeys and humans

1. Midget cells project to the parvocellular layers of LGN (P=small cells), 2. Parasol cells project to the magnocellular (M=large cells) 3. Small bistratified cells project to the interlaminar koniocellular (K) layers of the dorsal lateral geniculate nucleus [LGN] (Fig. 12). 4. Intrinsically photosensitive melanopsin containing cells project to the suprachiasmatic nucleus (SCN - circadian rhythms), intrageniculate leflet (IGL), and the olivary pretectum nucleus (OPN, pupil))

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RETINA II. PHOTORECEPTOR FUNCTION, RPE, OPTIC NERVE HEAD, BLOOD SUPPLY, ERG Photoreceptors I. Visual pigments (rod: rhodopsin and three cone opsins, and in the membranes of intrinsically photosensitive retinal ganglion cells, another visual pigment: melanopsin) A. Structure 1. Retinal: the chromophore 2. Opsin: the transmembrane protein that tunes the chromophore wavelength

sensitivity 4. Retinal-opsin bond

Fig. 1 B. Absorption spectra: [rod rhodopsin- 496 (~500) nm, (LW) red- 558 nm, (MW) green- 531 nm,

(SW) blue- (419) 420 nm, Melanopsin, 484 nm

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Fig. 2

C. Genetics: 1. Rod and SW opsins are encoded autosomally; rod on chromosome 3, SW on 7.

Rod and SW cone opsin show 40% homology. 2. MW and LW opsins are encoded by genes on Q-arm of X chromosome (97%

identical in amino acid sequence; only 40% identical to rod or SW cone). 3. There are >90 known genetic defects in rhodopsin, (many causing retinitis

pigmentosa). Fig. 3. Comparisons of the human visual pigment amino acid sequences. Colored dots are different amino acids

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II. The visual cycle: rhodopsin A. The bleaching and regeneration of visual pigment as first defined by spectroscopy. The

visual cycle includes all of the metabolic changes of vitamin A except the formation of retinoic acid which is not relevant to vision. (IRBP – interstitial retinoid binding protein shuttles retinol back and forth. (Fig. 4 below)

Fig. 5 More details of the visual cycle Fig. 5. More details of the visual cycle

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III. The phototransduction cascade (Fig. 6A and B below) A. When a photon is absorbed rhodopsin is isomerized, one transitional intermediate,

Metarhodopsin II, is = Rh* (activated, isomerized rhodopsin) B. Rh* catalyzes activation of the G-protein, transducin (exchange bound GDP for

GTP). This disinhibits the effector, cGMP-PDE. Activation of PDE causes C. C. Hydrolysis of cGMP - closure of cGMP activated cation channel. D. Termination of the response 1. Phosphorylation of Rh* and binding of arrestin 2. Hydrolyze GTP to GDP on transducin 3. Bind inhibitory subunit to PDE 4. Resynthesis of cGMP by guanylate cyclase (GC)

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Phototransduction summary

Phototransduction is the process by which a photon of light captured by a visual pigment molecule is transformed into an electrical response in the photoreceptor. The electrical response that occurs in vertebrate photoreceptors always is a hyperpolarization in response to photon absorption. Photoreceptors are depolarized in the dark, due to cation channels in the outer segment membrane being open, as shown in Fig. nn. In that depolarized state, they release the neurotransmitter, glutamate from their axon terminals (Yau, 1994). Light stimulation leads to interruption of the “dark current”, and the resulting hyperpolarization reduces neurotransmitter release from the photoreceptor cell terminal, causing transmission of visual signals from receptors to subsequent retinal neurons. Phototransduction in rods, in which the visual pigment is rhodopsin, is best studied, but the process is known to be very similar in cone photoreceptors. The peak absorption of rhodopsin is at 500nm, while the cones have one of three different visual pigments, long (red), medium (green) or short (blue) wavelength, with peak absorption at 564, 533, and 437nm respectively. The visual pigments are located in the membranes of the discs of the outer segment. They are composed of an opsin which is a transmembrane protein that is reversibly covalently bound to a chromophore, 11-cis-retinaldehyde. Opsins constitute most of the protein of the outer segment discs, and they determine the spectral sensitivity of the photoreceptor. The chromophore is derived from vitamin A that comes via the blood circulation from the liver, and reaches the eye via the choriocapillaris and then crosses the pigment epithelium to reach the photoreceptors (Lamb and Pugh, 2004). Absorption of a photon by a rod or cone pigment in the outer segment discs leads to isomerization of the pigment. In the case of the rods, when rhodopsin is isomerized, one critical transitional intermediate, Metarhodopsin II, is the activated rhodopsin (Rh*) that triggers an amplifying biochemical cascade of events that leads to closure of cation channels in the outer segment membrane, thereby hyperpolarizing the cell. (Lamb and Pugh, 2004). As shown in the figure below, in the first step of the cascade, Rh* catalyzes activation of the GTP-binding protein (G-protein), transducin, by exchanging bound GDP for GTP). Transducin then activates another protein, cGMP-PDE (cyclic GMP –phosphodiesterase), which hydrolyzes cGMP to 5’-GMP. cGMP is required to hold cation channels in the outer segment membrane open, so destruction of cGMP causes the channels to close, which hyperpolarizes the cell (Yau, 1994: Lamb and Pugh, 1992; Pugh and Lamb, 1993, Arshavsky et al., 2002; Lamb and Pugh, 2004). As important as the activation of the visual pigment is the termination of its catalytic activity so the photoreceptors will not continuously signal. (Rh*) is inactivated by a process that has two steps. First, activity of Rh* is lowered due to phosphorylation by rhodopsin kinase (GRK1). Second, to cap the residual activity, the protein arrestin (Arr1) binds to phosphorylated Rh*. It also is necessary to restore the transduction cascade by hydrolyzing GTP to GDP on transducin, to inactivate cGMP-PDE, and resynthesizing cGMP, a process for which guanylate cyclase is the catalyst (Lamb and Pugh, 2004; Burns et al., 2006) Following isomerization 11-cis retinal is converted to all-trans retinal, that is no longer bound to the opsin. The visual (or vitamin A) cycle restores functionality of the visual pigment through a series of steps. All-trans retinal is reduced to all-trans retinol and it travels back to the retinal pigment epithelium to be restored to the 11-cis configuration. It is esterified by lecithin-retinol acyltransferase (LRAT) and then converted to 11-cis retinol by the isomerohydrolase, RPE65. Mutations in RPE65 cause a blinding disease in infancy called Leber’s amaurosis (Redmond et al, 1998; Lamb and Pugh, 2004). The final oxidation to 11-cis retinal is catalyzed by 11-cis retinol dehydrogenase (RDH) (Lamb and Pugh, 2004) . Mutations in the active enzyme in humans, RDH5, leads to a disorder called fundus albipunctatus (Yammato et al., 1999). This is a form of congenital night blindness in which regeneration of rod and cone photopgiments is greatly delayed. In normal retinas, after oxidation to 11-cis retinal, the chromophore travels back to the rod outer segment where it can again be covalently bound to the opsin to form a functional visual pigment (rhodopsin) ready to absorb a photon.

References Arshavsky VY, Lamb TD, Pugh EN Jr G proteins and phototransduction. Annu Rev Physiol 64:153–187, 2002. Burns ME, Mendez A, Chen CK, et al. Deactivation of phosphorylated and nonphosphorylated rhodopsin by arrestin splice variants. J

Neurosci 26:1036–1044, 2006. Lamb TD, Pugh EN Jr A quantitative account of the activation steps involved in phototransduction in amphibian photoreceptors. J

Physiol 449:719–758, 1992. Lamb TD, Pugh EN Jr Dark adaptation and the retinoid cycle of vision, Progress in Retinal and Eye Research 23 307–380, 2004. Pugh EN, Lamb TD Amplification and kinetics of the activation steps in phototransduction. Biochim Biophys Acta 1141:111–49, 1993. Redmond, T.M., Yu, S., Lee, E., Bok, D., Hamasaki, D., Chen, N., Goletz, P., Ma, J.-M., Crouch, R.K. and Pfeifer, K., RPE65 is

necessary for production of 11-cis-vitamin A in the retinal visual cycle. Nat. Genet. 20, pp. 344–351, 1998. Yamamoto H, Simon A, Eriksson U, Eddie Harris E, Berson EL & Dryja TP, Mutations in the gene encoding 11-cis retinol

dehydrogenase cause delayed dark adaptation and fundus albipunctatus, Nature Genetics 22, 188 – 191, 1999. Yau KW (1994) Phototransduction mechanism in retinal rods and cones. The Friedenwald lecture. Invest Ophthalmol Vis Sci 35:9–32,

1994.

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IV. Photoreceptor membrane potentials (Fig. 7 below) A. Events that occur in the dark

1. The dark-current is flowing – it is mainly Na+ inward current into the outer segment (OS) via cGMP gated channels, completed by a K+ outward current from the inner segment (IS)

2. Resting membrane potential is relatively depolarized (approx. -40 mV) 3. Ionic currents and conductances: the cGMP-gated cation channels are open, and the Na+ -Ca2+ exchange is operating

4. Na+ -K+ -ATPase (pump) on the IS is working hard 5. Neurotransmitter (Glutamate) release - is continuous in the dark B. Events occurring in the light 1. Light-response: quanta are absorbed (principle of univariance - all quanta have the same effect, once absorbed), time-course is faster for cones than for rods. Spectral sensitivity affects possibility of absorption. The dark-current is interrupted due to closure of the cGMP gated channels 2. Membrane potential –hyperpolarized due to closure of the cGMP gated channels 3. Ionic currents and conductances - cGMP-gated channel closed, low [Na+]i, low

Ca2+/Na+ exchange 4. Na+/K+ pump - working 5. Neurotransmitter (glutamate) release - slowed or stopped Fig. 7A – Current flow around the photoreceptor OS Fig. 7B Rod and Cone photocurrents

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V. Retinal metabolism – the retina has the highest metabolic rate of any tissue in the body A. Mainly in photoreceptor inner segment - oxygen is delivered by “private” choriodal blood supply 1. Mitochondria in enlarged packs called ellipsoids

Functions: phosphorylation of opsins after bleaching, cGMP synthesis , Na+/K+ ATPase pump and synaptic release. Also transport and synthesis of glutamate B. Inner retina (all neurons proximal to photoreceptors) receives glucose and O2 via the inner retinal circulation. C. Nutrition 1. Continuous supply of Vit. A (retinol) required 2. Antioxidants (Vit. E) VI. Photoreceptor-retinal pigment epithelium interactions (Fig. 8) A. Roles of RPE (retinal pigment epithelium)

1. Epithelial layer with tight junctions to form selectively permeable outer blood retinal barrier that passes O2 and nutrients from the choriocapillaris; controls hydration by transport of fluids, promotes retinal adhesion

2. Glial cells that regulate ionic environment of extracellular space around photoreceptor OS

3. Phagocyte that removes shed photoreceptor disc outer segments Entire outer segment of human photoreceptor turns over every 9 to 13 days.

Rods shed at dawn light onset, cones at dusk light offset 4. Visual cycle - essential for pigment regeneration (transfer of retinal by IRBP) Fig. 8

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Optic nerve head – and blood supply, sections VII- XI will not be discussed in class, and will not be on the first midterm. There will be a lab on this material, and knowledge of the material is required for the lecture final. VII. Retinal nerve fiber layer (fiber bundles surrounded by Müller cells) (Fig. 9 below) A. Papillomacular bundle from macula and nasal fibers take straight course to edge of the optic nerve head

B. Arcuate bundles from the rest of the retina arch around fovea enter optic nerve at superior and inferior poles (most susceptible to glaucomatous damage).

C. Optic nerve head (center) is approx. 16 deg from the center of the foveola, and it is 5 deg (1.5 mm) in diameter.

VIII. Layers of the optic nerve head from anterior (surface of the retina) to posterior (where myelination begins behind the sclera) (Fig. 10 above) A. Surface nerve fiber layer - 90-95% axons, 5% astrocytes (optic disc or papilla)

B. Prelaminar region (also called anterior level of the lamina cribosa)- Nerve fibers are separated into approx. 1000 fascicle's or bundles by glial septa in which there is a capillary network - 23% astrocytes.

C. Lamina cribosa region - The nerve fascicle's pass through fenestrated sheets of scleral connective tissue (collagen) with occasional elastic fibers. The apertures (fenestrations) are lined by glial membrane.

D. Retrolaminar region - Characterized by a decrease in astroycytes (11% and acquisition of myelin, supplied by oligodendrocytes.

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IX. Supportive structure (Fig. 11) of the optic nerve head 1. Surface nerve layer (astroglial) a. ILM of Elschnig b. ILM of retina c. Central meniscus of Kuhnt 2. Prelaminar (astroglial) d. Intermediary of Kuhnt e. Border tissue of Jacoby f. Border tissue of Elschnig

(connective tissue) 3. g. Lamina cribosa (collagen) 4. Retrolaminar h. Meningeal sheaths X. Blood supply to the retina and optic nerve (Fig. 12 below) A. Ocular vessels are derived from the ophthalmic artery (OA), a branch of the internal carotid. 1. Central retinal artery (CRA)- 1st branch of OA, supplies inner 2/3 of neural retina. CRA enters the optic nerve 10 mm behind the eyeball and branches at disc to 4 quadrants of the retina. The inner retinal supply shows autoregulation 2. Posterior ciliary arteries (2 or 3) (choroid and optic nerve head), & several anterior ciliary arteries.

B. Arterial supply for the 4 layers of the optic nerve head

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1. Surface nerve fiber layer - arteriole branches of the CRA. These anastomose with vessels of the prelaminar region, and are continuous with peripapillary retinal and long radial peripapillary vessels. Occasionally ciliary-derived vessels from prelaminar region may enlarge to form cilioretinal arteries. 2. Prelaminar - precapillaries and capillaries of short posterior ciliary arteries from direct branches arising from arterioles surround the O.T. or indirectly from the peripapillary choroid. 3. Lamina cribosa region - vessels from posterior ciliary arteries form a dense plexus. Incomplete vascular system around the lamina is called the circle of Zinn-Haller (or Zinn). 4. Retrolaminar region - CRA and pial vessels (ciliary). XI. Venous return from retina and optic nerve A. Retina - central retinal vein (CRV) leaves the eye through the optic nerve and drains into the cavernous sinus. B. Choroid - the vortex veins, one in each quadrant of the posterior pole of the eye. C. Optic nerve head - mainly CRV, but some enters choroid system. Fig. 12

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