OPTO30007 Notes SM2 2024 - Anatomy of the Eye - PDF

OPTO30007 – NOTES SM2 2024 LECTURE 1 – WEEK 1 – ANATOMY OF THE EYE Problem 1: - 47yo lady experiences deep boring pain in the left eye whilst watching movie - Sees haloes - (Vision below 6/12 are not suitable for driving), the lady has a vision of 6/6 in the left eye Examination = narrow anterior chamber angle (a shallowing of drainage angle in the front part or anterior chamber of the eye) this menas that the angle between the iris and the cornea is narrow…. what does this mean for the lady? The symptoms and findings described in the scenario suggest that the 47-year-old lady may be experiencing an acute angle-closure glaucoma attack. Here’s what each finding typically indicates: 1. Deep boring pain in the left eye: This is a common symptom of acute angle-closure glaucoma. It occurs due to increased pressure inside the eye (intraocular pressure) which can lead to damage of the optic nerve. 2. Sees haloes: Haloes around lights are another classic symptom of acute angle-closure glaucoma. These haloes are caused by corneal oedema (swelling), which occurs due to the increased pressure in the eye. 3. Vision of 6/6 in the left eye: This means her visual acuity in the left eye is normal (6/6 is equivalent to 20/20 in Snellen notation). However, despite good vision, the other symptoms are concerning. 4. Examination finding of narrow anterior chamber angle: This is a critical finding. It suggests that the angle between the iris (colored part of the eye) and the cornea (clear front part of the eye) is narrow. In acute angle-closure glaucoma, this angle becomes even narrower or closes completely, leading to a sudden rise in intraocular pressure. What does this mean for the lady? Given these findings, the narrow anterior chamber angle indicates that the lady is at risk of developing or may already have acute angle-closure glaucoma. Acute angle-closure glaucoma is an emergency condition that requires immediate treatment to lower intraocular pressure and prevent permanent vision loss. If left untreated, acute angle-closure glaucoma can rapidly lead to irreversible damage of the optic nerve and severe vision loss. Therefore, the lady should be urgently referred to an ophthalmologist for further evaluation and management. Treatment typically involves medications to lower intraocular pressure and sometimes laser or surgical procedures to alleviate the angle closure. (EYE) Outer coat n Cornea and sclera - FUNCTION; Transparency & Strength Middle coat n Ciliary body, iris, choroid - FUNCTION; 1. Aqueous humour production, 2., Accommodation (focusing) 1. Aqueous humour production Important for the health of the Lense and cornea Creates intraocular pressure Formed by ciliary epithelial cells 2. Accommodation (focusing) Ligaments attach to ciliary processes Involves contraction of the ciliary muscle Inner Coat n Retina Optic nerve / optic disk Fovea Macula (are in which the fovea sits) Posterior pole Ora serrata HOW DO WE SEE? ð There are factors that can limit visual acuity, being neural factors and optical factors. (1) Neural factors (2) Optical factors Pupil size Clarity of optical media (cataracts, corneal opacities etc.) Refractive errors (blur) – due to; myopia, hypermetropia, astigmatism, presbyopia. In terms of optical factors, there are multiple which impact vision in general: (a) Cornea / tears: - Emmetropia – where the refractive state of an eye in which paralleled rays of light entering the eye are focused on the retina EXACTLY, creating a crisp image perceived in focus. (b) Axial length of the eye determines power (axial length is the length of the eye from the front to the back) - Myopia – short sightedness, where objects close look clear but far out are blurry - eye growth determines refractive state Myopia, although for short sightedness, is formed with a oblong (long) eye shape. - Hypermetropia – long sightedness, where objects far away look clear but up-close look burry. - Hypermetropia, although for long distance, s formed with a smaller circle eye (opposite of oblong). (c) Variable focus due to crystalline lens: - Balloon suspended in donut (ciliary) muscle - Thickens with age and less plastic - Presbyopia is loss of near focusing ability, at ages 40-45 – need reading glasses. PATHWAY for light: Through: PRCs > BCs > GCs Lateral interactions: 1. Horizontal cells 2. Amacrine cells n RODS: Night vision, ‘scotopic’, very sensitive, one type, no colour vision, 100 million of them, are absent from the fovea n CONES: Day vision, ‘photopic’, less sensitive, three types, allow for colour vision, 5 million of them, densest in the fovea. -THE NEURAL RETINA- => Contains 6 neurons: 1. HCs 2. BCs 3. ACs 4. GCs 5. Rods 6. Cones THE NEURONAL LAYERS OF THE EYE - There are 3 dieerent neuronal layers 1. Outer nuclear layer (ONL) 2. Inner nuclear layer (INL) 3. Ganglion cell layer (GCL) In the INL (inner nuclear layer) Ø Contains the cell bodies of horizontal, bipolar and amacrine cells Ø Contains Müller cells BIPOLAR CELLS in INL: - Important in ‘through’ pathway - 10 dieerent types of bipolar cells - 1 x rod bipolar cells - 9 x cone bipolar cells - Important for spatial vision and colour vision Lateral inhibition INL: HORIZONTAL CELLS - Input from photoreceptors - Provide output onto photoreceptors - Release inhibitory neurotransmitter GABA - Respond to light by hyperpolarizing AMACRINE CELLS in INL: - Many dieerent types - Have no axons (axon less) - Important in lateral inhibition like horizontal cells - Release inhibitory neurotransmitters like glycine & GABA, often co- express more than one neurotransmitter. GANGLION CELL LAYER (GCL) - Cell bodies of GCs and some displaced amacrine cells live in the ganglion cell layer - Ganglion cells are the main output neuron of the retina (many dieerent types: ON, OFF, M & P) - RGCs release Rtamate - RGCs ﬁre action potentials OPTIMIZING VISION: Retinal Structure Macula n Deﬁned by where pigment lies in the retina (where the fovea is), but retinal layers are ‘pushed’ to the side n 3mm diameter n Exists no cells or processes to impede light transfer. The central avascular zone is the optimized vision zone. OPTIC NERVE ð Formed by axons of ganglion cells as they exit the retina to pass visual information to higher cortical areas Note: Lamina cribosa is the area in which the ganglion axons begin to descend into one thick optical nerve, coming from the retina: THE VISUAL PATHWAY BEYOND TRHE RETINA 1. RGCs go to lateral geniculate nucleus (LGN – in thalamus) 2. 55% GC ﬁbres cross at the optic chiasm 3. Cortical cells need stimulation from both eyes to develop properly. If not, amblyopia can form (reduced vision in one eye caused by abnormal visual development early in life) caused from… in dieerence in refractive state, turned eye (squint), congenital cataract or from lid pathology abnormality Glaucoma - A name for a group of eye diseases where vision is lost due to damage to the optic nerve, typically irreversible vision loss. - Types: 1. Angle closure, 2. Angle open - Treatment: preventing RGC loss SUMMARY OF LECTURE: 1. Retina contains 3 neuronal layers (ONL, INL, GCL) 2. Retina contains 2 synaptic layers (OPL & IPL – outer plexiform layer and inner plexiform layer) 3. OPL consists of connections among PRCs, BCs and HCs 4. IPL consists of connections among BCs, ACs and GCs. 5. For vision at high light levels: - Visual acuity is highest at the fovea - Depends on density of cones - Avascular fovea LECTURE 2 – WEEK 1 – THE RETINA 2: PHOTORECEPTORS Lecture concept: What happened when light hits the retina? Problem 1: - 45-year-old lady - Has trouble seeing at night - Trips over her children’s toys - Has had many car accidents over the last 2 years - Can read OK In a photo taken of Joan’s fundus (fundus – is the inside, back surface of the eye), it shows many black spots that hinder the fundus’ natural colouration. What does this mean for the lady?... STRUCTURED OF PHOTORECEPTORS 1. Outer segment – (tip of the actual rod or cone (the shape)) where photopigment us housed 2. Inner segment (main body with nucleus and axon)– where mitochondria and golgi is housed 3. Synaptic terminal – (axon and synaptic terminal) where neurotransmitters are released and where the rod/cone connects via synapse to the bipolar and horizontal cells. THE RELATIONSHIP BETWEEN OUTER SEGMENT DISCS AND RHODOPSIN/OPSINS n Both rod and cone cells utilize their outer segment discs to house light-sensitive molecules – rhodopsin in rods and many dieerent types of opsins in cones. n RODS: the tip (actual rod part) of the rod is known as the outer segment, and it is where rhodopsin molecules are housed, molecules which are photopigments. It consists of a protein called an opsin, bound with a light-sensitive molecule called 11-cis retinal. ð When light enters eye and reaches the retina, it is absorbed by the rhodopsin molecules, triggering a biochemical cascade that leads to changes in membrane potential and ultimately to the generation of neural signals. n CONES: the actual cone tip of the cone is where opsins are housed, which can vary unlike rhodopsin, to dieerent types like red, green and blue cones. – opsin proteins are sensitive to dieerent wavelengths of light ð DiCerent cones respond to diCerent types of wavelengths of light due to the presence of diCerent opsins in their outer segment discs. How do photo receptors respond to light? ð Photoreceptors are hyperpolarized by light. ð Respond to light with graded changes in membrane potential (not action potentials) PHOTOTRANSDUCTION Phototransduction = The process of converting light into electrical signals in photoreceptor cells of the retina. 1. Activation of rhodopsin (opsins in cones) ð Light absorbed by rhodopsin, absorption causes structural change (conformation) in the bound retinal (11-cis retinal converts to all-trans retinal), leading to a conformational change in rhodopsin. 2. G-protein (Transducin) ð Conformationally changed rhodopsin allows for binding and activation of transducing, a G-protein consisting of subunits (a, B, and y). 3. Activation of phosphodiesterase ð Once transducin is activated, GMP bound to the a- subunit is exchanged for GTP (guanosine triphosphate). The now activated GTP subunit binds to and activates an enzyme called phosphodiesterase (PDE) located in photoreceptor outer segment. PDE is normally inhibited in the dark by high levels of [c]GMP. 4. Hydrolysis of cGMP and closure of cGMP gates channels for ion inﬂux. ð Activation of PDE results in hydrolysis (breaking down) of cGMP into 5’-GMP. Decreased levels of cGMP lead to the closure of the cGMP guarded sodium channels responsible for the cells’ depolarisation/hyperpolarisation. ð This in turn reduced the inﬂux of both Na+ & Ca2+ (calcium) into cell, leading to hyperpolarisation in the light. ð This reduces glutamate release from HOW DO PHOTORECEPTORS RECOVER photoreceptor cell onto bipolar cells, FROM LIGHT? which initiated the transmission of the visual signal to the retina and (1) cGMP Control subsequently to the brain. n cGMP controls the ion channels n cGMP is continuously produced by guancylate cyclase (dependent on calcium levels) (2) Restoration of rhodopsin n Activated rhodopsin is rapidly phosphorylated by rhodopsin kinase n This allows binding of arrestin to phosphorylated rhodopsin. n Bound arrestin prevents activated rhodopsin binding to transducin Importance of Calcium in RESTORATION OF THE cGMP activity SIGNAL: CALCIUM In DARK: DECREASED Ca: Ø Calcium passing into - Results In the increase in photoreceptors guanylate cyclase, which makes Via cGMP gated channels cGMP - Results in increase in rhodopsin VDCC kinase, which allows for more arrestin to bind. In LIGHT: - Results in increase in a\inity of Ø cGMP gates channels close cGMP for channels. Ø reduces Ca inﬂow - Arrestin produced prevents Ø with hyperpolarisation VDCC also activated rhodopsin from binding close to transducin. ð This means that calcium levels in the cell Is very low. RESTORATION OF 11-CIS RESPOSNE OF OPSINS TO RETINAL DIFFERENT WAVELENGTHS 11-cis retinal has been decreased as ð SHORT (S pigment) [Rhodopsin]: light impacts 11-cis retinal, changing Blue (435 nm) the conformation of rhodopsin. In ð MEDIUM (M pigment): Green (535 doing so, removing the 11-cis retinal nm) which is a protein attached to ð LONG (L pigment): rhodopsin. Purple/orangey yellow (550 nm) ð Restoration is done via the ð RODS (500 nm) Retinoid cycle, which converts all- trans retinal to all-trans retinol, to RPE via IRBP (interphotoreceptor retinoid binding protein) ð RPE stands for Retinal Pigment Epithelium, which reacts to light. RETINITIS PIGMENTOSA Ø Genetic disorder Ø Creates tunnel vision Ø Is formed by a genetic defect in rhodopsin or proteins involved in phototransduction Ø Is characterised by degeneration of the retina Ø Degeneration of photoreceptor cells (rods and cones) leading to vision loss over time Ø Please explain Retinitis pigmentosa, and talk about its genetic defect in rhodopsin/proteins involved in phototransduction. Talk about how it creates tunnel vision. Thanks LECTURE 3 – WEEK 2 – THE RETINA 3: PARALLEL PATHWAYS Lecture concept: How is information from photoreceptors carried to downstream neurons? Problem 1: Monica Woman aged 65 Noticed a disturbance in her vision Water splashing in front of her left eye ‘Flickering lights’ Trips over her grandchildren’s toys Husband concerned about her driving n Vision 6/9, 6/12 n Good health Grade 3 Melanoma exercised 4 years ago {Lymph nodes +ve} Why is this lady having trouble seeing? LUCA’s Attempt at Answer (not conﬁrmed as right answer): The most logical reason for vision trouble for Monica is potentially due to a Posterior Vitreous Detachment (PVD), an issue common in induvial over 50 where the vitreous gel in the eye detached from the retina. This is because over time the vitreous can shrink and become more liquid, eventually detaching from the retina. Floaters, ﬂashes of light, and visual disturbances are all symptoms of Monica’s that align with the symptoms associated with PVD: Floaters: - Described as ‘water splashing’ in front of the eye, ﬂoaters are small, shadowy shapes that appear in the ﬁeld of vision. They are caused by the vitreous get detaching from the retina, creating shadows on the retina from tiny clumps or strands of vitreous gel/humor. Flashes of light: - Referred to as ‘ﬂickering lights’, ﬂashes of light can occur when the vitreous gel tugs on the retina during detachment, the mechanical stimulation is perceived as ﬂashes of light. Visual Disturbances: - Di\iculty in seeing can lead to tripping over objects or concerns about driving, which is common as the ﬂoaters and ﬂashes can obscure vision. Diagnosis: - 6/9 & 6/12 vision indicated a slight to moderate visual impairment, which is consistent with symptoms of PVD, Monica should seek further medical assistance to determine whether her retina is detached or not. PROCESSING VISUAL INFORMATION (BIPOLAR CELLS) ð 2 di\erent types of bipolar cells exist: ð Both of which are meant to transmit signals from photoreceptors to ganglion cells ð IT IS IMPORTANT TO REMEMBER THAT THE PRESENCE OF LIGHT & GLUTAMATE ON OFF/ON BCs PRODUCE DIFFERENT OUTCOMES, WHERE GLUTAMATE PRESNECE CAN BE HYPERPOLARISING OR DFEPOLARISING DEPENDANT ON LIGHT PRESENCE. (1) On bipolar cells – cells depolarized by light falling on the retina (excited in day, inhibited in night) Are activated when light increases In darkness, continuously release neurotransmitter glutamate which inhibits ON BCs. When light hits ON BCs they hyperpolarize and reduce glutamate release, allowing the bipolar cell to depolarize. RECEPTOR: Have metabotropic glutamate receptors (mGluR6 – associated with TRPM1 channels), that when bound to glutamate, activate a cascade that keeps the cell hyperpolarised, when glutamate levels drop due to light, the cell depolarizes. > ON BCs hyperpolarise with increased glutamate. Less TRPM1 channels may lead to inability to e\ectively process visual signals in dim lighting. HOW DOES GLUTAMATE CAUSE HYPERPOLARISATION of ON BCS? - When light is present, this decreases the level of glutamate release, therefore mGluR6 activation is decreased with less available glutamate to bind to them. The downstream process of this lowers the IP3 & DAG levels, ultimately closing the TRPM1 channels. - Without the inﬂux of Na+ and Ca+ ions, the ON-BC become hyperpolarised. TRPM1 Channels Ø Appaloosa horses have reduced mRNA TRPM1 and have reduced b-wave and night blindness. Ø The b-wave of the electroretinogram (ERG) reﬂects activity of ON-BCs. Ø In Appaloosa horses with reduced TRPM1 channels, results in a weaker b-wave response in the ERG. Ø Nyctalopia (night blindness) – diLiculty seeing at night, occurs because the ON- BCs are not able to respond properly to reduced levels of glutamate during low- light condition. Less TRPM1 channels may lead to inability to eLectively process visual signals in dim lighting. (2) O` bipolar cells – cells hyperpolarized by light falling on the retina (inhibited in day, activated in night) Depolarise when light decreases, cells are excited by glutamate. In darkness, photoreceptors release glutamate continuously like ON BCs, which excite & depolarize the cell. When light is on retina, glutamate release reduces and hyperpolarises OFF bipolar cells RECEPTOR: Have ionotropic glutamate receptors (AMPA, NMDA & Kainate receptors), that open in response to glutamate allowing for depolarisation of GANGLION CELLS Ø Output neurons of the retina Ø Many di2erent types of cells (M ganglion cells & P ganglion cells), they both encode di2erent types of visual information Magnocellular GCs n Magni – large n Large receptor ﬁelds n Subserve motion detection, ﬂicker and analysis of gross features (quick recognition of and processing of the overall scene or object without focusing on ﬁne details) Ø Handle high frequency changes of light Ø Fast adaptions Parvocellular GCs Parvi = small More numerous Visual acuity and colour vision Ø Handle low frequency changes of light. Ø Slow adaptions ð Bipolar cells synapse with ganglion cells ð Ganglion cells can be ON or OFF types ð ON BCs can only synapse with ON GCs, and the same goes for OFF BC/GC. ð Communication between BC and GCs is done via iGluRs such as NMDA, AMPA, and Kainate receptors. GC Function Ku`ler, 1953 ð GCs function by integrating all the information signalled from the photoreceptors, allowing the process of conveying visual information like contrast and spatial details. ð Central and Surround are two separate pathways coming from the photoreceptor to the GC. ON CENTRE FIELD GC: ð These cells are excited when light falls onto the centre of their receptive ﬁeld, and inhibited when light falls on surrounding area OFF CENTRE FIELD GC: (Centre-Surround Organization) ð These cells are excited when light falls onto the surrounding area of their receptive ﬁeld, and inhibited when light falls onto the receptive ﬁled. ð BOTH these cells are important in detecting contrast, allowing them to detect edges and boundaries in the visual scene (important in spatial perception) CENTRAL & SURRUONDING PATHWAYS CENTRAL: n A pathway that provides a direct measure of light in the central part of the receptor ﬁeld, from photoreceptors, to BCs, to GCs. - This pathway contributes to detailed visual processing, the intensity of light and contrast. SURROUNDING: n A pathway that provides indirect pathway that involved horizontal cells that provide lateral inhibition to modulate activity of bipolar cells. The level of inhibition of BCs onto GCs is determined by level of surrounding light present. - This pathway contributes to enhancing contrast and edge detection, improving contrast and spatial resolution. - HCs release GABA *Centre-surround organization enhances our ability to see contrast at borders. WHAT WAS WRONG WITH MONICA? CONFIRMED ANSWER Ø Melanoma Associated Retinopathy Ø Rare complication of melanoma Ø Antibodies are produced directed against ON BCs (autoimmune disease) Ø Patient treated with oral prednisolone Ø There is less function of the ON BCs LECTURE 4 – WEEK 2 – THE RETINA 4: ROLE OF AMACRINE CELLS Lecture concept: What do amacrine cells do? Problem 1: What’s wrong with baby Jane? Jane is a 10-year-old twin At 3mo of age, father noticed horizontal eye movements (quivering eye movements) Examination: bilateral nystagmus Brain imaging all normal Genetic testing: FRMD7 mutation (on X chromosome) Why is this child having trouble seeing? LUCA’s Attempt at Answer (not conﬁrmed as right answer): (UN_ANSWERED) ACTUAL ANSWER BY INVIGILATOR: ð Jane’s bilateral nystagmus (rapid, uncontrolled eye movements) is associated with a mutation in the FRMD7 gene. This gene is critical for proper functioning in of starburst amacrine cells, which are responsible for horizontal motion detection. ð The mutation disrupts these cells, leading to the loss of optokinetic nystagmus and resulting in involuntary horizontal eye movement. This condition known as pendular nystagmus, aLects about 1 in 1500 people, with 70% of people attributed to FRMD7 mutations. FRMD7 Gene: required for horizontal motion detection. Ø Essential for proper function of horizontal motion detection mechanisms in the retina Ø FRMD7 deﬁciency: Leads to dysfunction in SACs, impairing horizontal motion detection, important in stabilizing gaze and smooth pursuit eye movements. Gene Expression: By Starburst Amacrine Cells Optokinetic Nystagmus (OKN): Eye movement in response to moving stimuli, crucial for tracking objects. - Impact of FRMD7 mutation > Loss of OKN due to impaired SAC function, leading to the inability to stabilize gaze during horizontal motion. TYPES OF AMACRINE CELLS: Ø Roughly 22 types Ø Neurotransmitters used - All contain Gaba or glycine - Some also contain Ach, Dopamine, neuropeptides [Amacrine cell inputs] * Amacrine cells modify bipolar cell response I.e., modify the retinal ‘though’ signal [Amacrine cell functions] Amacrine cells release GABA and glycine onto bipolar cells and ganglion cells, ‘ﬁne tuning’ the ‘through’ response. In terms of the rods and cones, all cone bipolar cells synapse onto ganglion cells, however rod bipolar cells only synapse with amacrine cells. In light, cones signal to bipolar cells, while at low light level the cones are not sensitive enough to signal, and so the rods signal to bipolar cells. Rod bipolar cells receives input from ONLY rods, where they express mGluR6 receptors, and depolarize when light falls on the retina. THE AMACRINE CELL & ROD PATHWAY ð 1 x Rod bipolar cells synapse with two amacrine cells. ð All amacrine cells express iGluRs (i.e will depolarize when RBCs depolarize), where glycine is the neurotransmitter. ROD PATHWAY n RBCs DO NOT synapse with GCs n Rod pathway ‘piggy backs’ onto the cone pathways. n The density of all amacrine cells limit the visual acuity at night (recap: both the OFF BCs and OFF GCs hyperpolarize to light, & ON BCs and ON GCs depolarize to light. OFF BCs hyperpolarize in response to glycine So, because RBCs do not synapse with GCs, they actually synapse with the All Amacrine Cells (AACs). The AACs then send signals received from RBCs, to CBCs. This actually allows the CBC pathways to integrate the RBCs neural signal into the ganglion cells. So yes, the Cone Bipolar Cell does the work for the Rod Bipolar Cell via Amacrine Cells. AMACRINE + GANGLION CELLS – DETECTING MOTION (1) Photoreceptor activation (2) Bipolar cells receive input from photoreceptors and transmit onto amacrine cells. (3) Amacrine cells, part of them includes a class of Starburst Amacrine Cells (SACs or SBACs). Starburst amacrine cells are a specialized type of amacrine cells involved in motion detection. They have dendrites that radiate in a starburst pattern and are sensitive to the direction of motion. SACs then release the neurotransmitters acetylcholine and GABA. Both these neurotransmitters are important in the modulation of activity onto DSGCs. Acetylcholine is released for an excitatory response, where it is released when motion is in the preferred direction of the DSGCs. This results in an increase in ﬁrring rate of DSGCs. GABA is released for an inhibitory response, where it is released when motion is in a non-preferred (null-direction) direction on DSGCs. This reduces the ﬁring rate of DSGCs and making them less responsive. SACs detect the direction of motion through their dendritic arbors (in a symmetrically radial like fashion). n The combination of inhibitory and excitatory inputs enhances the contrast between the preferred and non-preferred directions. (4) Direction-Selective Ganglion Cells (DSGCs) These are retinal ganglion cells that respond diLerently to motion in a speciﬁc direction. They receive input from BCs and SACs, to preferentially activate in a direction-selective manner. (5) Transmission to the Brain The output from DSGCs is transmitted through their axons to form the optic nerve, which innervated to the LGN of the brain and then to the visual cortex, where signals are then interpreted as motion. (YOSHIDA ET AL., 2001) ð Starburst cells were ablated (destroyed) using an Immunotoxin (saporin), and antibody against the protein ChAT which is in amacrine cells. When the SACs were injected with Saporin to target the ChAT, it would kill the starburst amacrine cells, and prove the need for starburst cells in perceiving motion. SACs/SBACs’ directionally tuned inhibition Ø This mechanism describes the ensuring that SBACs provide inhibitory input to DSGCs only when motion is detected in the null direction, thereby enhancing the direction selectivity of DSGCs, and providing accurate motion detection in the visual system. Ø Calcium responses: SBAC dendrites exhibit varying calcium responses along their length, the signal intensity (amount of calcium released) depends on the direction of the stimulus. Ø GABA release – is the inhibitory neurotransmitter which is released when the stimulus moves form the soma (cell body) outward along the dendrite (in a centrifugal direction), but NOT when it moved toward the soma (centripetal direction). THEREFORE, ð GABAergric inputs onto DSGCs come from SBAC dendrites that are orientated in the null direction. Why Care About Motion Detection? n DSGCs target 3 major pathways/regions of the brain: (1) LGN – Lateral Geniculate Nucleus - Is relay centre in thalamus for visual information from retina to visual cortex - It processes motion signals and contributes to the perception of movement, enabling the brain to interpret dynamic visual scenes (2) Superior Colliculus - Involved in orienting movements of the eyes and head towards stimuli - Helps coordinate rapid eye movements (saccades) to track moving objects, facilitating attention and focus on relevant visual information. (3) Accessory optic nucleus - Involved in stabilizing the visual ﬁeld during head movements - Process motion information to adjust eye movements and maintain stable vision, crucial for navigating the environment. LECTURE 5 – WEEK 2 – THE RETINA 5: ROLE OF GANGLION CELLS Lecture concept: Types of ganglion cells and their roles. TYPES OF GANGLION CELLS (1) PARASOL GANGLION CELLS (M-GCs) Ø Magni = large receptor ﬁelds Ø Allow for motion detection, low light vision, ﬂicker and analysis of gross features Ø Low spatial resolution, but high temporal resolution Ø Not densest in the fovea Ø Where multiple fovea cones synapse with multiple ON or OFF bipolar cells, which synapses with a single ON or OFF M-Ganglion cell. => Because of this, Midget ganglion cells (M-GCs) cannot produce great visual acuity. (2) MIDGET GANGLION CELLS (P-GCs) Ø Parvi = small receptor ﬁelds Ø More numerous in numbers than MGCs Ø Allow for visual acuity and colour vision Ø High spatial resolution, but low temporal resolution Ø Densest in the fovea Ø Where one parafovea cone synapses with one ON or one OFF bipolar cell, which synapses with a single ON or OFF M-Ganglion cell. => Because of this, P-Gcells (P-GCs) can produce great visual acuity. = The Midget System (3) INTRINSICALLY PHOTOSENSITIVE GCs Ø Bistratiﬁed Ø Small Ø Spare (very few) Ø Very important in the non-vision encoding aspects of light detection (tell our body when the lights are on) This speciﬁc class of GCs are in their own way, photosensitive just like photoreceptors. COLOUR DISCRIMINATION ð Within the eye, there are more red cones, then green cones, then blue cones There are: 1. Blue cones: respond well to wavelengths 430 2. Green cone: respond well to wavelengths 530 3. Red cones: respond well to wavelengths 560 Colour Comparisons by RGCs (retinal ganglion cells) n Midget ganglion cells exhibit a colour opponent centre-surround surface, with the bullseye being the best reaction for ON GCs, and the surrounding area around the bullseye being the best place for reaction for the OFFS GCs. n Some midget ganglion cells are excited by red falling on their centre and inhibited by green falling the surround (peripheral). n Others are excited by blue or yellow lights falling on their receptor ﬁeld centre. n Colour perceived is determined by the activity of ganglion cells. Targets/Pathway of Ganglion Cell Axons 1. LGN (thalamus) Visual perception - Major target of most GCs - Visual pathway 2. Suprachiasmatic nucleus (hypothalamus) - Cirdadian rhythm Non-image forming 3. Prectum (midbrain) functions - Pupil responses 4. Posterior nucleus of the thalamus - Photophobia 5. Superior colliculus - Eye movements INTRINSICALLY PHOTOSENSITIVE GCs ð Speciﬁcally, when light hits the melanopsin in the outer ð Respond to light by depolarising segment of the cone for a ð Have much much larger receptor intrinsically photosensitive ﬁelds than both the parasol and GC, the cascade leads to the midget GCs TRPC 6/7 channel to open Respond to light even in a blind Light causes the person. depolarisation of ganglion cells, while it causes the ð Have an opsin called melanopsin hyperpolarisation of ð Like rhodopsin for rods, when light cone/rod photoreceptor hits melanopsin, there is an cells. intracellular cascade that leads to a channel opening, to allow for the We know that melanopsin can create inﬂux and eLlux of sodium and intrinsic light sensitivity within GCs, potassium ions, forcing a because the RGC response in a blind depolarisation. mouse with melanopsin expressed, produces excitation (many ﬁring of action potentials) of (ISGCs) ganglion cells in response to light. FUNCTION 1 of ipGCs: Concentral Pupil Response ð When light is shone in one eye, it restricts both the eye that is being shone with light and the remaining eye (both eyes) ð When the sphincter puillae restricts, the pupil restricts. ð So when light is shone in the eye, an intracellular circuit will result in either the constriction of the sphincter pupillae muscles, or the relaxation of the dilator pupillae muscles. So, what is the pathway of photosensitive GCs in a pupil response from shining light? 1. Pathway involves the Melanopsin GCs projecting to the Optical Pretectal Nucleus (OPN) which is a pathway that DOES NOT cross at the optic chiasm, but stays to its’ own respective ‘eye lane’. Just before the LGN, the axon turns inward to project to the Pretectal nucleus. 2. This axon projects to the Pretectal nucleus which then sends a neuron projected to the Edinger Westfal nucleus in the midbrain on its own side and the opposite side as well. The Edinger Westfal nucleus is the parasympathetic innervation to the eye. 3. The Edinger Westfal Nucleus then send a neuron axon back to the iris of the eye, which is what allows for the constriction and relaxation of muscles within the eye for the pupil response IN BOTH EYES. THIS IS WHY WHEN YOU SHINE LIGHT INTO OTHER EYE, YOU CAUSE CONSTRICTION OF BOTH EYES. Therefore, the intrinsically photoreceptive GCs are very important in the constriction and relaxing of the pupil REGARDLESS of photoreceptors. FUNCTION 2 of ipGCs: Circadian Rhythm ipGCs target the Superchiasmatic nucleus of the thalamus. - Contains many nuclei that automate the unconscious functioning of the body, such as sleep & mood. LECTURE 6 – WEEK 3 – WHY DO WE NEED VISUAL ADAPTATIAN? Lecture concept: Our vision depends on our ability to detect small diCerences or changes in: - Intensity - Position - Colour …where this ability must be maintained under diCerent light levels n Sunrise to sunset What is adaptation in the visual system? ð Where the visual system adjusts its performance to changes in light levels (e.g ambient). Adaptation acts to match the limited neuronal response range to the visual input range. ð Most of the work is done by the scotopic (rods) & photopic (cones) systems. { VISUAL ADPATION } Light Adaption Dark Adaption Beneﬁcial Detrimental Increases visual sensitivity “recovery” in darkness following Rapid process (seconds) light exposure ð From darkness to light. Slow process (~ 40 mins) From light to darkness Weber’s Law: ‘The minimum increase of stimulus which will produce a perceptible increase of sensation is proportional to the pre-existent stimulus”. - The Lowest threshold in terms of illuminance adaption ability is greater for cones than it is for rods, although cones adapt faster than rods when the light is initially shone onto the retina. Rod Adaption: Rods adapt are restricted range of intensity before they saturate (1-2 log units) (so they reach saturation). Yet, they are still able to respond in backgrounds of moderate intensity, and quite well in red lighting. Cone Adaptation: Adapt over a great range of intensities, and DO NOT REACH saturation. [Ca2+ independent adaption] *Accounts for majority of light adaption response* ð cGMP opens the gate required for Ca2+ inﬂux in the light via the activation from PDE. Low PDE means less cGMP to open the Ca2+ channel open. ð Powerful negative feedback loop ð A drop in Ca2+ in response to light avoids saturation hence increases photoreceptor sensitivity. This is mediated by the unbinding of Ca2+ from GCAPs (Guanylyl cyclase activating protein), calmodulin & Recoverin. Guanylyl cyclase activation Recoverin and R* inactivation n Less Ca2+ leads to a shortened R* lifetime n R* is inactivated by rhodopsin kinase (GRK1) in a Ca2+ dependent manner Calmodulin & CNG channel gating n Less Ca2+ leads to an unbinding of Ca2+ from calmodulin n Has low eLect on rodfs Cones: - CNG-modulin modulates cGMP sensitivity The GCAPs & Recoverin are shared between rods and also cones. Ø CNG channel proteins (the proteins on the GCPAs that regulate Ca2+ binding) For rods: Cna1/Cnb1 For cones: Cnga3/Cngb3 ð In Cones the 11-cis retinal is recycled by RPE cells and Muller Glia Cells, while rods only get the recycled 11-cis retinal from the RPE cells. HAVING TROUBLE BATTING IN CRICKET AND TEMPORARY BLINDNESS WHEN LEAVING A CINEMA: = Bradyopsia Bradyopsia is known as ‘slow vision’, > caused by mutations in the RGS91 or R9AP genes, which results in diCiculty adapting to sudden changes in bright light and diCiculty in seeing moving objects at low contrast. - RGS91 (expressed in Retina)/R9AP (expressed in brain) are genes involved in the recovery of photoreceptors after light exposure. ð Interestingly, people with this mutation have better/faster night vision as people with no RGS91, PDE is more at low light levels -> shorter turnover of cGMP -> cGMP channels remain open at low light. ð LECTURE 7 – WEEK 3 – WHY DO WE NEED VISUAL ADAPTATIAN? Lecture concept: Our vision depends on our ability to detect small diCerences or changes in: What is the RPE? Location: The RPE is a single layer of pigmented cells that lies between the photoreceptors (rods and cones) of the retina and the choroid, which is the vascular layer of the eye that supplies blood to the retina. Pigmentation: The cells of the RPE are heavily pigmented with melanin, which gives them a dark appearance. This pigmentation helps to absorb excess light, preventing it from scattering within the eye and thus improving visual acuity Further functions; Absorption of light (heat); barrier function; transport of nutrients, ions and water; phagocytosis of shed OS; Photopigment recycling (all-trans-retinal to 11-cis-retinal) INHERITED RETINAL DISEASES Ø Rare genetic disorders Ø 300+ genes Ø DiLerent genes can cause the same disease phenotype, and then the same gene can cause diLerent disease phenotypes, even within the same family. TYEPS OF HERTITABLE EYE DISEASES (1) Monogenic (Mendelian) - Inherited retinal diseases with 1 pathogenic gene variant - DiLerent genes can be responsible for the same condition in another individual (2) Complex conditions - Condition caused by variants in more than 1 gene variant AND Environmental factors. - An example of this would be age-related macular degeneration, inﬂuences by both genetic factors and environmental and lifestyle factors (including diet, smoking, BMI) - Environmental factors can include chemical and radiation damage (UV light, smoking etc.) that may cause mutation in genes - Genetic factors include errors in DNA replication or errors that arise during recombination in meiosis. Genes encode for proteins, and when a mutated gene encodes a protein it results in either no protein, or an altered protein. Autosomal recessive: Where a parent who has one faulty gene copy and one normal gene copy, is a carrier and does not usually have symptoms. Ø ARIRD = Autosomal Recessive Inherited Retinal Dystrophy n For a child to be aeected, both parents would need to be carriers of a gene variant I the same gene. If both parents are carriers, child can be: - 25% chance aeected - 25% chance unaeected and not a carrier - 50% chance will be an unaeected carrier. EXAMPLE 1 of ARIRD: n ABCA4-related IRDs ð ATP-binding cassette transporter 4 ð Involved in the recycling of 11-cis retinal into all-trans retinal ð sits in the rim of the outer segment of the photoreceptor ð Does so by ﬂipping N-retinylidene phosophatidylethanolamine (NrPE) in photoreceptor disc membrane and prevents build up of vitamin A derivatives, which are toxic to the RPE ð Has 50 exons: An exon is a segment of a gene that codes for proteins. In the process of gene expression, the DNA sequence of a gene is transcribed into messenger RNA (mRNA). This mRNA transcript contains both exons and introns (non-coding regions). During a process called splicing, introns are removed, and the exons are joined together to form the final mRNA sequence that is translated into a protein. ABCA4-related IRDs > can cause Stargardt disease (muscle atrophy), retinitis pigmentosa (form of cone dystrophy), and cone-rod dystrophy. ð When ABCA4 is not present, it results in a build-up of vitamin A, which is where RPE breaks down (pigment epithelium cells in between the photoreceptors of retina and the choroid. Autosomal Dominannt: Where the phenotype trait it seen in those with one copy of the variant. Ø 50% chance of akected person to pass faulty gene onto child Mechanism of disease: 1. Loss of function – one copy of gene active means it is not enough to maintain normal function 2. Dominant negative & gain-of-function – if one gene is defective and other is not, the defective gene can still impede the normality function of the normal gene. EXAMPLE 1 of ADIRD: n Rho-related retinitis pigmentosa (rod-cone dystrophy) - Rho encodes for rhodopsin - 20-30% of AD rod-cone dystrophies - AD inherited retinal diseases less frequent than autosomal recessive (AR) diseases – also less severe than AR and X-linked. X-Linked: Faulty gene on the X chromosome Ø Female with faulty x chromosome usually do not show symptoms (x- inactivation) Ø Males will, as they only have 1 x X-chrome Ø No male to male transmission If mother is carrier but father is healthy: ð Each son has 50% chance of being aeected ð Each daughter has 50% chance of being a carrier lie mother. EXAMPLE 1 of X-linked-IRD: n Retinitis X-linked-pigmentosa GTPase regulator - Found in photoreceptor outer segments - Essential for the maintenance of photoreceptor viability Mitochondrial inheritance: Ø Mitochondria have their own DNA – 37 genes Ø Mutations in mitochondrial genes passed on through the mother (maternal inheritance) Ø Both males and females can be aeected TYPES OF GENETIC VARIATIONS (MUTATIONS) 1. Aneuploidy – deviation from normal diploid pairs of 23 chromosomes (downs) 2. Structural variants – aLect whole chromosome or large segment 3. Small variants at the single nucleotide level – aLect a deﬁned position, most inherited diseases are small variants. Diagnostic testing Find the variation in underlying condition Conﬁrm or rule out suspected genetic disorder TYPES OF GENETIC TESTING: 1. Karyotypes & Fish – are there 23 pairs of chromosomes, like for downs? 2. Microarrays – Better resolution than chromosome-level analysis and looks at thousands of genes simultaneously to study gene activity. Various biological markers can be associated with various health traits. 3. Sanger sequencing – ﬁrst generation sequence method. Can only do single genes, and is very speciﬁc (gene mutations linked to breast cancer) 4. Next generation sequencing – massive parallel sequencing, millions of DNA fragments. Gold standard for CHILD ONSET DISEASES. Standard diagnostic method of sequencing for inherited retinal diseases) typically, 6/10 people can get diagnosis by genetic testing for eye stuLs) Can pick up single nucleotide changes from within the millions of DNA fragments. Next generation sequencing 1. Targeted sequencing – single genes/small regions of DNA 2. Whole Exome Sequencing – sequences protein coding regions 3. Whole genome sequencing – analysis of whole genome Other applications of genetic testing Carrier testing (can mutation be passed to children) Parental testing (detect genetic abnormalities in fetal DNA before birth) Newborn Screening Predictive testing (like to search for genes for things like breast cancer) Pharmacogenetic testing (how genetic makeups respond to diLerent medications) BENEFITS & CONSIDERATIONS FOR GENETIC TESTING FOR IRDs Beneﬁts: Accurate diagnosis and management plan Phytological relief Informs risk to others including children Informs eligibility to targeted therapies Improves upon research and mechanisms for research Considerations Uncertain results Access & cost Psychological impact if there is no current treatment Insurance applications LECTURE 8 – WEEK 3 – THE BIONIC EYE Lecture concept: THE BIONIC EYE 1. Camera – capture images and transmits data to an external, body worn processing unit 2. Data processed – and sent to implanted system via external wire 3. Implanted receiver – passes signals onto retinal implant 4. Implanted electrode array – stimulates retina 5. Electrical signals sent from retina via visual pathway to vision processing centres in the brain. ð The electrodes of the bionic eye take over the natural job of the photoreceptors. Ø If the eye is blown out and there is no optic nerve to innervate, better to directly attach to the brain centres, otherwise you’d target the eye: RETINAL PROSTHESES A. Epiretinal electrode – on top of retina B. Subretinal electrode – behind retina C. Suprachoroidal electrode – behind blood supply at back of retina (most done in Australia) -> In surgery for this one, you make an insertions at the side of the eye to input the electrodes. D. Intrascleral electrode – pocket within the sclera PATIENT OUTCOMES – From ﬁrst bionic eye tests – for patients with END STAGE retinitis pigmentosa, with only BARE LIGHT PERCEPTION VISION Ø Some improvement in central vision but the vision resulted In general, devices are good for: was not good enough to warrant - Improving light detection and the risky surgery orientation So…Cortical implant! (people who do - Allowing some independent not have an eye – bypass straight to the mobility brain) - Improved object detection (tabletop items) n Second sight > cortical - Best performance can often prosthesis in blind participants recognize large letters or shapes. - Aid in obstacle avoidance OPTOGENETICS – gene therapy Ø In RPE photoreceptors are degenerated (die), but the other retina cells are intact, and so one could innervate them to make them light responsive. Ø One takes a light sensitive protein from an algae, and then takes the gene and inserts it into the DNA of speciﬁc neurons in the brain, to allow for bipolar and ganglion cells to become light responsive themselves. The protein from the algae, is an ion channel that opens in response to blue light and allows for depolarisation. Ø What does this mean? => This means that you can cause neurons to ﬁre just by ﬂashing blue light!! LECTURE 9 – WEEK 4 – NON-INVASSIVE ASSESSMENT OF THE EYE Lecture concept: NON-INVASIVE ASSESSMENT OF THE EYE Binocular vision = Sharing of visual pathways from both eyes in separate hemispheres (left side of each eye is processed on the right hemisphere & right side of each eye is processed on the left hemisphere) This gives us the ability to locate things in space much better. MEASURING RETINASL / CORTICAL ACTIVITY FOLLOWING LIGHT STIMULATION – METHODS: (A) EOG (electro-oculogram) Purpose: Measures the electrical potential difference between the front and back of the eye (RPE). n How well the RPE is working (how well RPE can recycle ‘retinal’, how well RPE can absorb light & how well it can phagocytose outer segments of photoreceptors n Vitreous & cornea has the same potential as the retina, so an active electrode can go on the cornea itself or very close to it, as the bone and skin around the eye also have the same potential as the outside of the eye (inactive electrode can be placed on skin) n Two electrodes, one on left side of eye and one on right side to measure the electrical potential di`erence. ð Because the voltage diLerence is directional from back to front of eye, and by moving the eye (+- 30°), you can produce a measurable diLerencing signal. ð So, you can take a sample of the resting potential of the eye after eye movement (left and right), and then take the diNerence from those two extremes. ∴ diUerence potential = diUerence between left and right gaze movements (potential). Ø Light peak/dark trough Ø = normally >2.0 Ø Maximal K+ buLering capacity of the RPE Arden Ratio is smaller when… - There is not enough vitamin A - Diseases of RPE - Diabetes when RPE and retina are sick - Retinal dystrophies Best’s Disease = ratio 1:3 A-waves = come from photoreceptors ð The size of the peak is determined by how many photoreceptors are activated by the light. ð The change in peak to asymptote (the steepness of the drop in a to the peak in b) is determined by how quickly the rhodopsin/opsins initiate the closing of cGMP channels (for NA+) ð The faster the ampliﬁcation, the faster the cGMP channels are closed. B-waves = come from bipolar cells CONTINUATION of A & B waves… ð If the A wave is normal (from trough to main peak), but the B wave is not (slow and shallow), there may be a problem with either the mGluR6 receptors of the bipolar cells, or the ability of the bipolar cells to transmit the signals onward. MEASURING RETINASL / CORTICAL ACTIVITY FOLLOWING LIGHT STIMULATION – METHODS: (B) ERG (electro-retinogram) Purpose: Measures the electrical activity of the retina in response to light stimuli. The scotopic threshold response (STR) is a key measure in assessing the sensitivity and function of rod photoreceptors in the retina under low-light conditions. It is used in diagnosing and monitoring retinal diseases, particularly those affecting night vision and rod function. The term “threshold” refers to the minimal level of light intensity required to elicit a detectable electrical response from the retina. The STR measures the response at this threshold level, providing insights into the sensitivity and function of the rods. ð If you were to block the bipolar cell response to light stimuli (by using L-AP4) what it shows is that there is only the a-wave left (no-b- wave) = Bipolar cells are required for the b-wave ð Blocking ganglion cells with tetrodotoxin does not impact the correct wave of amplitude of (a) or (b) waves. ð Injury to fovea, also results in a loss of signal. (C) VER/VEP (visual evoked response) Purpose: Measures the electrical activity of the visual cortex in response to visual stimuli. Can be useful for: OCULAR ALBINISM: n Visual acuity in children n Amblyopia n Albinism n Intactness of visual pathway (stroke, MS e.g.) ð Normally, 50% GCs axons crosses over to the other side of hemisphere, with the L or R signal No decussation – there is a giving the biggest signal in the centre electrode: lateral bias, where the GCs are not crossing over to other hemisphere. ð No decussation (decussation = crossing of GC axons) occurs in albinism, where the L or R signal give larger signals on the electrode. This is known as lateralisation of VEP signal. Another thing one can do with VEP > Record Vep amplitude as stimulus switches form black and white Keep making the check smaller until there Is no more signal The spatial frequency is converted to visual acuity ð SO basically, it means to make check sizes smaller and smaller until the person cannot really di`erentiate visually between the black and white squares = a measure of visual acuity: SUMMARY OF OCULAR ELECTODIAGNOSTICS – FOR EXAM! *HAND HELD ERG – Can be used to detect seizures and genetic predisposition to seizures in children* EXAM QUESTION – WHICH WILL BE ON THE EXAM!!! LECTURE 10 – WEEK 4 – VIRAL VECTORS & RETINAL GENE THERAPY Lecture concept: What is gene therapy, and what can it do? Gene therapy: Experimental technique that uses genes to treat or prevent disease. Can be used for: Replace mutate disease-causing gene Viral Vector: Gene inactivation A viral vector is a virus that has been Introduce a new gene to help ﬁght disease genetically engineered to carry Generate secreted proteins therapeutic genes into target cells without causing disease. These EX VIVO => Gene therapy where targeted cells are vectors are used to introduce new or removed from body, altered, and then transplanting modiﬁed genes into cells to correct them back not patient. genetic disorders or treat diseases. IN VIVO => Gene therapy where direct systemic n Vectors are made safe by administration (intravenous) to the patient wherein removing viral genes. the targeted cells remain the body. Choice of virus to use depends on: How well virus is understood How well virus can target certain cell types è Small How safe virus is to use è Replication-deﬁcient (without an adenovirus/helper virus, this virus cannot replicate = good thing) TYEPS OF AAV CAPSID INJECTIONS è Single-stranded DNA genome (RETINAL) è Around 120 human and non-human serotypes. 1. Subretinal injection - within the ‘bleb’ that is created between the PROS: retina and vitreous ð AAV8 (green ﬂuorescent protein) – highlights both the RPE layer and photoreceptor layer. 2. Intravitreal injection – Into the vitreous ð AAV2 (green ﬂuorescent protein – CONSIDERATIONS: highlights ganglion cell layer) - Developing A Gene Therapy – Understand the biology of the disorder Develop treatment approach Test eLectiveness in biological models of disease Establish safety [CURRENT CHALLENGES FOR OCULAR THERAPY] è Introducing the right of AAV into the body è Correct for introducing AAV è Speciﬁcity and eLicacy of current vectors è Delivery: Subretinal vs Intravitreal è Pre-existing immunity (PEI) and acquired immunity è Large genes è Clinical outcome measures Hard to assess how much of defective/missing protein we need in humans, but, visual rescue of animal models of retinal degeneration with mutations in high expressing genes is possible. How could we improve this? ð Better vector design (promotor, improved AAV capsids, gene sequencing optimization ð Clinical trials comparisons n PDE6B gene therapy (treated mice at 9 days of age, able to recover most of eye cells like rods and ganglion cells, where an increase in the PDE6B gene allowed for retinal recovering) – Same thing was done in the rods of dogs. Natural history of disease Onset Progression Mutation-speciﬁc eLects Inter-individual diLerences ELect of environmental factors. n CNGB3 gene therapy (treatment for cones with CNGB3 gene – complete loss of colour vision which is rescued by introducing greater CNGB3 proteins) – mice that were treated at day 30, recovered colour vision as well as the regular wild- type mice’s vision – while when treated at day 150, the recovery was only minimal and not a normal physiological colour vision) è Achromatopsia: Rare genetic disorder that results in the complete or partial loss of colour vision. Development of better vectors / AAV biology Alternative serotypes / species Physico-chemical modiﬁcation Capsid engineering (mutagenesis; rational design; directed evolution; in silico design) FIRST APPROVED IN VIVO GENE THERAPY WAS FOR AN IRD è LUXTURNA (voretigene neparvovec) - LCA è RPE65 mutations è Rare congenital inherited eye disease (1 in 40,000) è Symptoms appear at birth or in ﬁrst few months è Severe vision loss / blindness è First FDA approved therapy è Gene located in 1993 > therapy only into fruition in 2016 LECTURE 11 – WEEK 4 – RETINAL MOSAIC & SAMPLING Lecture concept: How does the retina sample visual space, and how does this relate to the anatomical structure of photoreceptors & ganglion cells? ð Smallest population of S-cones (only 10% of all cones in retina, the rest is M & L) ð Spatial resolving capacity of retina / visual system will be related to photoreceptor spacing. ð Resolution limit is approx. 40 to 60 cycles / degree. ð At 30 cycles/degree vision is at ‘20/20’ which means great resolving capacity and visual acuity, at 3 cycles/degree vision is at ‘20/200’ which means much lower resolving capacity. Angular Subtense Angular subtense refers to the angle that an object or feature subtends (or covers) at the eye from a given distance. Essentially, it describes how large an object appears to be from the observer's viewpoint. Measurement: Angular subtense is measured in degrees (°), minutes (′), or seconds (″) of visual angle. One degree is divided into 60 minutes, and one minute is divided into 60 seconds. Example: If you are looking at a coin that is held at arm's length, the angular subtense of the coin would be the angle formed between the lines of sight from your eyes to the edges of the coin. An increased spatial resolution can be seen where there is the greatest density of cones (fovea). > increasing amounts of blur in the vision as you wane of the central axis (degrees) – (into side vision – we have poorer vision oU to the side). GRATINGS = Wave of vision that intersect and create ‘gratings’, course grating has lower spatial frequency while ﬁne grating produces much great spatial frequency. Course grating = black and white stripes, more extreme contrast needed. Fine grating = feint grey stripes, no extreme contrast between black and white needed to see. Retinal Periphery > beyond 5 eccentricity, suggests there to be a resolution limit in retinal periphery set by neuronal factors (retina/brain). RGCs pool signal from multiple photoreceptors in retinal periphery – should set resolution limit, therefore, the GCs instead of photoreceptors in the periphery speciﬁcally that are the limiting factor of our spatial resolution. Between the photoreceptors and the GCs, where there is the lowest density correlates to the factor which is impeding spatial resolution. However, it is actually the midget-GCs (parvocellular) which limit our vision (80-90% of RGCs) - Smaller dendritic ﬁelds - Smaller centre-surround receptor feilds - Complete coverage of all eccentricities ‘In the context of vision, aliasing refers to the distortion or misrepresentation of visual information due to limitations in the spatial resolution of the visual system. It occurs when the frequency or detail of visual stimuli exceeds the resolving capacity of the eye or visual system, leading to artifacts or misleading perceptions.’ Aliasing – Ø Results from inadequate sampling of continuous function (e.g under sampling) Ø Signal reconstructed from samples differs from original, continuous signal Ø Alias typically has lower spatial frequency & different (i.e. non- veridical) orientation. So, you will get weird tiger-stripe like vision when the spatial frequency is too high to be able to resolve, so a spatial frequency that is ABOVE THE NYQUIST LIMIT. Samling frequency must be at least twice the highest spatial frequency to adequately represent signal. - in central fovea, L- & M- cone density high enough to capture all the information that survives the blurring by eye's optics - blurring by eye's optics helps prevent aliasing - in retinal periphery (lower cone density), effect of aliasing reduced by the disarray in sampling mosaic = produces noisy pattern rather than plausible object - aliasing (and other) studies have revealed an S-cone spacing of 10 minutes of arc, and L- & M-cone spacing of 30 seconds of arc WHY ARE S-CONES SO SPARSE IN COMPARISON TO M/L CONES? è Chromatic aberration in eye blurs short-wavelength image è Sparse sampling is adequate to capture retinal image seen by S-cone mosaic è Blue bends better (refracts better) – eyes are a little bit short sighted for blue light (little bit out of focus on the retina), always produces ‘fuzzy’ image, so why would the eye want more of them (s-cones)?? That’s the logic ^. LECTURE 12– WEEK 5 – IMAGING RETINAL CELLS Lecture concept: How do we take images of the retina and what does this show us? THE TRANSPARENT RETINA GOLGI STAINING è Silver nitrate è Golgi & Cajal – 1887 è Argued whether photoreceptor to GC pathway was a single cell or parts of the same reticular network or nervous tissue. è Contrast is essential to see things. è In the living eye the options for increased contrast are more limited n Photoreceptors are the easiest to image (with RBCs being the easiest cell to image n Retinal pigment epithelium (RPE) – beneath photoreceptors, and are hard to image. FOR BEST IMAGING: Bigger pupils = better to image ^ This is because of the eLects of ocular aberrations, and the eLects of diLraction….SO, what do we do about this? There exists a deformable mirror which can compensate for ‘wavefront aberrations’. This is done by the mirror being wavy in shape, and so the refracted light which hits it (in wavy form) the wavy mirror, then re-bends the light to a straight light image: What now? > Ok so the light has been corrected by the deformable mirror, and those waves of light are then ‘imaged’ (photo of this light is taken by a camera), to determine what image the light is producing (in this case imaging the surface of the retina) S-L-M cone identiﬁcation can be made with a VERY time-consuming bleaching method. So, how do we actually capture an image of the retina which does not take forever? -> We can image CHANGE (imaging contrast of change). SPEED is important in taking images to get proper contrast of movement. Need temporal & spatial resolution. = Motion contrast image. SCANNING LASER MICROSCOPY / OPTHALMOSCOPY Scanning laser ophthalmoscopes (SLOs) are fundamentally the same as scanning laser microscopes (SLM) Only diLerence is that in SLO the optics of the eye serve as the objective lens and the fundus is always the sample. (1) In a SLM/SLO light is focused to a point on the sample of interest (2) light that scatters from that focused point is measured with a sensitive light detectors (PMT or APD). (3) To form an image, the light is continually recorded while the focused spot scans across the sample, generally in a raster pattern (4) Positional information from the scanners is combines with the intensity information from the detector to form the image. SCANNING/DESCANNING è The fact that the returning light is rendered stationary after descanning allows us to place a confocal aperture prior to the PMT detector. What is the point of this? è Because... we are trying to illuminate only one spot on the retina, only one place of illumination, and try to focus it on a very very small point. Then the light is collected, and you only collect the light that is focused on the CONFOCAL APETURE, which is a tiny little whole which is projected onto a detector.

OPTO30007 Notes SM2 2024 - Anatomy of the Eye - PDF

Document Details

Tags

Related

Summary

Full Transcript

Upgrade to continue