Retinal Function & Visual Perception PDF

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Retinal function and visual perception Class Year 2 Semester 1 Course CNS Code CNS Title Retinal function and visual perception Lecturer Dr Omar Mamad (RCSI-IE): Dr. Colin Greengrass (RCSI-BH) Date...

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Retinal function and visual perception Class Year 2 Semester 1 Course CNS Code CNS Title Retinal function and visual perception Lecturer Dr Omar Mamad (RCSI-IE): Dr. Colin Greengrass (RCSI-BH) Date 30.11.2023 Learning objectives ALO374 Explain the processes of refraction, accommodation and visual acuity ALO375 Describe the characteristics of rods and cones ALO376 Explain the rhodopsin/transduction pathway in vision ALO377 Understand the basis for dark and light adaptation Describe the visual pathway and understand the basics of visual processing ALO378 in the cortex Overview of the Visual system Structure: (1) sensory organ (eye) Contains optical elements and photo (light) receptors (2) corresponding neural system in the brain Optic tract, nuclei & visual cortex Function: Detect and interpret photic stimuli Electromagnetic waves range between 400 and 750 nm Visible light Electromagnetic spectrum Higher Lower energy energy Electromagnetic spectrum  Structure of the eye (layer 1) Sclera (outer coat, connective tissue (white of eye), (layer 2) Choroid (nutritional layer & light absorbing pigments), (layer 3) Retina (photoreceptors & neurones) Iris: pupil diameter Cornea (most anterior Humours: aqueous (between Lens: focal length (ciliary contraction of radial & structure, transparent cornea & lens – produced by Pupil: changes amount of muscle contraction) Macula: longitudinal smooth muscle window, main light refraction ciliary), vitreous (behind lens light entering the eye region containing fovea fibres) (pigmentation point) – produced by retinal cells) (highest visual acuity) provides (eye’s colour)  Focal Point: Visual Optics & image formation The focal point in optics refers to the Light from close objects requires greater point where light rays converge after refraction than distant objects passing through a lens. In the context of the human eye, it is accommodation - process by which lens alters focal the point where light rays are length focused on the retina. Control: ciliary muscles distance vision (light is less divergent) need flattened lens → ciliary muscle relaxes: suspensory ligaments pull outwards near vision (light more divergent) more convex lens → ciliary muscle contracts: suspensory ligament tension reduced (mediated by parasympathetic) Image Formation on the Retina: The optical properties of the eye are responsible for forming an image of an object on the retina. Light waves from the object are directed towards the eye and need to be precisely focused. Focus and Refraction: For a clear image, light must be "focused" to a specific point on the retina. If light is not accurately focused, the image appears blurred. Distance and Divergence: When viewing objects closer than 6 metres, the light is still diverging as it reaches the eye. This divergence requires extra bending to focus the image properly. Refractive Power: The bending of light, known as refraction, is crucial for vision. The eye’s ability to bend light is referred to as its "refractive power." The eye adjusts its refractive power to focus light from objects at different distances. Visual Optics & Image Formation Visual Optics & image formation (2) Convex surfaces converge light rays to a focal point cornea (2/3) – convex shape, but cannot be altered lens (1/3) – biconvex, refractive power can be adjusted  Visual Optics & image formation (4) Image inversion: image on retina is inverted vertically and laterally part of image in top right produces image on bottom left of retina brain corrects this Visual Acuity Visual acuity refers to the sharpness or clarity of vision, which is the ability of the eye to see fine details. Ability to distinguish between two nearby points For clear vision, the focal point needs to be precisely on the retina. If the focal point falls before or behind the retina, it results in blurred vision. When the focal point is exactly on the retina, a person experiences maximum visual acuity. Snellen chart 20/200 Introduced by Herman Snellen (1862) Test of visual acuity Chart consists of letters of graduated sizes Chart is placed 20 feet away from the patient Distance label for smallest size of letter read is used 20/20 as measure of acuity The vision of a normal eye is 20/20 20/40 vision would be roughly equated to someone having “half” as good visual acuity as a person with normal visual acuity. Patients with poor visual acuity may be moved closer so that the distance changes, vision may then be recorded as e.g. 10/200.  Retina absence of large blood vessels Retina is thinner in Fovea than elsewhere because is marks the centre of the retina.  The retina: structure, cells & function Part of eye where image is focused Comprises several cell layers: 5) retinal ganglia cells  Fovea Region with highest visual acuity Fovea When looking directly at object image falls on fovea THE RETINA: STRUCTURE, CELLS & FUNCTION  Retina has ~130 million photoreceptors, 6 million bipolar cells, and 1 million ganglion cells. N.B. Horizontal and Amacrine cells contribute to signal integration. Ganglion cells are the only source of output from the retina, No other retinal cell type projects an axon through the optic nerve  Photoreceptor cells (1) Photoreceptors detect photons (quanta of electromagnetic radiation) Light energy is radiated in waves that have a characteristic wavelength Humans see in visible (VIS) part of the electromagnetic spectrum VIS spectrum constitutes ~400-700 nm of the spectrum Comprises the “ROYGBIV” portion Longer wavelengths interpreted as yellows/reds, shorter wavelengths blue/violet Rods – provide information on light intensity (not wavelength/colour) Cones – information on colour and provide sharper image ᷈ 100 million vs 3 million  Non-image-forming photoreceptors Intrinsically photosensitive retinal ganglion cells Circadian rhythms – timing system in the suprachiasmatic nucleus of hypothalamus Light is major entrainment mechanism but animals lacking rod and cone cells still entrain Non-image-forming photoreceptors discovered in early 2000s contribute to circadian rhythms, possibly pupillary light reflex Light-sensitive pigment is melanopsin (abs 470-480nm)  Photoreceptor cells (2) Distribution of rods & cones varies across retina Rods mainly in peripheral areas Cones have highest density in macula (fovea contains only cones) Outer segment: series of membranous plates (discs) – specialized receptor region Inner segment: mitochondria & other organelles Each photoreceptor synapses with bipolar cells Bipolar cells synapse with ganglion cells Ganglion axons form the optic nerve  Rods Region of synapse with bipolar cell High sensitivity to light Cell body can detect single photon Inner Low acuity segment ability to distinguish between two nearby points because many rods converge on same ganglion cell Do not contribute to Outer segment “colour” vision Discs No rods at fovea Discs containing photopigment  High rod density is necessary Regional differences in retinal structure for detecting very low levels of light – the rod pathway convergences a lot This means that visual acuity is high at the central retina and low at the periphery Central retina, few photoreceptors feed the information to the ganglion cell Peripheral retina, many photoreceptors provide input to Lower cone density is not a problem – the cone pathway ganglion cell. convergences very little and acuity is maintained  Phototransduction in rods Conversion of light to an electrical signal Photosensitive pigment rhodopsin on membranes of outer segment of photoreceptor cell (109 per rod cell!) chromophore (retinal ( small molecule derivative from vitamin A) combined with opsin) light triggers conformational change (activation) of rhodopsin rod then hyperpolarizes, altering release of NT (glutamate) to bipolar cells → increases firing of ganglion cell → signal sent down optic nerve Macula degeneration – leading cause of blindness in Western countries - Result of loss of photoreceptor cells Light Transduction 1.Structure of Rod Cells: 1. Rod cells containing stacks of disks, are rich in the photopigment rhodopsin. 2. Rhodopsin is made up of the protein opsin and a light-sensitive molecule called retinal (derived from Vitamin A). 2.Absorption of Light: 1. When light enters the eye, photons are absorbed by the retinal component of rhodopsin. 2. This absorption causes retinal to change from a bent shape (11-cis- retinal) to a straight shape (all-trans- retinal). 3.Activation of Rhodopsin: 1. The change in retinal's shape triggers a conformational change in the opsin protein, activating rhodopsin. SENSORY TRANSDUCTION Transduction Cascade: The activated rhodopsin then activates a G protein called IN ROD CELLS transducin. Transducin, in turn, activates an enzyme called phosphodiesterase (PDE). PDE lowers the concentration of cGMP (cyclic guanosine monophosphate) in the cell by converting it to GMP. SENSORY TRANSDUCTION IN ROD CELLS SENSORY TRANSDUCTION IN ROD CELLS Closing of Ion Channels Light Transduction Resetting the System: For the rod cell to be ready for another photon, the all- trans-retinal must be converted back to 11-cis- retinal, and the rhodopsin must be reformed. This process is part of the visual cycle and occurs in cells of the retinal pigment epithelium.  Phototransduction in rods Produced by a constitutively active guanylyl cyclase Darkness Rod Cells in the Dark: In the absence of light (dark conditions), rod cells are in a relatively depolarized state. This is due to the presence of cyclic guanosine monophosphate (cGMP), which keeps sodium (Na⁺) channels open, allowing a steady influx of Na⁺ ions. This depolarization leads to the continuous release of the neurotransmitter glutamate from the synaptic terminals of rod cells. Light Rod Cells in Light: When light hits the rod cells, it triggers a phototransduction cascade, leading to a decrease in cGMP levels. This decrease causes the Na⁺ channels to close, and the cell hyperpolarizes. As a result, the release of glutamate by the rod cells is reduced. IN BIPOLAR CELLS IN THE RETINA, GLUTAMATE IS Phototransduction in rods INHIBITORY 1 photon is capable of closing ~230 channels producing a ~2 % reduction in dark current!  Conveyance of signal from photoreceptor to ganglion cell Hyperpolarizing potential and reduction of NT release are graded according to light stimulus (brighter light→ more hyperpolarization) Glutamate released by photoreceptor cell ordinarily inhibits bipolar cell* Bipolar cell now depolarises Release of NT from bipolar cell increases which subsequently generates an AP in the ganglion cell *this is a simplification: photoreceptors synapse with two types of bipolar cell (“on-centre” and “off-centre”), and only the on- centre bipolar cell fires when the NT release from photoreceptor reduces) https://www.youtube.com/watch?v=GKaFjw8N8zQ On – fires in light; Off – doesn’t fire in light https://www.youtube.com/watch?v=1D_nIIevdzc  Characteristics of Cone Cells Low Sensitivity Require hundreds of photons for activation. Have less pigment and less signal amplification compared to rod cells. High Acuity Due to reduced convergence in wiring patterns, allowing for more detailed vision.  Colour vision & cones (2) Types of Cone Cells Three Classes Blue Cones (Short Wavelength): Peak absorption around 430 nm. Green Cones (Medium Wavelength): Peak absorption around 530 nm. Red Cones (Long Wavelength): Peak absorption around 600 nm. Opsin Variations: Each class of cone contains a different form of opsin (iodopsin), determining its sensitivity to specific wavelengths. COLOUR VISION & CONES - LIGHT ABSORPTION AND PERCEPTION Absorb Light in Different Spectrum Parts: Maximally sensitive to three distinct parts of the visual spectrum. Population Distribution: About 16% blue, 10% green, and 74% red cones in a typical human retina. With three overlapping pigments, incoming light stimulates each cone differently The brain can compare the relative stimulation of the three cones to decode the wavelength Colour blindness Inherited absence of one or more class of cone pigment - L and M cone pigment genes are on X chromosome X-linked, recessive (more common in men) Inability to distinguish between red & green  Scotopic vision is rod-mediated and lacks colour discrimination, as rods are Types of Vision more sensitive to light intensity rather than wavelength (colour). Vision Light Colour Sensitivity Photorecep Spatial Temporal Type Conditions Perception Acuity to Light tors Used Resolution Resolution Less Photopic Bright light Full colour High Cones High High sensitive Intermediate Limited Moderate Both rods Mesopic light Moderate Moderate Moderate colour sensitivity and cones (dawn/ dusk) Low light / More Scotopic Grayscale Low Rods Low Low Darkness sensitive  Eye’s sensitivity to light depends on amount of Dark Adaptation photosensitive pigment “available” Transition to Low Light Moving from high to low illumination levels requires visual adjustment. Light insufficient to activate cones Cones to Rods Initial reliance on cones is inadequate in dim light. Gradual activation of rod photoreceptors is necessary.  Eye’s sensitivity to light depends on amount of Dark Adaptation photosensitive pigment “available” Rod Rhodopsin Regeneration Rhodopsin, the photopigment in rods, must regenerate for optimal function. Full regeneration takes approximately 20-30 minutes. Scotopic Vision: Rod-mediated vision dominates, enabling vision in low illumination without colour discrimination. Vitamin A and Night Vision Essential for rhodopsin synthesis. Deficiency leads to impaired scotopic vision or "night blindness."  Eye’s sensitivity to light depends on amount of Light Adaptation photosensitive pigment “available” Transition to High Light: Moving from low to high illumination levels necessitates rapid ocular adjustment. Rod Saturation: Rod cells become quickly saturated and their photopigment, rhodopsin, bleaches in bright light.  Eye’s sensitivity to light depends on amount of Light Adaptation photosensitive pigment “available” Cone Recruitment: Visual processing shifts to cone photoreceptors, which are less sensitive to light and responsible for high-acuity colour vision. Cone-Driven Processing: The brain prioritizes cone-based information for detailed and colour vision. Adaptation Speed: Cone adaptation to increased light intensity occurs more rapidly than rod adaptation to darkness. Visual pathways Huge Complexity!!! auditory nerve contains ~30,000 fibres vs optic nerve ~1,000,000 Pupil reflexes  Light Detection Visual pathways Photoreceptor cells in the retina detect light and convert it into neural signals. Left Right Visual Action potential Visual field generated at GC field Optic Nerve Transmission Signals travel from the retina through the optic nerve. (In the thalamus) Optic Chiasma Crossing At the optic chiasma, fibres L R from the nasal retina cross while temporal fibres to the opposite brain continue ipsilaterally. hemisphere  LGN Processing Visual pathways Signals reach the lateral geniculate nucleus (LGN) of the thalamus, which relays visual information. Left Right Visual Action potential Visual field generated at GC field Cortical Interpretation The LGN sends processed signals to the primary visual cortex for initial interpretation. (In the thalamus) Field Processing Each brain hemisphere processes the opposite L R visual field, integrating the information for complete visual perception.  Visual pathways (PL) (M L) P Layers and M layers project to different areas of the primary visual cortex Visual analysis Hierarchical organization of the visual system This visual pathway diagram illustrates the parallel processing streams in visual perception, where the dorsal stream (involving M cells and related pathways) is concerned with "where" (spatial location and movement) aspects, and the ventral stream (involving P cells and related pathways) is concerned with "what" (shape, colour, and detail) aspects.  Further visual processing Parietal lobe: perception of movement Temporal lobe: perception of shape and colour Parietal Lobe - Dorsal Stream Function Dorsal Stream Function: Pathway from primary visual cortex to parietal lobe. Termed the "where" pathway. Processes spatial location, motion, and control of eye and arm movements. Importance in Object Location: Crucial for locating and grasping objects. Assists in navigating through the environment.  Parietal Lobe - Perception of Movement Role in Movement Perception: Key in perception and interpretation of movement. Integrates visual information with sensory data and motor commands. Essential for spatial awareness and navigation. Spatial Orientation and Coordination: Understands spatial relationships of objects. Aids in eye-hand coordination and visually guided movements. Focuses attention on spatial locations. Temporal Lobe - Perception of Shape and Colour Shape Recognition: Inferior temporal cortex critical for complex shape recognition. Neurons respond to size, shape, texture, and orientation. Colour Perception: Integrates colour information with shape and pattern recognition. Enhances perception of detailed and colourful scenes. Ventral Stream and Memory: "What" pathway for object recognition. Links to memory in recognizing and remembering objects and colours.  Visual pathways Ganglion cell axons form optic nerve (blind spot) Axons cross midline at optic chiasm - medial (nasal) decussate, temporal do not Signals from left visual field of each eye brought together on right side of brain (and vice versa) Tracts synapse in lateral geniculate nucleus (LGN) in thalamus Optic radiations then project to visual cortex (occipital lobe) - hundreds of millions of neurons participate in visual processing  Visual pathways from LGN, optic radiation projects different visual information (e.g. shape, movement) to different regions of visual cortex Cortical neurons fire when they receive a particular pattern of illumination (e.g. vertical/horizontal bar) Cortical processing (highly complex) (Re)-inversion of image LGN and cortex have topographical maps of the retina point for point (fovea is heavily overrepresented) Cortical Processing in Visual System Cortical Neurons and Pattern Recognition Neurons in the visual cortex respond to specific visual stimuli. Specialized as feature detectors for orientations and shapes (e.g., vertical/horizontal bars). Essential in early stages of visual processing. Image (Re)-Inversion in Visual Processing Processing of Visual Input Images on the retina are inverted and reversed. The brain re-inverts these images for correct perception. Inversion occurs through complex processes in the visual cortex. Topographical Maps in LGN and Cortex Lateral Geniculate Nucleus (LGN) Relay centre in the thalamus for visual information from the retina. Maintains a topographical map of the retina. Topographical Maps in LGN and Cortex Cortical Representation The visual cortex also preserves retinal topographical mapping. Topographical Maps in LGN and Cortex Foveal Overrepresentation Fovea, crucial for high-acuity vision, is heavily represented. Reflects the high density of photoreceptors in the fovea. Larger cortical area devoted to processing detailed central vision.  Human Physiology Medical Physiology Bibliography by Sherwood by Boron & Boulpaep