Retina PDF - Level 3 Semester 5 Module

Level 3 Semester 5 Module (CNS) Conjunctiva Covers the inner surface of the eyelids and the anterior surface of the eye. Membrane which produces mucous that lubricates the eye and prevents dryness. Pr...

Level 3 Semester 5 Module (CNS) Conjunctiva Covers the inner surface of the eyelids and the anterior surface of the eye. Membrane which produces mucous that lubricates the eye and prevents dryness. Protects the eye. Fibrous Tunic Vascular Tunic Vascular Tunic Ciliary Muscles Ciliary Processes Sensory Tunic Retina Optic Disc  Retina Fovea Centralis Other areas of Retina Contains only closely Contain mainly rods packed cones Provide night, dim light Provides acute color & peripheral vision vision in bright light Shades of grey only Macula Lutea Optic Disc Contains more widely Contains no receptors spaced cones Blind spot Normal Opthalmoscopic View of Eye Physician Information Histological layers of retina The Organization of the Retina Figure 17.6b, c Photoreceptors  Retina Pigmented Layer function Neural Layer 1. Absorbs light Contains photoreceptors 2. Carries out phagocytosis (rods and cones) for 3. Stores Vitamin A visual perception Contains bipolar cells & ganglion cells for visual impulse transmission  Retina: photoreceptors 100,000,000 rods 5,000,000 cones Cones Rods Fovea Periphery High light levels Low light levels Colors Monochromatic Good acuity Poor acuity  Cones Are located in macula lutea but are most highly concentrated in the fovea centralis. Are sensitive to bright light (daylight) situations in which light is very intense. Each cone synapses with a single bipolar cell which synapses with a single ganglion cell. The axons of ganglion cells form the optic nerve to conduct visual images to the brain. Provide acute (sharp) color images (vision).  Rods Most highly concentrated in the retina outside the macula lutea Many rods synapse with a single bipolar cell Many bipolar cells may synapse with a single ganglion cell which carries stimuli to brain More sensitive & function only in dim light, night and peripheral vision Images are blurry and only in shades of gray  Visual Pigments Composed of two components – Retinal - light absorbing molecule (made from Vitamin A) – Opsin (four types made from protein) Opsin combined with retinal = visual pigment OPSIN + RETINAL = Visual Pigment Depending on the type of opsin retinal is bound to, each of the four pigments will only absorb certain wavelengths of light.  Visual Pigments: RODS Retinal + Opsin = Rhodopsin (visual purple) Absorbs light throughout entire visible light spectrum (most sensitive to green) Functions only in dark, dim light & peripheral vision Light causes: 1. Retinal to change shape & 2. Separate from opsin causing nerve impulse Regenerate only in dark or dim light situations (Light) RHO DOPSIN OPSIN RETINAL Impulse  Visual Pigments: Cones Retinal + Red, Green or Blue Opsin = Red, Green or Blue visual pigments Each Opsin absorbs light only in specific area of the visible light spectrum to which it is sensitive, ie, red cones, green cones & blue cones Function only in bright light (daylight) Provide sharp color images (Light) Red Cone Red Opsin RETINAL Impulse Green Cone Green Opsin RETINAL Impulse  Retina 100,000,000 rods 5,000,000 cones 1,000,000 ganglion cells Convergence Low Convergence Cone-Fed Circuits Retinal Bipolar Cone ganglion cell cell High Convergence Rod-Fed Circuits Retina Bipolar Rod ganglion cell cell Convergence rod/cone cells The fovea centralis (central pit) INL Light ONL Foveola Human foveal pit  The fovea centralis (central pit) A depression in the center of the macula lutea 0.5mm in diameter. Its structure allows maximal degree of distinct vision than at any other part of the retina, this is because: 1- It is thinned out (0.1 mm) due to missing of some layers while the others are displaced to the sides, such arrangement allows the high rays to reach the cones directly without passing through various layers of the retina. 2- Contain only cones. Each foveal cone is connected to a single bipolar cell which is also connected to a single ganglion cell. Therefore, each cone is connected to optic nerve fibers (no convergence) and has a private pathway to the cerebral cortex allowing for a maximal degree of sharp vision. 3- The fovea is encircled by blood capillaries. the absence of blood vessels crossing the fovea also accounts for acute vision. 4- The pigmented layers of the retina is highly developed in the fovea. The pigments absorb light thus light reflection at this region is greatly prevented. In Darkness Photoreception-cont.  Electrophysiology of Vision Electric recording in Retinal cells: Rods & Cones: Hyperpolarization Bipolar cells: Hyper- & Depolarization Ganglion cells: Depolarizing potential  Light Change in photopigment Metarhodopsin II Activation of transducin Activation of phophodiesterase Decrease IC cyclic GMP Closure of Na channels Hyperpolarization of receptor Decrease release of synaptic tramitter Action potential in optic nerve fibres