Eye Lecture Notes PDF

The eye Dr. Liao Ping Objectives • • • • • Structures of eye Common eye diseases Photoreceptors and retina Visual pathway to the brain Visual field and injury analyse Gross Anatomy of the Eye • Cornea: glassy transparent external surface of the eye • Pupil: opening where light enters the eye •...

The eye Dr. Liao Ping Objectives • • • • • Structures of eye Common eye diseases Photoreceptors and retina Visual pathway to the brain Visual field and injury analyse Gross Anatomy of the Eye • Cornea: glassy transparent external surface of the eye • Pupil: opening where light enters the eye • Iris: sphincter muscle, gives color to eyes • Sclera: white of the eye • Optic nerve: bundle of axons from the retina Lacrimal glands and dry eye Three layers of the eye Outer: The sclera and cornea make up the exterior layers. Middle (uvea): iris, ciliary body, and choroid. Inner (nervous tunic): retina. Pink eye is an inflammation of the conjunctiva Light refraction and cornea function • Optics – Study of light rays and their interactions • Reflection – Bouncing of light rays off a surface • Absorption – Transfer of light energy to a particle or surface • Refraction – Bending of light rays from one medium to another Refraction of Light by the Cornea • Eye collects light, focuses on retina, and forms image. Accommodation by the Lens • Changing shape of lens provides extra focusing power. Eye relaxed Or cornea and/or lens are too curved Presbyopia • Gradual loss of the ability to focus on nearby objects. • An aging process. • May caused by:  Loss of elasticity of eye lens and becoming rigid  Weakening of ciliary muscle Cataract: clouding of the lens caused by Aging, UV, cigarette. Etc. Aqueous humor and glaucoma Strabismus斜视 Diplopia 复视 (double vision) Mild torticollis 斜颈 Conversion of light energy to neural activity Ophthalmoscopic Appearance of Retina Cross-Sectional Anatomy of the Eye pinhole principle Laminar Organization of the Retina • Inside-out layers • Light passes through ganglion cells and bipolar cells before reaching photoreceptors. Many nocturnal animals have reflective layer beneath photoreceptors. Microscopic Anatomy of the Retina • Direct (vertical) pathway – Ganglion cells  – Bipolar cells  – Photoreceptors M-type ganglion cell: large, 5% P-type ganglion cell: small, 90% nonM-nonP-type ganglion cell: 5% Photoreceptor Structure • Four main regions – Outer segment – Inner segment – Cell body – Synaptic terminal • Types of photoreceptors – Rods and cones Photoreceptor Structure—(cont.) • Rods: long, cylindrical outer segment with many disks, gray tones • Cones: shorter, tapering outer segment with fewer disks, three different pigments • Rods over 1000 times more sensitive to light than cones Regional Differences in Retinal Structure • Structure varies from fovea to retinal periphery. • Peripheral retina – Higher ratio of rods to cones – Higher ratio of photoreceptors to ganglion cells – More sensitive to weak light Regional Differences in Retinal Structure— (cont.) • Cross section of fovea: pit in retina where outer layers are pushed aside – Maximizes visual acuity • Central fovea: all cones (no rods) – Area of highest visual acuity Phototransduction in Rods Rhodopsin: light sensitive receptor protein consisting of retinal and opsin. Retinal: one form of Vit. A Phototransduction in rods – Light energy interacts with rhodopsin. • Produces a change in membrane potential – Analogous to activity at G-protein-coupled neurotransmitter receptor • Causes a change in second messenger Phototransduction in Rods • Dark current: Rod outer segments are depolarized in the dark because of steady influx of Na+. • Photoreceptors hyperpolarize in response to light. ON bipolar cells and OFF bipolar cells ON bipolar cells have G protein coupled glutamate receptor, hyperpolarize to glutamate stimulation. OFF bipolar cells have ionotropic glutamate receptor, hypopolarize to glutamate stimulation. ON bipolar cells in the dark In the dark, a photoreceptor (rod/cone) cell will release glutamate, which inhibits (hyperpolarizes) the ON bipolar cells and excites (depolarizes) the OFF bipolar cells. ON bipolar cells in the light Microscopic Anatomy of the Retina • Retinal processing also influenced by lateral connections – Horizontal cells • Receive input from photoreceptors and project to other photoreceptors and bipolar cells – Amacrine cells • Receive input from bipolar cells and project to ganglion cells, bipolar cells, and other amacrine cells Ganglion Cell Receptive Fields • Ganglion cells different in firing action potentials – ON-center and OFF-center ganglion cells – Responsive to differences in illumination Hermann grid and lateral inhibition Phototransduction in Cones • Similar process to rod phototransduction • Different opsins – Red (long wavelength), green (medium wavelength), blue (short wavelength) • Color perception – Contributions of blue, green, and red cones to retinal signal Mixing Colors • Mixing of red, green, and blue light causes equal activation of the three types of cones. • The perception of “white” results The Pupillary Light Reflex • Connections between retina and brain stem neurons that control muscles around pupil • Continuously adjusting to different ambient light levels • Consensual • Pupil similar to the aperture of a camera Dark and Light Adaptation Over minutes to an hour All-cone daytime vision All-rod nighttime vision • Factors in dark/light adaptation  Dilation of pupils in dark  Regeneration of unbleached rhodopsin in dark  Adjustment of functional circuitry, more rods activated in dark • Calcium’s role in light adaptation  Calcium concentration changes in photoreceptors  Indirectly regulates levels of cGMP channels  Light to dark, Ca2+ influx via cGMP channel is reduced, more cGMP is produced, open cGMP channel again. Visual pathway to brain • • • • Optic nerve Optic chiasma Optic tract Visual cortex The Visual Field • Amount of space viewed by retina when eye is fixated straight ahead • Visual field: 150-170o, Wiki: Nasal 60o + Temporal 107o, above 70o + below 80o Right and Left Visual Hemifields • Left hemifield projects to right side of brain. • Ganglion cell axons from nasal retina cross in optic chiasm. Visual Field Deficits from Lesions in the Retinofugal Projection Retinal Inputs to the relay center in thalamus Lateral geniculate nucleus (LGN) Nonthalamic Targets of the Optic Tract • Hypothalamus: role in biological rhythms, including sleep and wakefulness, e.g sunset. • Pretectum in midbrain: control size of the pupil • Superior colliculus in midbrain: orients the eyes in response to new stimuli—move fovea to objects of interest Light reflex Stereopsis Holly Bridge, 2016 Organization of the LGN • Inputs segregated by eye and ganglion cell type Outputs of the LGN • Primary visual cortex, also named Brodmann’s area 17, V1, striate cortex. Nonretinal Inputs to the LGN • Primary visual cortex provides 80% of the synaptic input to the LGN—role not clearly identified. – “Top–down” modulation may gate “bottom-up” input from LGN back to cortex. • Brain stem neurons provide modulatory influence on neuronal activity. Inputs to the Striate Cortex • Magnocellular LGN neurons project primarily to layer IVC. • Parvocellular LGN neurons project to layer IVC. • Koniocellular LGN axons make synapses primarily in layers I and III. Outputs of the Striate Cortex • Layer II, III, and IVB cells project to other cortical areas. • Layer V cells project to the superior colliculus and pons. • Layer VI cells project back to the LGN. Parallel Pathways • Magnocellular, blob, and parvo-interblob pathways Inputs to the Striate Cortex: enlarged macula Beyond the Striate Cortex • Dorsal stream – Analysis of visual motion and the visual control of action (where) • Ventral stream – Perception of the visual world and the recognition of objects (what) Visual Areas in Human Brain Visual perception Identifying and assigning meaning to objects Parallel processing and perception Groups of cortical areas contribute to the perception of color, motion, and object meaning.