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Eye Physiology and Vision

Eye Physiology and Vision. Tabitha Schrufer PhD Candidate Penn State College of Medicine Dept of Cellular and Molecular Physiology. Outline. ๑ Overview of eye anatomy ๑ The iris and pupil ๑ The lens and accommodation ๑ The retina ๑ Photoreceptor function ๑ The visual pathway.

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Eye Physiology and Vision

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  1. Eye Physiology and Vision Tabitha Schrufer PhD Candidate Penn State College of Medicine Dept of Cellular and Molecular Physiology

  2. Outline • ๑Overview of eye anatomy ๑ The iris and pupil • ๑ The lens and accommodation ๑ The retina • ๑ Photoreceptor function • ๑ The visual pathway

  3. The Anatomy of the Eye • ๑Sclera • “white of the eye” • Dense fibrous connective tissue • Collagen and elastic fibers • Contains BVs and insertion site for extrinsic eye muscles • ๑Cornea • Thin, transporting epithelium • Continuous with the sclera, covering iris and pupil

  4. The Anatomy of the Eye • ๑Iris • “colored part” of the eye • Creates and adjusts the pupil • Contains BVs, pigment cells, loose connective tissue and pupillary muscle • Regulates amount of light • ๑Lens • Focuses image on the retina • Closely-packed concentric columnar cells

  5. The Anatomy of the Eye • ๑Ciliary Body • Ring of smooth muscle around lens • Regulates the shape of the lens for accommodation • ๑Suspensatory Ligaments • Attach the ciliary body to the lens

  6. The Anatomy of the Eye • ๑Retina • Lines the posterior part of the eye • Held in place by the vitreous • Contains light-sensitive photoreceptors • ๑Choroid • Pigmented to absorb excess light • Supplies the retina with nutrients and oxygen via blood supply

  7. The Iris • ๑Surface endothelium- Single layer continuous with the posterior lining of the cornea • ๑Stroma- closely packed radiating connective tissue • nerve filaments, BVs, lymph vessels and large, irregularly branched connective tissue cells • ๑Sphincter muscles- in the deep layers of the stroma, involuntary smooth muscle about 1mm in width • ๑Basement membrane- strong connective tissue support for the innermost layers of the iris • ๑Pigment layer- 2 rows of epithelial cells of a dark purple color

  8. Eye Color Distribution • ๑ Determined by multiple factors • Melanin content of iris pigment cells • Cellular density of iris stroma • Genetics/inheritance-2 major genes and 1 minor, EYCL1, EYCL2, EYCL3 • Sun exposure • ๑ Brown eyes- high amounts of melanin which serves to absorb light at the shorter wavelengths • ๑ Blue eyes- little to no melanin within the iris stroma; longer wavelengths are absorbed, shorter wavelengths reflected • ๑ Sunlight triggers melanin production in the eye, as it does to the skin

  9. Intrinsic Eye Muscles • ๑Parasympathetic activation by bright light causes constriction (miosis) • ๑Sympathetic activation in response to dim light causes dilation (mydriasis) • ๑An increase in light in one retina results constriction of its pupil (direct light response); it also causes identical constriction in the other eye (consensual light response) • ๑This response not only regulates the total amount of light that enters the eye, but affects the quality of the retinal image

  10. Basic Lenses and Refraction • ๑ The bending of light upon entering a substance of different index of refraction • ๑ The degree of refraction depends upon • Difference in indices • Angle the light enters • ๑ Simple convex lenses control refraction so that that converge on a distant surface (F) • ๑ Simple concave lenses control refraction so that the light diverges from it but appears to be originating from a point in front of the lens (F)

  11. Accommodation • ๑ The process of changing the shape of the lens while focusing on an object • ๑ At rest focused on distant objects, needs to increase focal power to focus on close objects • ๑ The ciliary muscles contract and release tension in the ligaments and the lens becomes rounder • ๑ The increased curvature increases the focal power and shifts the focal point closer to the eye

  12. Problems Associated with the Lens • ๑Presbyopia- • loss of accommodation with age • occurs as a result in the loss of elasticity of the lens • increases the near-point of accommodation • ๑Astigmatism- • uneven curvature of the refractive surfaces of the eye • point source of light cannot be brought to a precise focus on the retina, results in a blurry image

  13. Lens Correction Far-sightedness Near-sightedness

  14. The Retina • ๑ Displaced portion of the CNS contains the light-sensitive cells • ๑ Transduces light into neural signals and passes on the information to other regions of the brain • ๑ In addition to PRs contains 4 additional types of neurons that form an orderly intricate neural circuit • ๑ While ganglion cells communicate via action potentials, the PRs and other neurons communicate via graded synaptic potentials that are conducted electronically • ๑ Arranged with the light-sensing PRs at the very posterior edge of the eye where they are located near the PE and choroid which assists in regeneration

  15. The Retinal Anatomy • ๑Ganglion Cells • Axons generate the optic nerve • Send action potentials to the thalamus • 1x106 cells in the retina • ๑Amacrine Cells • Synapse within the inner layer of the retina • Interconnect bipolar cells and ganglion cells • Regulate communication between the ganglion cells and the photoreceptors

  16. The Retinal Anatomy • ๑Bipolar Cells • Directly connect PRs and ganglion cells in a radial direction • ๑Horizontal Cells • Synapse within the outer layer of the retina • Interconnect PRs with bipolar cells in a horizontal direction • Mediate interactions over a wide area of the retina

  17. The Retinal Anatomy • ๑Photoreceptors • Rods- detect amount of light present • Cones- detect color • Highest density of cones at the fovea • ๑Pigment Epithelium • Absorb excess light • Involved in photoreceptor regeneration

  18. The Fovea • ๑Central area of the retina with cone-mediated vision • ๑Neurons are displaced laterally to the fovea to min scattering • ๑Very high resolution vision, due to 1:1:1 ratio of PR:bipolar cell:ganglion cell • ๑At the periphery the ratio of PRs to ganglion cells is high, cover a large receptive field • ๑Cone levels fall to a very low density outside the fovea, where the rods take over • ๑Peripheral vision is mediated by both rods and cones

  19. Photoreceptor Population and Distribution • ๑ One type of rod monochromatic dark-adapted vision; rods outnumber cones 16:1 • ๑ 3 subtypes of cones (S, M, L) responsible for color-sensitive vision in brighter light • ๑ Elongated cells with synaptic terminals and inner segment and outer segments • Inner segment- nucleus and mitochondria, synthesizes rhodopsin, funnels photons to the outer segment • Outer segment- transduction site, last part to see the light, highly modified cilium • Contains 1000 disk membranes- flattened organelles pinched off from outer membrane that contain the rhodopsin

  20. Photoreceptor Function • ๑ Phototransduction involves a cascade of chemical and electrical events to detect, amplify, and signal a response to light • ๑ Use receptor potentials to carry the visual signal from the outer segment to their synapses • ๑ Occurs through the process of hyperpolarization, changing the voltage potential of the membrane to be more negative than its resting potential • ๑ Hyperpolarization directly modulates the rate of transmitter (glutamate) release from the PR onto its postsynaptic neurons • ๑ The receptors are set up to release more glutamate upon depolarization and less during hyperpolarization, which results in higher PR activity in the dark

  21. Photoreceptor Function • 1. In the dark, each PR produces a current that flows out of the inner and into the outer segment • Current carried by inwardly-directed Na ions in the outer segment and outwardly-directed K ions from the inner segment • Na flows through a nonselective cation channel, which light indirectly regulates, K flows through a K channel which light does not regulate • Na carries 90% of the dark current and 10% by Ca • 2. Na-K pumps in the inner segment removes the Na and imports K. A Na-Ca exchanger removes Ca from the outer segment • 3. Absorption of photons leads to closure of the nonselective cation channels in the outer segment, while the K channels remain open

  22. Photoreceptor Function • 4. The total conductance of the cell decreases as K continues to flow out of the cell, creating an outward current that causes the cell to hyperpolarize • ๑ The number of cation channels that close depends on the number of photons that are absorbed • ๑ The range if one rod’s sensitivity is from one to 1000 photons-cones are less sensitive, but they are faster and are unsaturatable

  23. Rhodopsin • ๑The photon is absorbed by rhodopsin, the light receptor molecule tightly-packed in the disk membranes • ๑One rod contains 10 billion rhodopsin molecules, which ensures an optimum photon capture rate • ๑Rhodopsin has 2 components • Retinal- Vitamin A aldehyde • Opsin- single polypeptide containing 7 transmembrane domains

  24. Transduction • 1. Kinked 11-cis retinal is covalently bound to opsin in the dark • 2. When retinal absorbs a photon it isomerizes to all-trans retinal which triggers opsin to form metarhodopsin II • 3. Metarhodopsin II activates transducin that carries the signal forward in the cascade and causes a reduction in Na conductance • 4. All-trans retinal separates from opsin in a process called bleaching which causes rhodopsin to change from red (rhodopsin) to yellow (opsin)

  25. Defects in Color Vision • ๑Dichromacy- lack of 1 type of normally-function cones • ๑Anomalous trichromacy- shift in the normal spectrum, typically from homologous recombination on the chromosome • Red-green color blindness- men that only have 1 copy of the L-cone gene typically have 3 copies of M, HR results in an L-M hybrid • ๑Monochromacy- lack of 2 of the 3 types of cones, very rare, less than 0.001% of the population • S-cone monochromacy- rare X-linked disorder in which the L and M cones are missing because of mutations on the X chromosome

  26. The Visual Pathway-CNS • ๑ Info about an image transmitted to the brain along the optic nerve • ๑ About 90% of the ganglion cell axons go to the lateral geniculate nucleus (LGN) in the thalamus • ๑ Another population sends info to both the superior colliculi in the midbrain, to control eye movements • ๑ A final population of ganglion cells sends info to the pretectum (pupillary reflex), and to several nuclei involved in the control of circadian rhythms and sleep • ๑ The optic nerves from both eyes meet and cross at the optic chiasm, at the base of the hypothalamus of the brain

  27. The Visual Pathway-CNS • ๑Info from both eyes is combined and splits according to the visual field • the right side of the visual cortex deals with the left half of the field of view from both eyes • the left side of the visual cortex deals with the right half of the field of view from both eyes • small region in the center processed by both halves of the brain • ๑The neurons of the LGN relay the visual image to the visual cortex which is located at the rear of the brain in the occipital lobe above the cerebellum • ๑The visual cortex is the most massive system in the human brain and is responsible for higher-level processing of the visual image

  28. Retinal Pathology

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