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THE EYE

THE EYE. A Specialized Organ for Photoreception. ACCESSORY EYE STRUCTURES. Eyebrows Shade Protection from sweat and other substances. Eyelids ( Palpebrae ) Protection from foreign objects Moisten the eyes (blinking) Levator palpebrae superioris muscles raise the eyelids.

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THE EYE

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  1. THE EYE A Specialized Organ for Photoreception

  2. ACCESSORY EYE STRUCTURES • Eyebrows • Shade • Protection from sweat and other substances • Eyelids (Palpebrae) • Protection from foreign objects • Moisten the eyes (blinking) • Levatorpalpebraesuperioris muscles raise the eyelids • Palpebral Fissure • Space between upper and lower eyelids

  3. ACCESSORY EYE STRUCTURES • Medial and Lateral Canthi • Eye angles • Caruncle • Fleshy, elevated area in medial canthus • Contains sebaceous and sweat glands • Eyelashes • Reflex blinking when touched • Richly innervated

  4. ACCESSORY EYE STRUCTURES • Tarsal or Meibomian Glands • Embedded in eyelids • Openings posterior to eyelashes • Secrete oil to lubricate eyelids • Ciliary Glands • Small sebaceous glands • Between hair follicles of eyelashes

  5. ACCESSORY EYE STRUCTURES • Conjunctiva • Mucous membrane • Lines inside of each eyelid (palpebral conjunctiva) • Covers anterior surface of eye except over cornea (bulbar or ocular conjunctiva) palpebral conjunctiva ocular conjunctiva

  6. ACCESSORY EYE STRUCTURES lacrimal gland • Lacrimal Apparatus • Lacrimal gland + ducts that drain tears into nasal cavity • Releases dilute saline solution onto eyes • Contains mucus, antibodies, lysozyme • Solution drains via: • Lacrimal puncta lacrimal canals  lacrimal sac  nasolacrimal duct  nasal cavity lacrimal sac lacrimal canal nasolacrimal duct

  7. EXTRINSIC EYE MUSCLES • Superior Rectus • Moves eyes superiorly • Inferior Rectus • Moves eyes inferiorly • Lateral Rectus • Moves eyes laterally • Inferior Oblique • Moves eyes up and out [R] Lateral View

  8. EXTRINSIC EYE MUSCLES • Superior Oblique • Moves eyes down and out • Medial Rectus • Moves eyes medially Superior View

  9. EYE MUSCLE INNERVATION • (LR6SO4)O3 • Lateral Rectus Cranial nerve VI (abducens) • Superior Oblique Cranial nerve IV (trochlear) • All Others Cranial nerve III (oculomotor)

  10. STRUCTURE of the EYE FIBROUS TUNIC Sclera Cornea

  11. STRUCTURE of the EYE VASCULAR TUNIC Choroid Ciliary body Iris

  12. STRUCTURE of the EYE SENSORY TUNIC • Outer pigmented • Inner neural layer Retina

  13. STRUCTURE OF THE EYE Sclera Ora serrata Choroid Suspensory ligament Cornea Pupil Iris Ciliary body

  14. STRUCTURE of the EYE Anterior Segment Anterior chamber Lens Posterior chamber Vitreous humor

  15. STRUCTURE of the EYE Optic disc Optic nerve Optic disc Fovea centralis Macula lutea

  16. STRUCTURE of the EYE Canal of Schlemm Central artery & vein

  17. PHYSIOLOGY OF VISION: REFRACTION • Refraction • Bending of light rays • Light rays change speed as they pass through substances of different densities • Light is refracted in 3 areas of the eye: • Cornea • Entrance of lens • Exit of lens Close vision Distant vision

  18. PHYSIOLOGY OF VISION: REFRACTION lens bulges • Lens shape can change • Can be used to refract light to a specific distance • Allows light to focus on retina • No focusing is necessary for distant objects (over 6m away) Close vision lens flattens Distant vision

  19. FOCUSING FOR NEAR VISION • Accommodation • Ciliary muscle contracts • Suspensory ligaments release tension on lens • Lens thickens and bulges to focus image on retina • Constriction of Pupils • Constrictor muscle in iris makes pupil smaller • Focuses light in a finer point • Convergence of Eyeballs • Medial rectus muscles contract • Each eye moves medially • Focuses on near object

  20. VISION: DISORDERS OF REFRACTION • Emmetropia • Normal vision • Myopia • Nearsightedness • Object focuses in front of the retina • Eyeball is too long • Hyperopia • Farsightedness • Object focuses behind the retina • Eyeball is too short

  21. PHOTORECEPTION: THE RETINA pigmented layer • Pigmented Layer • Outer layer of retina • Dark pigment to prevent light from scattering • Neural Layer • Inner layer of retina • Contains photoreceptors • Modified neurons called rods and cones • Composed of three layers neural layer

  22. THE RETINA: INNER NEURAL LAYER • Three Layers • Photoreceptor layer • Bipolar cell layer • Ganglion cell layer

  23. THE RETINA: INNER NEURAL LAYER • Photoreceptor Layer • Rods and cones adjacent to the outer pigmented layer cone • Rods • More numerous • Used in dim light • Used for peripheral vision • Do not detect color rod • Cones • Less numerous • Provide color vision • Used for acute vision Light

  24. NEURAL LAYER: RODS AND CONES • Rods and Cones • Contain an inner and outer segment • Outer segment • Receptor region • Contains visual pigments inner segment outer segment

  25. NEURAL LAYER: BIPOLAR CELLS • Bipolar Cells • Just inside the photoreceptor cell region • Pass impulse from the receptor cells to the inner ganglion cells Light

  26. NEURAL LAYER: GANGLION CELLS • Ganglion Cells • Innermost retinal layer • Generate an action potential in response to photoreceptor and bipolar cells • Action potential travels to optic nerve (composed of axons from these cells) Light

  27. VISUAL PIGMENTS • Retinal • Light absorbing molecule • Made from Vitamin A • Combines with proteins call opsins to form 4 types of visual pigments • When bound to opsin, retinal is bent • When light strikes retinal it causes the molecule to straighten and release opsin • Series of reactions result in electrical impulses down the optic nerve

  28. PHOTODISSOCIATION OF RHODOPSIN Retinal 1 Light 4 2 Rhodopsin active site 3 Energy

  29. STIMULATION OF PHOTORECEPTORS • Excitation of Rods • Visual pigment in rods is rhodopsin (retinal + a form of opsin) • Stored in discs in outer segments of rods • Light strikes rhodopsin  retinal changes shape  releases opsin • Triggers a reaction that initiates an impulse • Retinal is converted back to its original form in the pigmented layer of the retina • Rhodopsin is reformed when the retinal attaches to opsin again

  30. STIMULATION OF PHOTORECEPTORS • Excitation of Cones • 3 types of Cones • Each contains a pigment with a different opsin • Each pigment is sensitive to a different wavelength of light • Each detects a different color of light (red, green and blue) • Breakdown and regeneration of visual pigments in the cones is the same as for rhodopsin • Threshold for activation of cones is much higher so they respond to intense light

  31. VISION DISORDERS • Night Blindness • Nyctalopia • Caused by impairment of rod function • Most commonly caused by Vitamin A deficiency • Color Blindness • Due to lack of one or more cone types at birth • Most commonly red/green color blindness

  32. VISUAL PATHWAY Left eye Right eye Optic nerve Optic chiasma Optic tract Lateral geniculate body of thalamus Optic radiations Visual cortex

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