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  1. 1. 15 The Special Senses P A R T A
  2. 2. Palpebrae (Eyelids) Figure 15.1b
  3. 3. Lacrimal Apparatus Figure 15.2
  4. 4. Extrinsic Eye Muscles Figure 15.3a, b
  5. 5. Structure of the Eyeball Figure 15.4a
  6. 6. Anterior Segment Figure 15.8
  7. 7. Cornea <ul><li>Clear window in the anterior part of the eye that allows the entrance of light. </li></ul><ul><li>It is a major part of the light-bending apparatus of the eye </li></ul><ul><li>Covered by epithelial sheets on both faces (anterior and posterior) </li></ul><ul><ul><li>External layer – stratified squamous ET – for protection; merges with the ocular conjuntiva </li></ul></ul><ul><ul><ul><li>provide a smooth surface that absorbs oxygen and other needed cell nutrients that are contained in tears. </li></ul></ul></ul><ul><ul><ul><li>This layer is filled with thousands of tiny nerve endings that make the cornea extremely sensitive to pain when rubbed or scratched. </li></ul></ul></ul><ul><ul><ul><li>Moist and being nourished by tears </li></ul></ul></ul><ul><ul><li>Deep epithelial layer – This single layer of cells is located between the stroma and the aqueous humor. </li></ul></ul><ul><ul><li>Because the stroma tends to absorb water, the endothelium's primary task is to pump excess water out of the stroma (by having sodium pumps). </li></ul></ul><ul><ul><li>Without this pumping action, the stroma would swell with water, become cloudy, and ultimately opaque </li></ul></ul>
  8. 8. Cornea <ul><li>Bowman’s layer </li></ul><ul><ul><li>a tough layer that protects the corneal stroma, consisting of irregularly-arranged collagen fibers </li></ul></ul><ul><li>Stroma </li></ul><ul><ul><li>Located behind the external epithelium </li></ul></ul><ul><ul><li>A thick, transparent middle layer, consisting of regularly-arranged collagen fibers </li></ul></ul><ul><ul><li>It consists primarily of water (78 percent); layered protein fibers (16 percent) </li></ul></ul><ul><ul><li>that give the cornea its strength, elasticity, and form </li></ul></ul>
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  10. 10. Lens <ul><li>A biconvex, transparent, flexible, avascular structure that: </li></ul><ul><ul><li>Allows precise focusing of light onto the retina </li></ul></ul><ul><ul><li>Is composed of epithelium and lens fibers </li></ul></ul><ul><li>Lens is avascular because blood vessels interfere with transparency. </li></ul><ul><li>The lens depends entirely upon the aqueous and vitreous humors for nourishment. </li></ul><ul><li>Lens has 2 regions </li></ul><ul><ul><li>Lens epithelium – cuboidal cells found at the anterior surface of the lens. These cells differentiate into lens fibers </li></ul></ul><ul><ul><li>Lens fibers – cells filled with the transparent protein crystallin. These cells are packed in layers and contain no nuclei. </li></ul></ul><ul><ul><ul><li>New lens fibers are added continuously the lens enlarges, become denser, less elastic. </li></ul></ul></ul>
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  13. 13. Pupil Dilation and Constriction Figure 15.5
  14. 14. Sensory Tunic: Retina <ul><li>A delicate two-layered membrane </li></ul><ul><ul><li>Pigmented layer – the outer layer that absorbs light and prevents its scattering </li></ul></ul><ul><ul><li>Neural layer , which contains: </li></ul></ul><ul><ul><ul><li>Photoreceptors that transduce light energy </li></ul></ul></ul><ul><ul><ul><li>Bipolar cells and ganglion cells </li></ul></ul></ul><ul><ul><ul><li>Amacrine and horizontal cells </li></ul></ul></ul>
  15. 15. The Retina: Ganglion Cells and the Optic Disc <ul><li>Ganglion cell axons : </li></ul><ul><ul><li>Run along the inner surface of the retina </li></ul></ul><ul><ul><li>Leave the eye as the optic nerve </li></ul></ul><ul><li>The optic disc: </li></ul><ul><ul><li>Is the site where the optic nerve leaves the eye </li></ul></ul><ul><ul><li>Lacks photoreceptors (the blind spot) </li></ul></ul>
  16. 16. The Retina: Photoreceptors <ul><li>Rods : </li></ul><ul><ul><li>Respond to dim light </li></ul></ul><ul><ul><li>Are used for peripheral vision </li></ul></ul><ul><li>Cones : </li></ul><ul><ul><li>Respond to bright light </li></ul></ul><ul><ul><li>Have high-acuity color vision </li></ul></ul><ul><ul><li>Are found in the macula lutea </li></ul></ul><ul><ul><li>Are concentrated in the fovea centralis </li></ul></ul>
  17. 17. Functions of the retinal cell type <ul><li>Horizontal cells – converge signals from several receptors: “decide” how many receptors each ganglion “see”. </li></ul><ul><li>Bipolar cells . Connect the receptor to ganglion cells </li></ul><ul><li>Amacrine cells process aspects of light information such as motion, contrast </li></ul><ul><li>Ganglion cells encode light information within action potentials to be processed and reconstructed by the visual cortex via the LG </li></ul><ul><li>RETINA PRESENTATION FROM WEBSITE </li></ul><ul><li> </li></ul>
  18. 18. Light <ul><li>Electromagnetic radiation – all energy waves from short gamma rays to long radio waves </li></ul><ul><li>Our eyes respond to a small portion of this spectrum called the visible spectrum </li></ul><ul><li>Different cones in the retina respond to different wavelengths of the visible spectrum </li></ul>
  19. 19. Light Figure 15.10 The wavelength is the distance between repeating units of a wave pattern
  20. 20. Refraction and Lenses <ul><li>When light passes from one transparent medium to another its speed changes and it refracts (bends) </li></ul><ul><li>Light passing through a convex lens (as in the eye) is bent so that the rays converge (join) to a focal point </li></ul><ul><li>When a convex lens forms an image, the image is upside down and reversed right to left </li></ul>
  21. 21. Focusing Light on the Retina <ul><li>Pathway of light entering the eye: cornea, aqueous humor, lens, vitreous humor, and the neural layer of the retina to the photoreceptors </li></ul><ul><li>Light is refracted : </li></ul><ul><ul><li>At the cornea </li></ul></ul><ul><ul><li>Entering the lens </li></ul></ul><ul><ul><li>Leaving the lens </li></ul></ul><ul><li>The lens curvature and shape allow for fine focusing of an image </li></ul>
  22. 22. Focusing Light on the Retina <ul><li>In the normal resting state : </li></ul><ul><ul><li>our ciliary muscle is relaxed </li></ul></ul><ul><ul><li>the elastic lens tends to become thick </li></ul></ul><ul><ul><li>aqueous & vitreous humour push outward on the sclerotic coat </li></ul></ul><ul><ul><li>ligaments become extended / tensed </li></ul></ul><ul><ul><li>lens pulled into a thin shape </li></ul></ul>
  23. 23. Focusing for Distant Vision <ul><li>Light from a distance needs little adjustment for proper focusing </li></ul><ul><li>Far point of vision – the distance beyond which the lens does not need to change shape to focus (20 ft.) or: </li></ul><ul><li>The object distance at which the eye is focused with the eye lens in a neutral or relaxed state . </li></ul>Figure 15.13a
  24. 24. Focusing Light on the Retina - short focal length <ul><li>contraction of ciliary muscle </li></ul><ul><li>distance between edges of ciliary body decreases </li></ul><ul><li>relaxation of suspensory ligament </li></ul><ul><li>lens becomes thicker </li></ul><ul><li>focal length shortens </li></ul><ul><li>light rays converge earlier; image formed on retina </li></ul>
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  26. 26. Focusing for Close Vision <ul><li>Close vision requires: </li></ul><ul><ul><li>Accommodation – changing the lens shape by ciliary muscles to increase refractory power </li></ul></ul><ul><ul><li>Constriction – the pupillary reflex constricts the pupils to prevent divergent light rays from entering the eye </li></ul></ul><ul><ul><li>Convergence – medial rotation of the eyeballs toward the object being viewed so both eye are focused on the object </li></ul></ul>
  27. 27. Focusing for Close Vision Figure 15.13b LENS AND IRIS PRESENTATION FROM WEBSITE
  28. 28. Problems of Refraction <ul><li>Emmetropic eye – normal eye with light focused properly </li></ul><ul><li>Myopic eye ( nearsighted ) – the focal point of far object is in front of the retina . Myopic people see close objects clearly but distant objects are blurred </li></ul><ul><ul><li>Corrected with a concave lens </li></ul></ul><ul><li>Hyperopic eye ( farsighted ) – the focal point is behind the retina. See far objects clear but not close ones </li></ul><ul><ul><li>Corrected with a convex lens </li></ul></ul>
  29. 29. Problems of Refraction Figure 15.14a, b
  30. 30. Photoreceptors <ul><li>Photoreceptors are modified neurons </li></ul><ul><li>They absorb light and generate chemical or electrical signals </li></ul><ul><li>2 cell types – rods and cones – that produce visual images </li></ul><ul><ul><li>Outer segment - points towards the wall of the eye (towards the pigmented layer of the retina) </li></ul></ul><ul><ul><li>Inner segment – facing the interior </li></ul></ul><ul><ul><li>2 segments are separated by a narrow constriction - cilium </li></ul></ul><ul><li>The inner segment connects to the cell body which is continuous of the inner fiber that has the synaptic terminal </li></ul>
  31. 31. Photoreceptors <ul><li>Photoreceptors are modified neurons </li></ul><ul><li>They absorb light and generate chemical or electrical signals </li></ul><ul><li>2 cell types – rods and cones – that produce visual images </li></ul>
  32. 32. <ul><li>Outer segment - points towards the wall of the eye (towards the pigmented layer of the retina) </li></ul><ul><li>Inner segment – facing the interior </li></ul><ul><li>2 segments are separated by a narrow constriction - cilium </li></ul><ul><li>The inner segment connects to the cell body which is continuous of the inner fiber that has the synaptic terminal </li></ul>Figure 15.15a, b
  33. 33. Photoreception: Functional Anatomy of Photoreceptors <ul><li>Photoreception – process by which the eye detects light energy </li></ul><ul><li>Rods and cones contain visual pigments ( photopigments ) </li></ul><ul><ul><li>Embedded in areas of the plasma membrane that is arranged in a stack of disc-like infoldings </li></ul></ul><ul><ul><li>change shape as they absorb light </li></ul></ul><ul><ul><li>This foldings increase surface area that is available for trapping light </li></ul></ul><ul><ul><li>In rods – the discs are discontinuous while in the cones they are continuous </li></ul></ul>
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  35. 35. Photoreceptors - Rods functional characteristics <ul><li>Sensitive to dim light and best suited for night vision </li></ul><ul><li>Absorb all wavelengths of visible light </li></ul><ul><li>Perceived input is in gray tones only </li></ul><ul><li>Sum of visual input from many rods feeds into a single ganglion cell </li></ul><ul><li>Results in fuzzy and indistinct images </li></ul>
  36. 36. Photoreceptors - Cones functional characteristics <ul><li>Need bright light for activation (have low sensitivity) </li></ul><ul><li>Have pigments that furnish a vividly colored view </li></ul><ul><li>Each cone synapses with a single ganglion cell </li></ul><ul><li>Vision is detailed and has high resolution </li></ul>
  37. 37. Chemistry of Visual Pigments <ul><li>Retinal is a light-absorbing molecule </li></ul><ul><ul><li>Combines with proteins called opsins to form 4 types of visual pigments </li></ul></ul><ul><ul><li>Similar to and is synthesized from vitamin A. </li></ul></ul><ul><ul><ul><li>Vitamin A is stored in the liver and transported by the blood to the cells of the pigmented layer (local reservoir of vitamin A) </li></ul></ul></ul><ul><ul><li>Retinal has two 3D forms/isomers: </li></ul></ul><ul><ul><ul><li>11- cis – a bent structure when connected to opsin </li></ul></ul></ul><ul><ul><ul><li>all- trans – when struck by light and change the shape of opsin to its active form </li></ul></ul></ul><ul><ul><ul><li>Transforming fro 11-cis to all-trans is the only light dependent stage </li></ul></ul></ul><ul><li>Isomerization of retinal initiates electrical impulses in the optic nerve </li></ul>
  38. 38. <ul><li>The visual pigment of rods is called rhodopsin – a deep purple pigment </li></ul><ul><ul><li>Each molecule consists of two major parts - opsin and 11- cis retinal </li></ul></ul><ul><li>Rhodopsin molecules are arranged in a single layer in the membrane of each of the discontinuous discs in the outer segment </li></ul><ul><li>Rhodopsin is formed and accumulates in the dark </li></ul>Excitation of Rods Figure 15.15a, b
  39. 39. Excitation of Rods <ul><li>Light phase </li></ul><ul><li>When rhodopsin absorb light retinol is changed to its all-trans isomer </li></ul><ul><li>Rhodopsin breaks down into all-trans retinal + opsin (this process is called bleaching of the pigment ) </li></ul><ul><li>Dark phase </li></ul><ul><li>Vitamin A oxidized to the 11-cis retinal form and combined with opsin to form rhodopsin. </li></ul>
  40. 40. CH 3 C C H H H 2 C H 2 C C C CH 3 H CH 3 C H C CH 3 C H C H C H C CH 3 C O H C H C C H H H 2 C H 2 C H 3 C C C C CH 3 CH 3 H C H C C O C H C H C C C H C CH 3 H CH 3 H Oxidation Rhodopsin Opsin All -trans retinal – 2H +2H Reduction Vitamin A Regeneration of the pigment: Slow conversion of all- trans retinal to its 11- cis form occurs in the pig- mented epithelium; requires isomerase enzyme and ATP. Dark Light 11- cis retinal All- trans isomer 11- cis isomer Bleaching of the pigment: Light absorption by rhodopsin triggers a series of steps in rapid succession in which retinal changes shape (11- cis to all- trans) and releases opsin. Figure 15.16
  41. 41. Excitation of Cones <ul><li>Visual pigments in cones are similar to rods (retinal + opsins) </li></ul><ul><li>Cones are less sensitive – need more light to be activated </li></ul><ul><li>There are three types of cones: </li></ul><ul><ul><li>Blue – wave length 420nm, </li></ul></ul><ul><ul><li>Green – wave length 530nm, </li></ul></ul><ul><ul><li>Red – 560nm </li></ul></ul><ul><li>The absorption spectra overlap giving the hues - activation of more than one type of cone </li></ul><ul><li>Method of excitation is similar to rods but the cones need higher-intensity (brighter) light because they are less sensitive </li></ul>COLOR PRESENTATION FROM WEBSITE
  42. 43. Phototransduction <ul><li>The outer segments of the photoreceptor has ligand-regulated sodium gates that bind to cGMP on the intracellular side. </li></ul><ul><li>In the dark, cGMP opens the gate and permits the inflow of sodium which reduces the membrane potential from -70mv to -40mv </li></ul><ul><li>This depolarized current is called the dark current and it results in in continuous NT (glutamate) release by the photoreceptors in the synapse with the bipolar cells </li></ul><ul><li>Light stops the dark current </li></ul>
  43. 44. Phototransduction <ul><li>Light energy splits rhodopsin into all- trans retinal, releasing activated opsin </li></ul><ul><li>The freed opsin activates the G protein transducin </li></ul><ul><li>Transducin catalyzes activation of phosphodiesterase (PDE) </li></ul><ul><li>PDE hydrolyzes cGMP to GMP and releases it from sodium channels </li></ul><ul><li>Without bound cGMP, sodium channels close but potassium channels in the outer segment remain open </li></ul><ul><li>The photoreceptor membrane hyperpolarizes, and neurotransmitter cannot be released </li></ul>
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  45. 46. Phototransduction Figure 15.18 TRUNSDUCTION PRESENTATION FROM WEBSITE
  46. 47. Phototransduction <ul><li>Photoreceptors do not generate action potential and neither do the bipolar cells </li></ul><ul><li>they generate graded potential </li></ul><ul><li>The ganglion cells generate action potential </li></ul>
  47. 48. Signal Transmission in the Retina
  48. 49. Adaptation to bright light (going from dark to light) <ul><li>As long as the light is low intensity, relatively little amount of rhodopsin is bleached. </li></ul><ul><li>In high intensity light, rhodopsin is bleached as fast as it is re-formed </li></ul><ul><li>Going from dark/dim light to light - first we see white light because the sensitivity of the retina is “set” to dim light </li></ul><ul><li>Both rods and cones are strongly stimulated and large amounts of the pigments are broken, producing a flood of signals that are responsible for the white light </li></ul><ul><li>Rods system is turned off and the cones system adapts </li></ul><ul><li>By switching from the rod to the cone system – visual acuity is gained </li></ul>
  49. 50. Adaptation to dark <ul><li>Initially we do not see nothing </li></ul><ul><li>Cones stop functioning in low light </li></ul><ul><li>Rhodopsin accumulates in the dark and retinal sensitivity is restored </li></ul><ul><li>When we move from a lit room to a dark room, we cannot see clearly, because: </li></ul><ul><li>It takes about 20-30 minutes for enough rhodopsin to reform for us to see properly </li></ul>
  50. 51. Visual Pathways <ul><li>Axons of retinal ganglion cells form the optic nerve </li></ul><ul><li>Medial fibers of the optic nerve decussate at the optic chiasm </li></ul><ul><li>Most fibers of the optic tracts continue to the thalamus </li></ul><ul><li>Other optic tract fibers end in superior colliculi in the midbrain (initiating visual reflexes) </li></ul><ul><li>Optic radiations travel from the thalamus to the visual cortex </li></ul>