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  • 1. 15 The Special Senses P A R T A
  • 2. Palpebrae (Eyelids) Figure 15.1b
  • 3. Lacrimal Apparatus Figure 15.2
  • 4. Extrinsic Eye Muscles Figure 15.3a, b
  • 5. Structure of the Eyeball Figure 15.4a
  • 6. Anterior Segment Figure 15.8
  • 7. Cornea
    • Clear window in the anterior part of the eye that allows the entrance of light.
    • It is a major part of the light-bending apparatus of the eye
    • Covered by epithelial sheets on both faces (anterior and posterior)
      • External layer – stratified squamous ET – for protection; merges with the ocular conjuntiva
        • provide a smooth surface that absorbs oxygen and other needed cell nutrients that are contained in tears.
        • This layer is filled with thousands of tiny nerve endings that make the cornea extremely sensitive to pain when rubbed or scratched.
        • Moist and being nourished by tears
      • Deep epithelial layer – This single layer of cells is located between the stroma and the aqueous humor.
      • 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).
      • Without this pumping action, the stroma would swell with water, become cloudy, and ultimately opaque
  • 8. Cornea
    • Bowman’s layer
      • a tough layer that protects the corneal stroma, consisting of irregularly-arranged collagen fibers
    • Stroma
      • Located behind the external epithelium
      • A thick, transparent middle layer, consisting of regularly-arranged collagen fibers
      • It consists primarily of water (78 percent); layered protein fibers (16 percent)
      • that give the cornea its strength, elasticity, and form
  • 9.
  • 10. Lens
    • A biconvex, transparent, flexible, avascular structure that:
      • Allows precise focusing of light onto the retina
      • Is composed of epithelium and lens fibers
    • Lens is avascular because blood vessels interfere with transparency.
    • The lens depends entirely upon the aqueous and vitreous humors for nourishment.
    • Lens has 2 regions
      • Lens epithelium – cuboidal cells found at the anterior surface of the lens. These cells differentiate into lens fibers
      • Lens fibers – cells filled with the transparent protein crystallin. These cells are packed in layers and contain no nuclei.
        • New lens fibers are added continuously the lens enlarges, become denser, less elastic.
  • 11.
  • 12.
  • 13. Pupil Dilation and Constriction Figure 15.5
  • 14. Sensory Tunic: Retina
    • A delicate two-layered membrane
      • Pigmented layer – the outer layer that absorbs light and prevents its scattering
      • Neural layer , which contains:
        • Photoreceptors that transduce light energy
        • Bipolar cells and ganglion cells
        • Amacrine and horizontal cells
  • 15. The Retina: Ganglion Cells and the Optic Disc
    • Ganglion cell axons :
      • Run along the inner surface of the retina
      • Leave the eye as the optic nerve
    • The optic disc:
      • Is the site where the optic nerve leaves the eye
      • Lacks photoreceptors (the blind spot)
  • 16. The Retina: Photoreceptors
    • Rods :
      • Respond to dim light
      • Are used for peripheral vision
    • Cones :
      • Respond to bright light
      • Have high-acuity color vision
      • Are found in the macula lutea
      • Are concentrated in the fovea centralis
  • 17. Functions of the retinal cell type
    • Horizontal cells – converge signals from several receptors: “decide” how many receptors each ganglion “see”.
    • Bipolar cells . Connect the receptor to ganglion cells
    • Amacrine cells process aspects of light information such as motion, contrast
    • Ganglion cells encode light information within action potentials to be processed and reconstructed by the visual cortex via the LG
  • 18. Light
    • Electromagnetic radiation – all energy waves from short gamma rays to long radio waves
    • Our eyes respond to a small portion of this spectrum called the visible spectrum
    • Different cones in the retina respond to different wavelengths of the visible spectrum
  • 19. Light Figure 15.10 The wavelength is the distance between repeating units of a wave pattern
  • 20. Refraction and Lenses
    • When light passes from one transparent medium to another its speed changes and it refracts (bends)
    • Light passing through a convex lens (as in the eye) is bent so that the rays converge (join) to a focal point
    • When a convex lens forms an image, the image is upside down and reversed right to left
  • 21. Focusing Light on the Retina
    • Pathway of light entering the eye: cornea, aqueous humor, lens, vitreous humor, and the neural layer of the retina to the photoreceptors
    • Light is refracted :
      • At the cornea
      • Entering the lens
      • Leaving the lens
    • The lens curvature and shape allow for fine focusing of an image
  • 22. Focusing Light on the Retina
    • In the normal resting state :
      • our ciliary muscle is relaxed
      • the elastic lens tends to become thick
      • aqueous & vitreous humour push outward on the sclerotic coat
      • ligaments become extended / tensed
      • lens pulled into a thin shape
  • 23. Focusing for Distant Vision
    • Light from a distance needs little adjustment for proper focusing
    • Far point of vision – the distance beyond which the lens does not need to change shape to focus (20 ft.) or:
    • The object distance at which the eye is focused with the eye lens in a neutral or relaxed state .
    Figure 15.13a
  • 24. Focusing Light on the Retina - short focal length
    • contraction of ciliary muscle
    • distance between edges of ciliary body decreases
    • relaxation of suspensory ligament
    • lens becomes thicker
    • focal length shortens
    • light rays converge earlier; image formed on retina
  • 25.
  • 26. Focusing for Close Vision
    • Close vision requires:
      • Accommodation – changing the lens shape by ciliary muscles to increase refractory power
      • Constriction – the pupillary reflex constricts the pupils to prevent divergent light rays from entering the eye
      • Convergence – medial rotation of the eyeballs toward the object being viewed so both eye are focused on the object
  • 27. Focusing for Close Vision Figure 15.13b LENS AND IRIS PRESENTATION FROM WEBSITE
  • 28. Problems of Refraction
    • Emmetropic eye – normal eye with light focused properly
    • 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
      • Corrected with a concave lens
    • Hyperopic eye ( farsighted ) – the focal point is behind the retina. See far objects clear but not close ones
      • Corrected with a convex lens
  • 29. Problems of Refraction Figure 15.14a, b
  • 30. Photoreceptors
    • Photoreceptors are modified neurons
    • They absorb light and generate chemical or electrical signals
    • 2 cell types – rods and cones – that produce visual images
      • Outer segment - points towards the wall of the eye (towards the pigmented layer of the retina)
      • Inner segment – facing the interior
      • 2 segments are separated by a narrow constriction - cilium
    • The inner segment connects to the cell body which is continuous of the inner fiber that has the synaptic terminal
  • 31. Photoreceptors
    • Photoreceptors are modified neurons
    • They absorb light and generate chemical or electrical signals
    • 2 cell types – rods and cones – that produce visual images
  • 32.
    • Outer segment - points towards the wall of the eye (towards the pigmented layer of the retina)
    • Inner segment – facing the interior
    • 2 segments are separated by a narrow constriction - cilium
    • The inner segment connects to the cell body which is continuous of the inner fiber that has the synaptic terminal
    Figure 15.15a, b
  • 33. Photoreception: Functional Anatomy of Photoreceptors
    • Photoreception – process by which the eye detects light energy
    • Rods and cones contain visual pigments ( photopigments )
      • Embedded in areas of the plasma membrane that is arranged in a stack of disc-like infoldings
      • change shape as they absorb light
      • This foldings increase surface area that is available for trapping light
      • In rods – the discs are discontinuous while in the cones they are continuous
  • 34.
  • 35. Photoreceptors - Rods functional characteristics
    • Sensitive to dim light and best suited for night vision
    • Absorb all wavelengths of visible light
    • Perceived input is in gray tones only
    • Sum of visual input from many rods feeds into a single ganglion cell
    • Results in fuzzy and indistinct images
  • 36. Photoreceptors - Cones functional characteristics
    • Need bright light for activation (have low sensitivity)
    • Have pigments that furnish a vividly colored view
    • Each cone synapses with a single ganglion cell
    • Vision is detailed and has high resolution
  • 37. Chemistry of Visual Pigments
    • Retinal is a light-absorbing molecule
      • Combines with proteins called opsins to form 4 types of visual pigments
      • Similar to and is synthesized from vitamin A.
        • Vitamin A is stored in the liver and transported by the blood to the cells of the pigmented layer (local reservoir of vitamin A)
      • Retinal has two 3D forms/isomers:
        • 11- cis – a bent structure when connected to opsin
        • all- trans – when struck by light and change the shape of opsin to its active form
        • Transforming fro 11-cis to all-trans is the only light dependent stage
    • Isomerization of retinal initiates electrical impulses in the optic nerve
  • 38.
    • The visual pigment of rods is called rhodopsin – a deep purple pigment
      • Each molecule consists of two major parts - opsin and 11- cis retinal
    • Rhodopsin molecules are arranged in a single layer in the membrane of each of the discontinuous discs in the outer segment
    • Rhodopsin is formed and accumulates in the dark
    Excitation of Rods Figure 15.15a, b
  • 39. Excitation of Rods
    • Light phase
    • When rhodopsin absorb light retinol is changed to its all-trans isomer
    • Rhodopsin breaks down into all-trans retinal + opsin (this process is called bleaching of the pigment )
    • Dark phase
    • Vitamin A oxidized to the 11-cis retinal form and combined with opsin to form rhodopsin.
  • 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. Excitation of Cones
    • Visual pigments in cones are similar to rods (retinal + opsins)
    • Cones are less sensitive – need more light to be activated
    • There are three types of cones:
      • Blue – wave length 420nm,
      • Green – wave length 530nm,
      • Red – 560nm
    • The absorption spectra overlap giving the hues - activation of more than one type of cone
    • Method of excitation is similar to rods but the cones need higher-intensity (brighter) light because they are less sensitive
  • 42.  
  • 43. Phototransduction
    • The outer segments of the photoreceptor has ligand-regulated sodium gates that bind to cGMP on the intracellular side.
    • In the dark, cGMP opens the gate and permits the inflow of sodium which reduces the membrane potential from -70mv to -40mv
    • 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
    • Light stops the dark current
  • 44. Phototransduction
    • Light energy splits rhodopsin into all- trans retinal, releasing activated opsin
    • The freed opsin activates the G protein transducin
    • Transducin catalyzes activation of phosphodiesterase (PDE)
    • PDE hydrolyzes cGMP to GMP and releases it from sodium channels
    • Without bound cGMP, sodium channels close but potassium channels in the outer segment remain open
    • The photoreceptor membrane hyperpolarizes, and neurotransmitter cannot be released
  • 45.
  • 46. Phototransduction Figure 15.18 TRUNSDUCTION PRESENTATION FROM WEBSITE
  • 47. Phototransduction
    • Photoreceptors do not generate action potential and neither do the bipolar cells
    • they generate graded potential
    • The ganglion cells generate action potential
  • 48. Signal Transmission in the Retina
  • 49. Adaptation to bright light (going from dark to light)
    • As long as the light is low intensity, relatively little amount of rhodopsin is bleached.
    • In high intensity light, rhodopsin is bleached as fast as it is re-formed
    • Going from dark/dim light to light - first we see white light because the sensitivity of the retina is “set” to dim light
    • 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
    • Rods system is turned off and the cones system adapts
    • By switching from the rod to the cone system – visual acuity is gained
  • 50. Adaptation to dark
    • Initially we do not see nothing
    • Cones stop functioning in low light
    • Rhodopsin accumulates in the dark and retinal sensitivity is restored
    • When we move from a lit room to a dark room, we cannot see clearly, because:
    • It takes about 20-30 minutes for enough rhodopsin to reform for us to see properly
  • 51. Visual Pathways
    • Axons of retinal ganglion cells form the optic nerve
    • Medial fibers of the optic nerve decussate at the optic chiasm
    • Most fibers of the optic tracts continue to the thalamus
    • Other optic tract fibers end in superior colliculi in the midbrain (initiating visual reflexes)
    • Optic radiations travel from the thalamus to the visual cortex