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    Eye Eye Presentation Transcript

    • 15 The Special Senses P A R T A http://www.physpharm.fmd.uwo.ca/undergrad/medsweb/L1Eye/m1eye.swf
    • Palpebrae (Eyelids) Figure 15.1b
    • Lacrimal Apparatus Figure 15.2
    • Extrinsic Eye Muscles Figure 15.3a, b
    • Structure of the Eyeball Figure 15.4a
    • Anterior Segment Figure 15.8
    • 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
    • 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
    • http://dels.nas.edu/ilar_n/ilarjournal/40_2/40_2Cowellfig1.jpg http://www.siumed.edu/~dking2/intro/IN022b.htm
    • 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.
    • http://www.bu.edu/histology/p/08001loa.htm
    • http://www.siumed.edu/~dking2/ssb/EE011b.htm
    • Pupil Dilation and Constriction Figure 15.5
    • 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
    • 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)
    • 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
    • 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
      • RETINA PRESENTATION FROM WEBSITE
      • http://www.physpharm.fmd.uwo.ca/undergrad/medsweb/L1Eye/m1eye.swf
    • 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
    • Light Figure 15.10 The wavelength is the distance between repeating units of a wave pattern
    • 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
    • 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
    • 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
    • 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
    • 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
    • http://230nsc1.phy-astr.gsu.edu/hbase/vision/accom.html
    • 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
    • Focusing for Close Vision Figure 15.13b LENS AND IRIS PRESENTATION FROM WEBSITE
    • 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
    • Problems of Refraction Figure 15.14a, b
    • 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
    • Photoreceptors
      • Photoreceptors are modified neurons
      • They absorb light and generate chemical or electrical signals
      • 2 cell types – rods and cones – that produce visual images
      http://library.thinkquest.org/28030/physio/perceive.htm
      • 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
    • 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
    • http://thebrain.mcgill.ca/flash/i/i_02/i_02_m/i_02_m_vis/i_02_m_vis.html
    • 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
      http://thebrain.mcgill.ca/flash/d/d_02/d_02_m/d_02_m_vis/d_02_m_vis.html
    • 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
      http://thebrain.mcgill.ca/flash/d/d_02/d_02_m/d_02_m_vis/d_02_m_vis.html
    • 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
      • 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
    • 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.
    • 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
    • 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
      COLOR PRESENTATION FROM WEBSITE
    •  
    • 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
    • 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
    • http://thebrain.mcgill.ca/flash/d/d_02/d_02_m/d_02_m_vis/d_02_m_vis.html#2
    • Phototransduction Figure 15.18 TRUNSDUCTION PRESENTATION FROM WEBSITE
    • Phototransduction
      • Photoreceptors do not generate action potential and neither do the bipolar cells
      • they generate graded potential
      • The ganglion cells generate action potential
    • Signal Transmission in the Retina
    • 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
    • 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
    • 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