eye
TRANSCRIPT
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Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Human Anatomy & PhysiologySEVENTH EDITION
Elaine N. MariebKatja Hoehn
PowerPoint® Lecture Slides prepared by Vince Austin, Bluegrass Technical and Community College
C H
A P
T E
R
15The Special Senses
P A R T A
http://www.physpharm.fmd.uwo.ca/undergrad/medsweb/L1Eye/m1eye.swf
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Palpebrae (Eyelids)
Figure 15.1b
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Lacrimal Apparatus
Figure 15.2
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Extrinsic Eye Muscles
Figure 15.3a, b
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Structure of the Eyeball
Figure 15.4a
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Anterior Segment
Figure 15.8
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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
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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
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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.
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Pupil Dilation and Constriction
Figure 15.5
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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
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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)
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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
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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
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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
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Light
Figure 15.10
The wavelength is the distance between repeating units of a wave pattern
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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
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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
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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
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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
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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
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Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummingshttp://230nsc1.phy-astr.gsu.edu/hbase/vision/accom.html
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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
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Focusing for Close Vision
Figure 15.13b
LENS AND IRIS PRESENTATION FROM WEBSITE
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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
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Problems of Refraction
Figure 15.14a, b
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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
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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
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Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 15.15a, b
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
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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
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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
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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
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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
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Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 15.15a, b
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
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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.
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CH3
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Oxidation
Rhodopsin
Opsin
All-trans retinal
–2H
+2HReduction
Vitamin A
Regeneration ofthe pigment:Slow conversionof all-trans retinalto its 11-cis formoccurs in the pig-mented epithelium;requires isomeraseenzyme and ATP.
Dark Light
11-cis retinal
All-trans isomer
11-cis isomer
Bleaching of thepigment:Light absorptionby rhodopsintriggers a seriesof steps in rapidsuccession inwhich retinalchanges shape(11-cis to all-trans)and releasesopsin.
Figure 15.16
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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
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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
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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
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Phototransduction
Figure 15.18
TRUNSDUCTION PRESENTATION FROM WEBSITE
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Phototransduction
Photoreceptors do not generate action potential and neither do the bipolar cells
they generate graded potential
The ganglion cells generate action potential
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Signal Transmission in the Retina
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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
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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
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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