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Special sense of vision
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SPECIAL SPECIAL SENSES
EXPERT VISION ACADEMY
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The Senses
Special senses
Smell
Taste
Sight
Hearing
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Vision
Dominant sense 70% of all sensory receptors are in the eyes Converts the energy of light into electrical
nerve impulses that are interpreted by the brain as sight
Visual receptor cells (photoreceptors) sense and encode patterns of light that enter the eye
Brain uses these signals to give us images
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sclera
irispupil
teardrainage canal
cornea
Medial commisure
lateral commisure
palpabre
palpabre
Lacrimal caruncle
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The Eye
Each eye has over a million nerve fibers Protection for the eye
Most of the eye is enclosed in a bony orbit
A cushion of fat surrounds most of the eye
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The Eye
Eye acts much like a camera Lens of eye adjusts to bring object into
clear focus Pupil of eye constricts to allow less light
to enter in bright setting or dilates to allow more light to enter in darker setting
Through bending of light rays, image reaches retina
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Accessory Structures of the Eye
Eyelids and eyelashes
Conjunctiva
Lacrimal apparatus
Extrinsic eye muscles
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Accessory Structures of the Eye
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Accessory Structures of the Eye
Eyelids Eyelids (palpebrae) Tarsal glands lubricate the eye The tarsal glands (modified sebaceous gland) secrete a
fluid to keep the eye lids from adhering to each other. The tarsal plate gives form and support to the eyelids.Eyelashes Eyelashes double/triple row of hairs. Eyelashes are innervated by nerve ending – trigger
blinking Eyebrows Shade inhibit sweat help protect the eyeballs from foreign objects, perspiration,
and the direct rays of the sun.
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Accessory Structures of the Eye
Conjunctiva Membrane that lines the eyelids Connects to the surface of the eye Secretes mucus to lubricate the eye
Functions: major
produce a lubricating mucus prevents the eyes from drying out
other protection
prevents foreign objects from penetrating beyond the confines of its sac
Conjunctivitis is inflammation, produces a pinkish discoloration (pinkeye)
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Accessory Structures of the Eye
Lacrimal apparatus
Lacrimal gland—produces lacrimal fluid Lacrimal canals—drain lacrimal fluid from eyes Lacrimal sac—provides passage of lacrimal
fluid towards nasal cavity Nasolacrimal duct—empties lacrimal fluid into
the nasal cavity
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FLOW OF TEARS
Lacrimal gland
Lacrimal ducts
Sup. or inf. lacrimal canal
Lacrimal sac
Nasolacrimal duct
Nasal cavity
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Accessory Structures of the Eye
Function of the lacrimal apparatus Protects, moistens, wash and lubricates the
eye Empties into the nasal cavity
Properties of lacrimal fluid Dilute salt solution (tears) Contains antibodies and lysozyme
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Accessory Structures of the Eye
Muscles Two types:
Extrinsic Are skeletal muscles Move the eye voluntarily
Intrinsic Are smooth involuntary muscles Located within the eye
Iris muscle Ciliary muscle
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Accessory Structures of the Eye
Extrinsic eye muscles Six muscles attach to the outer surface of
the eye Produce eye movements
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Accessory Structures of the Eye
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Structure of the Eye
Three Layers or coats or tunics forming the wall of the eyeball
Fibrous layer Outside layer
Vascular layer Middle layer
Sensory layer Inside layer
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Structure of the Eye
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Structure of the Eye: The Fibrous Layer
Sclera White connective tissue layer Seen anteriorly as the “white of the eye”
Cornea Transparent, central anterior portion Allows for light to pass through The only human tissue that can be transplanted
without fear of rejection Both Composed of dense avascular connective
tissue
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Structure of the Eye: Vascular Layer
Choroid Is a blood-rich nutritive layer in the posterior of the eye Pigment prevents light from scattering Modified anteriorly into two structures Ciliary body—smooth muscle attached to lens that
change the lense shape Iris—regulates amount of light entering eye• Pigmented layer that gives eye color• Composed of circular and radial muscles• Reflex contraction of circular muscle in bright light (small
dia of pupil)• Reflex contraction of radial muscle in dim light (large dia
of pupil)
Pupil—rounded opening in the iris
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bright light normal light dim light
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Layers of Eye
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Structure of the Eye: Sensory Layer
Retina contains two layers Outer
pigmented layer Inner neural
layer Contains
receptor cells (photoreceptors)
Rods Cones
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Structure of the Eye: Sensory Layer
Three layers of neurons make up a major portion of the retina. Named in the order in which the conduct impulses: Photoreceptor neurons Bipolar neurons Ganglion neurons
Signals pass from photoreceptors via a two-neuron chain Bipolar neurons Ganglion cells
Signals leave the retina toward the brain through the optic nerve
Optic disc (blind spot) is where the optic nerve leaves the eyeball Cannot see images focused on the optic disc
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Layers of Retina
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Structure of the Eye: Sensory Layer
Rods Most are found towards the edges of the retina Allow dim light vision and peripheral vision Objects are seen in shades of gray not color Contains pigment Rhodopsin Rhodopsin is highly sensitive to light, light
causes a rapid breakdown of the pigment into its scotopsin and retinal components
Cones Allow for detailed color vision Densest in the center of the retina Fovea centralis—area of the retina with only cones No photoreceptor cells are at the optic disc, or blind spot
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Cone sensitivity Three types of
cones red, green, and blue
Different cones are sensitive to different wavelengths
Color blindness is the result of the lack of one cone type
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Lens
Biconvex crystal-like structure, flexible Held in place by a suspensory ligament attached
to the ciliary body Focuses image onto retina Changes lens thickness to allow light to be
properly focused onto retina Cataracts result when the lens becomes hard and
opaque with age Vision becomes hazy and distorted Eventually causes blindness in affected eye
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Segments, or Chambers, of the Eye
Anterior chamber Posterior chamber Vitreous segment
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Watery fluid found between lens and cornea Similar to blood plasma Helps maintain intraocular pressure Provides nutrients for the lens and cornea Helps to maintain the shape of the eye Prevent the eyeball from collapsing
Aqueous humor
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Vitreous humor
Posterior to the lens Gel-like substance with fine fibrils Prevents the eye from collapsing Helps maintain intraocular pressure Transmits light
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How do we see?
For vision to occur: An image must be formed on the retina to
stimulate the rods and cones Nerve impulses must be conducted to the
visual areas of the cerebral cortex Interpretation occurs
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Formation of a retinal image
Four processes focus light rays so that they form clear images on the retina
Refraction of light rays Accommodation of the lens Constriction of the pupils Convergence of the eyes
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Process of image formation on the retina
1) Refraction:
bending of light as medium changes to focus light
2) Accommodation of lens for near/distance vision:
shape of lens changed by ciliary muscle to make light focus on retina
3) Dilation /Constriction of pupil:
ANS reflex to prevent scattering of light through edges of lens
4) Convergence of eyes:
to focus both eyes on same object and provide vision
Images are focused on the retina upside‑down and mirror‑image, and the brain then translates this information.
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Function of the Lens: Light Refraction
Bending of light Occurs when light
passes at an angle from a medium of one optical density into a medium of a different density
Convex surfaces cause light to converge
Concave surfaces cause light to diverge
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• Convex lenses cause light waves to converge
• Concave lenses cause light waves to diverge
Air
Glass
(a) (b)
Diverginglight waves
Convexsurface
Lightwave
Converginglight waves
Concavesurface
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Pathway of Light Through the Eye
Light must be focused to a point on the retina for optimal vision
The eye is set for distance vision Accommodation—the lens must change
shape to focus on closer objects
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Pathway of Light Through the Eye
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Focusing on a Near Object
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Accomodation
Focusing on a Far Object
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Pathway of Light Through the Eye
Image formed on the retina is
Reversed from left to right Upside down Smaller than the object
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Images Formed on the Retina
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Visual Nerve Pathways
Ganglion cell axons leave the eye to form the optic sensory nerve
The nerve fibers cross anterior to pituitary gland, the optic chiasma.
Nerve fibers travel to the thalamus and to reflex centers.
Optic radiations are nerve pathways that lead to the visual association area in the occipital lobe.
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Principal visual pathways from the eyes to the visual cortex.
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Visual Fields and Visual Pathways
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Emmetropia—eye focuses images correctly on the retina
Myopia (nearsighted) Distant objects appear blurry Light from those objects fails to reach the
retina and are focused in front of it Results from an eyeball that is too long
Errors of Refraction
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Errors of Refraction
Hyperopia (farsighted) Near objects are blurry while distant
objects are clear Distant objects are focused behind the
retina Results from an eyeball that is too short or
from a “lazy lens”
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Errors of Refraction
Astigmatism Images are blurry Results from light focusing as lines, not
points, on the retina due to unequal curvatures of the cornea or lens
Correction special lenses with uneven curvature
compensates for eye’s asymmetry allows the image to focus evenly on retina
laser
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Errors of Refraction
Cataracts When lens becomes hard
and opaque, our vision becomes hazy and distorted
Clouding of lens
Inadequate nutrient delivery to deeper lens fibers
Causes: diabetes mellitus, smoking, UV damage, congenital, age-related hardening, thickening of lens
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Night blindness Inhibited rod function that hinders the ability to
see at night Hampers one’s ability to drive safely at night most common cause-----prolonged vitamin A
deficiency leads to rod degeneration
treatment vitamin A supplements
restore function if administered before degenerative changes occur
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Color blindness Genetic conditions that result in the inability to see
certain colors Due to the lack of one type of cone (partial color
blindness) sex-linked more common in males most common type
red-green deficit or absence of red or green cones seen as same color
either red or green
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Glaucoma Can cause blindness due to increasing
pressure within the eye Normally 15 (12-20) mm Hg Above 25mm Hg an individual has glaucoma
which if untreated can lead to retinal damage and blindness
Pressure can increase due to decreased drainage or excess drainage of the aqueous humor which is normally re-circulated via vessels into vitreous humor