Download special senses - eye and vision

Accessory structures of the eye
• Eyelids (palpebrae) separated by the palpebral fissure
– protect the anterior surface of the eye
• Conjunctiva covers most of eye – stratified squamous
ET
• Palbebral conjunctiva – lines the inner surface of the
eyelid
• Ocular conjunctiva – lines the anterior surface of the
eyball
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Accessory Structures of the Eye – lacrimal apparatus
• Structures that are associate
with the production, secretion
and removal of tears
• Secretions from the lacrimal
gland contain lysozyme (to
destroy bacteria)
• Tears form in the
glands wash across
pass through the
canaliculi, lacrimal
nasolacrimal duct
lacrimal
the eye,
lacrimal
sac and
• Lacrimal apparatus – keeps
the surface of the eye moist
• Lacrimal gland –
produces lacrimal fluid
• Lacrimal sac – fluid
empties into nasal cavity
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Figure 16.5b
Extrinsic Eye Muscles
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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 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.
• The lens is held in place by its ciliary zonule (suspensory
ligaments)
• The lens and ciliary zonules divide the eye posterior and anterior
cavities
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Internal Chambers and Fluids
• Posterior segment (cavity)
• Filled with vitreous humor
• Clear, jelly-like substance
• Transmits light
• Supports the posterior surface of the lens
• Helps maintain intraocular pressure
• Anterior segment
• Filled with aqueous humor
• Renewed continuously
• Formed as a blood filtrate
• Supplies nutrients to the lens and cornea
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Eye’s tunics (layers)
Fibrous tunic
Sclera and Cornea
Vascular tunic
Choroid, Ciliary body, and Iris
Sensory tunic
Pigmented layer
Neural layer
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The Fibrous Tunic
• Most external layer of the eyeball
• Composed of two regions of connective tissue
• Sclera – “white of the eye”
• White, opaque region
• Provides shape and an anchor for eye muscles
• Cornea – transparent part that allow passage of light
• Limbus – junction between sclera and cornea
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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
• 4 layers
• External layer – stratified squamous ET – for protection; merges with the
ocular conjuntiva
• absorbs oxygen and other needed cell nutrients that are contained in
tears.
• filled with thousands of tiny nerve endings that make the cornea
extremely sensitive to pain when rubbed or scratched.
• Deep epithelial layer –single layer of cells
• primary task is to pump excess water out of the stroma (by having
sodium pumps).
• Bowman’s layer
• a tough layer consisting of irregularly-arranged collagen fibers
• Stroma
• Located behind the external epithelium
• A thick, transparent middle layer, consisting of regularly-arranged
collagen fibers
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http://dels.nas.edu/ilar_n/ilarjournal/40_2/40_2Cowellfig1.jpg
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http://www.siumed.edu/~dking2/intro/IN022b.htm
Vascular Tunic (Uvea)
Choroid - highly vascular, dark brown (melanin) membrane that
forms the posterior portion of the uvea
Ciliary body - thickened ring of tissue encircling the lens,
composed of smooth muscle bundles (ciliary muscles) which
control lens shape.
Suspensory ligament extends from ciliary processes to the lens to
hold it in place
Iris - colored part of the eye between
cornea and lens Pupil - central opening
of the iris that regulates amount of light
entering the eye
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The Sensory Tunic (Retina)
• Composed of two layers
• Pigmented layer – single layer of melanocytes
• Neural layer – sheet of nervous tissue
• Contains three main types of neurons
• Photoreceptor cells
• Bipolar cells
• Ganglion cells
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The Retina: Photoreceptors
Rods:
Respond to dim light, more sensitive to light than cones, but
do not provide sharp images or color vision. Used for
peripheral vision.
Cones:
Respond to bright light, have high-acuity color vision.
Found in the macula lutea (lateral to optic disc).
Concentrated in the fovea centralis of the macula lutea.
Macula lutea – contains mostly cones
Fovea centralis – contains only cones, Region of highest visual
acuity
Optic disc – blind spot – no photo receptor, exit of optic nerve
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How the eye focus light
• The eye must focus light onto a
precise spot on the retina (macula
lutea)
• The cornea accounts for about 80%
of the focusing that the eye does.
• The lens does the rest of the
focusing.
• The lens is also able to change its
shape to focus so things at different
distances can come into focus (like
an auto-focusing camera).
• The closer that an object is to the
eye, the more focusing the lens has
to do in order to make the image
clear
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Nearsightedness (myopia)
• Light is focused in front of the
retina rather than directly on
it. This leads to a naturally
closer point of focus.
• People with myopia can see
close objects without
difficulty, but distant objects
are blurred.
• The correction requires a
concave lens (minus power),
which causes the light
reaching the eye to diverge.
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Farsightedness
• Light is focused behind
the retina.
• These people do not
have problems with
distant vision but need
glasses to with convex
lenses to augment the
converging power of
the lens to close vision.
• There is a natural
decline in the ability of
the lens to focus as one
ages.
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• The notation of visual acuity is written as a fraction,
with normal vision being 20/20 (twenty/twenty
vision)
• At a 20 foot distance, (the top number in the fraction,
or testing distance), a person with normal vision
should be able to read the small 20/20 line on an eye
chart.
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