Download the special senses

Lecture Material is adapted from © 2013 Pearson Education, Inc. Human
Anatomy and Physiology
Dr. Henrik Pallos
THE SPECIAL SENSES
Special Senses
• Special sensory receptors
– Distinct, localized receptor cells in head
•
•
•
•
•
Vision
Taste
Smell
Hearing
Equilibrium
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The Eye and Vision
• 70% of body's sensory receptors in eye
• Visual processing by nearly half cerebral cortex
– Primary visual cortex
– Visual association area
• Most of eye protected by cushion of fat and
bony orbit
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Accessory Structures of the Eye
• Protect the eye and aid eye function
1.
2.
3.
4.
5.
Eyebrows
Eyelids (palpebrae)
Conjunctiva
Lacrimal apparatus
Extrinsic eye muscles
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The eye and accessory structures.
My wife’s epicanthic fold
Eyebrow
Eyelid
(palpebrae)
Eyelashes
Site where
conjunctiva
merges with
cornea
Palpebral
fissure
Lateral
commissure
Iris
Eyelid
Pupil
Lacrimal Medial
Sclera
(covered by caruncle commissure
conjunctiva)
Surface anatomy of the right eye
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Eyebrows
• Overlie supraorbital margins
• Function
1. Shade eye from sunlight
2. Prevent perspiration from reaching eye
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Eyelids
• Protect eye anteriorly
• Separated at palpebral fissure
• Meet at medial and lateral commissures
• Lacrimal caruncle
– At medial commissure
– Contains oil and sweat glands
• Tarsal plates—
supporting connective
tissue
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Eyelid Muscles
• Levator palpebrae superioris
– Gives upper eyelid mobility
• Raises eyelid to
to open eye
• Blink reflexively
every 3-7 seconds
– Protection
– Spread secretions
to moisten eye
• (Orbicularis oculi)
– Closes eye when
contracts
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Eyelids
• Eyelashes
– Nerve endings of follicles initiate reflex blinking
• Lubricating glands associated with eyelids
1. Tarsal (Meibomian) glands
• Modified sebaceous glands
• Oily secretion lubricates lid and eye
2. Ciliary glands between hair follicles
• Modified sweat glands
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The eye and accessory structures.
Levator palpebrae
superioris muscle
Orbicularis
oculi muscle
Eyebrow
Tarsal plate
Palpebral
conjunctiva
Tarsal glands
Cornea
Palpebral
fissure
Eyelashes
Bulbar conjunctiva
Conjunctival sac
Orbicularis
oculi muscle
Lateral view; some structures shown in sagittal section
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© McGraw Hill Anatomy and Physiology
Conjunctiva
• Transparent mucous membrane
– Produces a lubricating mucous secretion
1. Palpebral conjunctiva lines eyelids
2. Bulbar conjunctiva covers white of eyes
Conjunctival sac between palpebral and
bulbar conjunctiva
- Where contact lens rests
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Lacrimal Apparatus
• Lacrimal gland and ducts that drain into nasal cavity
• Lacrimal gland in orbit above lateral end of eye
• Lacrimal secretion (tears) – 1ml/day
– Dilute saline solution containing mucus, antibodies, and
lysozyme:
• cleanses and protects the eye surface as it lubricates it
– Blinking spreads tears toward medial commissure
• Tears enter paired lacrimal canaliculi via lacrimal puncta
• Then drain into lacrimal sac and nasolacrimal duct
• Taste of eye drops
– Lacrimal secretion increases:
• Eyes are irritated (washes away irritant)
• Emotional (Why we have, we do not know.)
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The lacrimal apparatus.
Lacrimal sac
Lacrimal gland
Excretory ducts
of lacrimal glands
Lacrimal punctum
Lacrimal canaliculus
Nasolacrimal duct
Inferior meatus
of nasal cavity
Nostril
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Homeostatic imbalance
• Nasal cavity mucosa is continuous with lacrimal duct
system
• Cold/nasal inflammation: swelling of lacrimal mucosa
• Constriction of
nasolacrimal duct
• No tear draining
• Watery eyes
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Extrinsic Eye Muscles
• 6 straplike extrinsic eye muscles
– Originate from bony orbit; insert on eyeball
1. enable eye to follow moving objects
2. maintain shape of eyeball
3. hold in orbit
• 4 rectus muscles originate from common tendinous
ring; names indicate movements
– Superior, inferior, lateral, medial rectus muscles
• 2 oblique muscles move eye in vertical plane and
rotate eyeball
– Superior and inferior oblique muscles
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H test
http://emedicine.medscape.com/article/1189759-overview
Extrinsic eye muscles.
Trochlea
Superior oblique muscle
(CN 4)
Superior oblique tendon
Superior rectus muscle
(CN 3)
Axis of
rotation
of eye
Inferior
rectus muscle (CN 3)
Medial
rectus muscle (CN 3)
Lateral
rectus muscle ( CN 6)
Common
tendinous ring
Superior view of the right eye
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Extrinsic eye muscles.
Superior oblique muscle
Superior oblique tendon
Superior rectus muscle
Lateral rectus muscle
Inferior
Inferior
oblique
rectus
muscle muscle (CN 3)
Lateral view of the right eye
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Extrinsic eye muscles.
Muscle
Action
Controlling cranial nerve
Lateral rectus
Moves eye laterally
VI (abducens)
Medial rectus
Superior rectus
Inferior rectus
Moves eye medially
III (oculomotor)
Elevates eye and turns it medially
III (oculomotor)
Depresses eye and turns it medially
III (oculomotor)
Elevates eye and turns it laterally
III (oculomotor)
Depresses eye and turns it laterally
IV (trochlear)
Inferior oblique
Superior oblique
Summary of muscle actions and innervating cranial nerves
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Homeostatic imbalances
• Movements of external muscles of the two
eyes are not perfectly coordinated
• Image is not properly focused on the same
area of the visual field in each eye
• Two images are seen instead of one
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• Diplopia: double vision
– paralysis, weakened muscle, temporary
consequences of alcohol
• Strabismus (cross-eyed)
– Congenital weakness of muscle
– Affected eye rotates medially or laterally
• The eyes may alternate to focus and compensate
• Or disregard deviant eye: functional blindness
• Exercising muscle, surgery
http://www.aapos.org/terms/conditions/100
Structure of the Eyeball
• Wall of eyeball contains 3 layers
• Internal cavity filled with fluids called humors
• Lens separates internal cavity into
1. anterior segment
2. posterior segment (cavities)
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Structure of the Eyeball
• Wall of eyeball contains 3 layers
1. Fibrous
1.
2.
Sclera
Cornea
2. Vascular
1.
2.
3.
Choroid
Ciliary body
Iris
3. Inner (Retina)
1.
2.
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Pigmental layer
Neural layer
Internal structure of the eye (sagittal section).
Ora serrata
Ciliary body
Sclera
Ciliary zonule
(suspensory
ligament)
Choroid
Cornea
Iris
Pupil
Anterior
pole
Anterior
segment
(contains
aqueous humor)
Lens
Scleral venous sinus
Posterior segment
(contains vitreous humor)
Retina
Macula lutea
Fovea centralis
Posterior pole
Optic nerve
Central artery and
vein of the retina
Optic disc
(blind spot)
Diagrammatic view. The vitreous humor is illustrated only in the bottom part of the eyeball.
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Internal structure of the eye (sagittal section).
Ciliary body
Ciliary
processes
Vitreous humor
in posterior
segment
Iris
Margin
of pupil
Anterior
segment
Lens
Cornea
Ciliary zonule
(suspensory
ligament)
Retina
Choroid
Sclera
Fovea centralis
Optic disc
Optic nerve
Photograph of the human eye.
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Fibrous Layer
• Outermost layer
• Dense avascular connective tissue
• Two regions: sclera and cornea
1. Sclera
– Opaque posterior region
– Protects, shapes
eyeball
– Anchors extrinsic
eye muscles
– Continuous with
dura mater of brain
posteriorly
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Fibrous Layer
2. Cornea
– Transparent anterior 1/6 of fibrous layer
– Bends light as it enters eye
– Numerous pain receptors contribute to blinking
and tearing reflexes
– Transplantation
• Avascular
• One person to
another with
minimal risk of
rejection
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Vascular Layer (Uvea)
• Middle pigmented layer
• 3 regions: choroid, ciliary body, and iris
1. Choroid region
– Posterior portion of uvea
– Supplies blood to all
layers of eyeball
– Brown pigment
absorbs light to
prevent light
scattering and
visual confusion
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Vascular Layer
2. Ciliary body
– Ring of tissue surrounding lens
– Smooth muscle bundles (ciliary muscles) control
lens shape
– Capillaries of ciliary processes secrete fluid
– Ciliary zonule
(suspensory
ligament) holds
lens in position
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Vascular Layer
3. Iris
• Colored part of eye
• 2 smooth muscle layers +
elastic fibers
• Random pattern before birth
• Pupil
central opening that regulates
amount of light entering eye
• Color of iris: only brown pigment
• Lot of pigment: brown/black
• Small amount: blue, green, gray
• Unpigmented area scatter shorter wavelengths light
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1. Close vision and bright light
sphincter pupillae (circular muscles) contract; pupils constrict
parasympathetic fibers
2. Distant vision and dim light
dilator pupillae (radial muscles) contract; pupils dilate –
sympathetic fibers
3. Changes in emotional state
pupils dilate when subject matter is appealing such as in your
eyes during Anatomy and Physiology lecture ;-)
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Inner Layer: Retina
• Originates as outpocketing of brain
• Delicate 2-layered membrane
1. Outer Pigmented layer
• Single-cell-thick lining
1. Absorbs light and prevents its scattering
2. Phagocytize photoreceptor cell fragments
3. Stores vitamin A
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Microscopic anatomy of the retina.
Neural layer of retina
Pigmented
layer of
retina
Choroid
Pathway of
light
Sclera
Optic disc
(blind spot)
Central artery
and vein of retina
Optic
nerve
Posterior aspect of the eyeball
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Inner Layer: Retina
2. Inner Neural layer
– Transparent
– Composed of 3 main types of neurons
1. Photoreceptors
2. Bipolar cells
3. Ganglion cells
– Signals spread from photoreceptors to bipolar cells
to ganglion cells
– Ganglion cell axons exit eye as optic nerve
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Microscopic anatomy of the retina.
Neural layer of retina
Pigmented
layer of
retina
Choroid
Pathway of
light
Sclera
Optic disc
(blind spot)
Central artery
and vein of retina
Optic
nerve
Posterior aspect of the eyeball
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The Retina
• Optic disc (blind spot)
– Site where optic nerve leaves eye
– Lacks photoreceptors
• Quarter-billion photoreceptors of two types
1. Rods
2. Cones
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Why we do not see blind spot…
• When dot/white gap disappears:
1. Visual filling
• The black bar fills the space (or white space in case of
the dark dot)
• Our brain use the surrounding image to fill in the area
with something similar
• Brain assumes that it is better something similar than
disturbing
2. Eye flickering movements (saccades)
•
•
•
Same area does not always project to the same area
What an eye does not see now, it will see a millisec
later when the saccades redirect the visual axis
Fastest muscular movements
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Microscopic anatomy of the retina.
Ganglion
cells
Axons
of
ganglion
cells
Bipolar
cells
Photoreceptors
• Rod
• Cone
Amacrine cell
Horizontal cell
Pathway of signal output
Pathway of light
Pigmented
layer of retina
Cells of the neural layer of the retina
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Photoreceptors
• Rods
– Dim light, peripheral vision
receptors
– More numerous, more sensitive
to light than cones
– No color vision or sharp images
– Numbers greatest at periphery
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Photoreceptors
• Cones
1. Vision receptors for bright
light
2. High-resolution color vision
– Macula lutea exactly at
posterior pole
• Mostly cones
• Fovea centralis
– Tiny pit in center of macula with all
cones; best vision
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Blood Supply to the Retina
• 2 sources of blood supply
1. Choroid supplies outer third (photoreceptors)
2. Central artery and vein of retina supply inner
two-thirds
• Enter/exit eye in center of optic nerve
• Vessels visible in living person
• Hypertension can be detected
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Hypertensive retinopathy
• damage to the retina from high blood
pressure.
– It occurs as the existing high blood pressure
causes changes to the microvasculature of
the retina.
– first findings: flame hemorrhages
and cotton wool spots
– progression: hard exudates
can appear around the
macula
– causing impairment of vision
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Part of the posterior wall (fundus) of the right eye as seen with an ophthalmoscope.
Central
artery
and vein
emerging
from the
optic disc
Optic disc
Macula
lutea
Retina
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Internal Chambers and Fluids
• The lens and ciliary zonule separate eye into
two segments
– Anterior segment
– Posterior segments
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Internal Chambers and Fluids
• Posterior segment contains vitreous humor that
–
–
–
–
–
Transmits light
Supports posterior surface of lens
Holds neural layer of retina firmly against pigmented layer
Contributes to intraocular pressure
Forms in embryo; lasts lifetime
• Anterior segment composed of two chambers
– Anterior chamber— between cornea and iris
– Posterior chamber— between iris and lens
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Internal Chambers and Fluids
• Anterior segment contains aqueous humor
– Plasma like fluid continuously formed by capillaries of
ciliary processes
– Drains via scleral venous sinus (canal of Schlemm) at
sclera-cornea junction
– Supplies nutrients and oxygen mainly to lens and
cornea but also to retina, and removes wastes
• Glaucoma: blocked drainage of aqueous humor
increases pressure and causes compression of
retina and optic nerve  blindness
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Circulation of aqueous humor.
Cornea
Lens
Posterior
segment
(contains
vitreous
humor)
Iris
Lens epithelium
Lens
Cornea
2
Corneal epithelium
Corneal endothelium
Aqueous humor
1 Aqueous humor
forms by filtration
from the capillaries
in the ciliary
processes.
2 Aqueous humor
flows from the
posterior chamber
through the pupil into
the anterior chamber.
Some also flows
through the vitreous
humor (not shown).
3 Aqueous humor is
reabsorbed into the
venous blood by the
scleral venous sinus.
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Anterior
segment
(contains
aqueous
humor)
Anterior chamber
Ciliary zonule
(suspensory
ligament)
Posterior chamber
Scleral venous
sinus
Corneoscleral
junction
3
1
Ciliary
processes
Ciliary
muscle
Bulbar
conjunctiva
Sclera
Ciliary
body
Lens
• Biconvex, transparent, flexible, and avascular
• Changes shape to precisely focus light on retina
• Two regions
– Lens epithelium anteriorly
– Lens fibers form bulk of lens
– Lens fibers filled with transparent protein crystallin
• Lens becomes more dense, convex, less elastic with
age
• Lens can be replaced surgically with artificial lens
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Cataracts
• Clouding of lens
– Consequence of aging
– Diabetes mellitus
– Heavy smoking: STOP SMOKING!
– Frequent exposure to intense sunlight! Sunglasses
– Some congenital
– Crystallin proteins clump
– Large doses of Vitamin C increases cataract
formation
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BREAK
Functions of the Eye
1. Light/energy strikes the retina
2. Converts energy into action potentials
3. Relayed to brain for processing
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Light And Optics: Wavelength And Color
• Eyes respond to visible light
– Small portion of electromagnetic spectrum
– Wavelengths of 400-700 nm
• Light
– Packets of energy (photons or quanta) that travel
in wavelike fashion at high speeds
– Color of objects:
1. They absorb some wavelength
2. They reflect some wavelength
– Objects reflect determines color eye perceives
• Red apple reflects red, green grass reflects green
• Black: do not reflect anything, but absorbs everything
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The electromagnetic spectrum and photoreceptor sensitivities.
10–5
nm
10–3
Gamma
rays
103
nm 1 nm
X rays
UV
nm
106
Infrared
(109 nm =)
nm
1m
103 m
Micro- Radio waves
waves
Light absorption (percent of maximum)
Visible light
Blue
cones
(420 nm)
100
50
0
400
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Green Red
Rods
cones cones
(500 nm) (530 nm) (560 nm)
450
500
550
600
Wavelength (nm)
650
700
Light And Optics: Refraction And Lenses
• Refraction
– Bending of light rays
• Due to change in speed when light passes from one
transparent medium to another
• Occurs when light meets surface of different medium
at an oblique angle
– Curved lens can refract light
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Refraction.
As you sight through the side of the glass at the portion of the straw located above the water's surface,
light travels directly from the straw to your eye (to air to glass, glass to air, but this refraction we can ignore)
Since this light does not change medium, it will not refract.
As you sight at the portion of the straw that was submerged in the water, light travels from water to air
(or from water to glass to air). This light ray changes medium and subsequently undergoes refraction.
As a result, the image of the straw appears to be broken.
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Refraction and Lenses
• Light passing through convex lens (as in eye)
is bent so that rays converge at focal point
– Image formed at focal point is upside-down and
reversed right to left
• Concave lenses diverge light
– Prevent light from focusing
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Light is focused by a convex lens.
Point sources
Focal points
Focusing of two points of light.
C
A
D
C
B
A
D
B
The image is inverted—upside down and reversed.
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Focusing Light on The Retina
• Pathway of light entering eye:
cornea, aqueous humor, lens, vitreous humor, entire
neural layer of retina, photoreceptors
• Light refracted 3 (-4) times along pathway
1. Entering cornea
2. Entering lens
3. Leaving lens
• Majority of refractory power in cornea
• Change in lens curvature allows for fine focusing
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Focusing For Distant Vision
• Eyes best adapted for distant vision
• Far point of vision
– Distance beyond which no change in lens shape
needed for focusing
• 6m for emmetropic (normal) eye
• Cornea and lens focus light precisely on retina
• Ciliary muscles relaxed
• Lens stretched flat by tension in ciliary zonule
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Focusing for distant and close vision.
Nearly parallel rays
from distant object
Sympathetic activation
Lens
Ciliary zonule
Ciliary muscle
Inverted
image
Lens flattens for distant vision.
Sympathetic input relaxes the ciliary muscle
tightening the ciliary zonule, and flattening the lens.
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Focusing For Close Vision
• Light from close objects (<6 m) diverges as
approaches eye
– Requires eye to make active adjustments using 3
simultaneous processes
1. Accommodation of lenses
2. Constriction of pupils
3. Convergence of eyeballs
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Focusing For Close Vision
1. Accommodation
– Changing lens shape to increase refraction
– Near point of vision
• Closest point on which the eye can focus
– Presbyopia—loss of accommodation over age 50
2. Constriction
– Accommodation pupillary reflex constricts pupils to prevent
most divergent light rays from entering eye
– (an image is always blurry around edges)
3. Convergence
– Medial rotation of eyeballs toward object being viewed
• If can’t converge: diplopia – double vision
• Image falls different parts of the 2 retinas, brain see 2 images
– Press gently on one eyelid as you look at the slide
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Focusing for distant and close vision.
Parasympathetic activation
Divergent rays
Inverted
from close object
image
Lens bulges for close vision.
Parasympathetic input contracts the ciliary muscle
loosening the ciliary zonule, allowing the lens to bulge.
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Focusing for distant and close vision.
View
Ciliary muscle
Lens
Ciliary zonule
(suspensory
ligament)
The ciliary muscle and ciliary zonule are arranged
sphincterlike around the lens.
As a result, contraction loosens the ciliary zonule fibers
and relaxation tightens them.
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Problems Of Refraction
1. Myopia (nearsightedness)
– Focal point in front of retina, e.g., eyeball too long
– Close object: no problem
– Corrected with a concave lens
2. Hyperopia (farsightedness)
– Focal point behind retina, e.g., eyeball too short
– Far object: no problem
– Corrected with a convex lens
3. Astigmatism
– Unequal curvatures in different parts of cornea or lens
– Corrected with cylindrically ground lenses or laser
procedures
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Problems of refraction. (1 of 3)
Emmetropic eye (normal)
Focal
plane
Focal point is
on retina.
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Problems of refraction. (2 of 3)
Myopic eye (nearsighted)
Myopia
Focal point in
front of retina,
e.g., eyeball too
long
– Close object:
no problem
– Corrected with a
concave lens
Eyeball
too long
Uncorrected
Focal point is in
front of retina.
Corrected
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Concave lens moves focal
point further back.
Problems of refraction. (3 of 3)
Hyperopia
(farsightedness)
– Focal point behind
retina, e.g.,
eyeball too short
– Far object:
no problem
– Corrected with a
convex lens
Hyperopic eye (farsighted)
Eyeball
too short
Uncorrected
Focal point is
behind retina.
Corrected
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Convex lens moves focal
point forward.
Functional Anatomy Of Photoreceptors
• Rods and cones
– Modified neurons
– Receptive regions called
outer segments
• Contain visual pigments
(photopigments)
Molecules change shape as
absorb light
– Inner segment of each
joins cell body
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Photoreceptors of the retina.
Process of
bipolar cell
Synaptic terminals
Rod cell body
Inner fibers
Rod cell body
Nuclei
Cone cell body
Mitochondria
The outer segments
of rods and cones
are embedded in the
pigmented layer of
the retina.
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Pigmented layer
Inner
Outer segment segment
Outer fiber
Melanin
granules
Connecting cilia
Apical microvillus
Discs containing
visual pigments
Discs being
phagocytized
Pigment cell nucleus
Basal lamina (border
with choroid)
Photoreceptor Cells
•
•
•
•
Vulnerable to damage
Degenerate if retina detached
Destroyed by intense light
Outer segment renewed every 24 hours
– Tips fragment off and are phagocytized
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Rods
• Functional characteristics
– Very sensitive to light
– Best suited for night vision and peripheral vision
– Contain single pigment
• Perceived input in gray tones only
– Pathways converge
• causing fuzzy, indistinct images
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Cones
• Functional characteristics
– Need bright light for activation (have low sensitivity)
– React more quickly
– Nonconverging pathways result in detailed, highresolution vision
• Have 1 of 3 pigments for colored view
• Color blindness–lack of one or more cone pigments
For those red/green deficient, they will see a 5.
Comparison of Rods and Cones
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Chemistry Of Visual Pigments
• Retinal
– Light-absorbing molecule that combines with one of
four proteins (opsins) to form visual pigments
• Synthesized from vitamin A
– Retinal has 2 forms: bent form and straight form
• Bent form change to straight form when pigment absorbs
light
• Conversion of bent to straight initiates reactions 
electrical impulses along optic nerve
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Phototransduction: Capturing Light
• Deep purple pigment of rods–rhodopsin
– Bent retinal + opsin  rhodopsin
– Three steps of rhodopsin
formation and breakdown
1. Pigment synthesis
2. Pigment bleaching
3. Pigment regeneration
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Phototransduction: Capturing Light
1. Pigment synthesis
– Rhodopsin forms and accumulates in dark
2. Pigment bleaching
– When rhodopsin absorbs light, retinal changes to
straight form
– Retinal and opsin separate (rhodopsin breakdown)
3. Pigment regeneration
– Straight retinal converted to bent
– Rhodopsin regenerated in outer segments
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Phototransduction In Cones
• Similar as process in rods
• Cones far less sensitive to light
– Takes higher-intensity light to activate cones
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Events of phototransduction.
Slide 6
G protein signaling mechanisms
are like a molecular relay race.
2nd
Light Receptor G protein Enzyme
messenger
(1st
messenger)
1 Retinal absorbs light
and changes shape.
Visual pigment activates.
Phosphodiesterase (PDE)
Visual
pigment
All-trans-retinal
Light
cGMP-gated
cation
channel
open in
dark
11-cis-retinal
Transducin
(a G protein)
2 Visual pigment
activates
transducin
(G protein).
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3 Transducin
activates
phosphodiesteras
e (PDE).
4 PDE converts
cGMP into GMP,
causing cGMP
levels to fall.
cGMP-gated
cation
channel
closed in
light
5 As cGMP levels fall,
cGMP-gated cation
channels close, resulting
in hyperpolarization.
Light Transduction Reactions
• Light-activated rhodopsin activates G protein
transducin
• In dark, cGMP holds channels of outer
segment open  Na+ and Ca2+ depolarize cell
• In light cGMP breaks down, channels close,
cell hyperpolarizes
– Hyperpolarization is signal!
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Information Processing In The Retina
• Photoreceptors and bipolar cells only generate
graded potentials (EPSPs and IPSPs)
• When light hyperpolarizes photoreceptor cells
1. Stop releasing inhibitory neurotransmitter
glutamate
2. Bipolar cells (no longer inhibited) depolarize,
release neurotransmitter onto ganglion cells
3. Ganglion cells generate APs transmitted in optic
nerve to brain
© 2013 Pearson Education, Inc.
In the dark
Signal transmission in the retina (1 of 2).
In the dark
1 cGMP-gated channels
open, allowing cation influx.
Photoreceptor depolarizes.
Na+
Ca2+
2 Voltage-gated Ca2+
channels open in synaptic
terminals.
Photoreceptor
cell (rod)
−40 mV
3 Neurotransmitter is
released continuously.
Ca2+
4 Neurotransmitter causes
IPSPs in bipolar cell.
Hyperpolarization results.
5 Hyperpolarization closes
voltage-gated Ca2+ channels,
inhibiting neurotransmitter
release.
Bipolar
Cell
6 No EPSPs occur in
ganglion cell.
7 No action potentials occur
along the optic nerve.
© 2013 Pearson Education, Inc.
Ganglion
cell
Slide 8
In the light
Signal transmission in the retina. (2 of 2).
Below, we look at a tiny column of retina.
The outer segment of the rod, closest to the
back of the eye and farthest from the
incoming light, is at the top.
In the light
1 cGMP-gated channels
close, so cation influx
stops. Photoreceptor
hyperpolarizes.
Light
Light
Photoreceptor
cell (rod)
−70 mV
2 Voltage-gated Ca2+
channels close in synaptic
terminals.
3 No neurotransmitter
is released.
4 Lack of IPSPs in bipolar
cell results in depolarization.
5 Depolarization opens
voltage-gated Ca2+ channels;
neurotransmitter is released.
Bipolar
Cell
Ca2+
Ganglion
cell
© 2013 Pearson Education, Inc.
6 EPSPs occur in ganglion
cell.
7 Action potentials
propagate along the
optic nerve.
Slide 8
Light Adaptation
• Move from darkness into bright light
– Both rods and cones strongly stimulated
• Pupils constrict
– Large amounts of pigments broken down
instantaneously, producing glare
– Visual acuity improves over 5–10 minutes as:
• Rod system turns off
• Retinal sensitivity decreases
• Cones and neurons rapidly adapt
© 2013 Pearson Education, Inc.
Dark Adaptation
• Move from bright light into darkness
– Cones stop functioning in low-intensity light
– Rod pigments bleached; system turned off
– Rhodopsin accumulates in dark
– Transducin returns to outer segments
– Retinal sensitivity increases within 20–30 minutes
– Pupils dilate
© 2013 Pearson Education, Inc.
Night Blindness
• Nyctalopia
– Not able to see in low light
• Rod degeneration / rod damage
– Commonly caused by vitamin A deficiency
– If administered early vitamin A supplements
restore function
– Also caused by retinitis pigmentosa
• Degenerative retinal diseases that destroy rods
© 2013 Pearson Education, Inc.
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