Transcript Anatomy/Neuro-Anatomy of the Visual System

Anatomy/Neuro-Anatomy of
the Visual System
Medical ppt
http://hastaneciyiz.blogspot.com
Learning Objectives
• Describe the function of major structures of
the visual system
• Describe major milestones in development of
the visual system
• Describe normal age related changes in
vision and their impact on occupational
performance
• Describe changes in visual function
associated with pathology
Eye as a camera
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
Structures of the Eye and
Orbit
The anterior visual system
Orbit
• Eyeball
• Optic nerve
• Extraocular
muscles
• Other nerves
• Blood vessels
• Lacrimal gland
• Fat
• Connective tissue
Eyelids and Eyelashes
• Protect eye from foreign bodies
• Help limit light into the eye
• Functions as part of the lacrimal system
• Blinking squeezes tears from lacrimal
gland
• Tears fill in uneven surfaces of cornea
• Nourishes and protects cornea
Eyeball has three layers
Outer protective layer
• Sclera and cornea
Middle vascular layer
• Uveal tract
• Consists of iris, ciliary
body and choroid
Inner sensory layer
• Retina
Sclera
•
•
•
•
•
Encloses eyeball except for cornea
Extension of the dura mater of CNS
Protects inner contents of eye and
Helps maintain shape of the eye
Extraocular muscles attach to its
surface
Cornea
QuickT ime™ and a
•
•
•
•
Avascular
Transparent
5 layers
Protects inner
contents of eye
• Refracts light
P hoto CD Decompressor
are needed to use this pict ure
Aqueous
• Continuously produced
& drained away
• trabecular meshwork
• canal of Schlemm
• Maintains health of lens
and cornea
• Maintains shape &
pressure within eye
Iris
• Pigmentation
protects retina
• Controls pupil
aperture
• Dilator muscle
sympathetic control
• Spincter muscle
Lens
• 65% water 35%
protein
• Avascular
• Refracts light to
focus image onto
retina
• Fibers form
throughout life
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
Ciliary Body
• Ciliary muscle
• Shapes lens
• Controlled by CN III
• Ciliary process
• Secretes aqueous
Vitreous
• Maintains
transparency and
form of eye
• Holds retina in place
Conjunctiva
• Thin transparent membrane covering
sclera and inner eyelid
• Provides protection and moisture
• Many blood vessels, few pain fibers
• Conjunctivitis common condition
Choroid
• Vascular supply for
eye
• Capillaries and
veins
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
Retina
Direction of light
•
•
•
•
Lines posterior 2/3rd of eye
Distant receptor organ
5 layers
Inside out arrangement
Rod Receptor Cells
Rod Cells
•
•
•
•
Concentrated in periphery
Activate in low illumination
Detect general form, not details
Provide background information
Cone Receptor Cells
Cone Cell
•
•
•
•
Capture detail and color
Require direct stimulation
Bright light’
Concentrated in fovea
Retinal Pigment Epithelium (RPE)
RPE Layer
• Works with Bruchs membrane and
choroid layer
• Maintains health of receptor cells
• Breakdown causes build up of cellular
debris
Retinal Processing Pathway
Axons form
Optic nerve
Ganglion Bipolar
cells
cells
• Impulses converge onto bipolar cells
• Converge again onto ganglion cells
• Axons of ganglion cells merge and exit
at optic disc
Optic nerve
• CN II
• Each nerve contains
1 million plus heavily
myelinated ganglion
axons
• Macular fibers inside
peripheral fibers
outside
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
Visual Field
• Visual field
• 160-180 degrees horizontally
• 120 degrees vertically
• Practical field of vision
• Head and eye movement
• 270 degrees
Hill of Vision Concept
Visual Field Diagram
Extraocular Muscles (EOM)
•
•
•
•
•
•
Medial rectus
Lateral rectus
Superior rectus
Inferior rectus
Superior oblique
Inferior oblique
Sup. oblique
Sup. rectus
Med. rectus
Lat. rectus
Inf. rectus
Inf. oblique
Cranial Nerves Controlling
Extraocular Eye Muscles
• CN III Oculomotor
• CN IV Trochlear
• CN VI Abducens
Oculomotor Nerve (3)
• Innervates 5 muscles
• Medial, superior,and inferior rectus
muscles, inferior oblique
• Levator palpebrae superioris
• Internal musculature of the eye
• Ciliary muscle (lens)
• Spincter muscle (pupil)
Trochlear Nerve (4)
• Innervates superior oblique
• Down and out muscle of eye
Abducens Nerve (6)
• Innervates lateral rectus
• Abducts eye
Birds Eye View of Visual Pathways
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
Development of the
Visual System
Visual system develops from
three types of tissue
• Neuroectodermal from brain
• Becomes retina, iris and optic nerve
• Surface ectoderm of head
• Forms lens
• Mesoderm
• Forms vascular supply and sclera
Embryonic Eye Development
3-4 weeks gestation
The eye begins as
a groove in the
neural fold on the
cranial end of the
embryo
Over the next
week, the groove
turns inside out
and balloons
outward and
creates a hollow
bulb projecting
from each side
of the neural tube
At 4 weeks, the bulb and stalk are fully
formed. The lateral surface of the bulb
begins to flatten and the ectoderm
thickens to become the lens placode
The placode turns in on itself to form
a deep indentation (the lens pit). The
ends of the pit come to together to form
the lens vesicle, which then is pinched
off to become the lens.
At the same time, the optic vesicle
begins to fold in on itself to form a
double walled, bowl shaped structure
called the optic cup. The two walls
fuse together, the outside wall
becomes the RPE, the inside wall becomes the sensory retina. The axons
of the ganglion cells converge into the
optic stalk to become the optic nerve.
Retina
Cornea
Lens
Optic
nerve
• Rim of optic cup eventually becomes
the ciliary body and muscle, iris, dilator
and sphincter muscles
• Mesenchyme cells develop into the
choroid and sclera-both are extensions
of vascular and fibrous structures within
brain
• Sclera-continuation of dura mater
• Choroid-continuation of pia arachnoid
• Form a sheath around the optic n.
The relationship between these
structures explains why an increase
in cerebral spinal fluid after brain
injury can be diagnosed by
observing the optic disc for
papilledema
Maturation of Face and Eyes
• As the embryo develops, the eyes
migrate from the sides to the front as
the face matures
• Face is formed by 14 weeks
• During development, structures may fail
to fully form or to close completely
• Creates many of the congenital eye
conditions observed in children
Maturation of Visual System
Pre-natal
Post-natal
• Rods and Cones
• 25 wks-both begin to
develop
• Optic Tract
• 28-38 wks-begins to
myelinate
• Superior Colliculus
• Basic structure
develops 16-28 wks
• Rods and Cones
• 4 mos-complete with
rods finishing first
• Optic Tract
• Rapid myelination
first 2 mos continued
for 2 years
• Superior Colliculus
• Myelination
completed at 3 mos
Maturation of Visual System
Pre-natal
Post-natal
• LGN
• Matures after birth
• GC Tracts
• Myelination begins at
birth
• LGN
• Process takes 9 mos
• Stereoscopic vision
at 3-4 mos
• GC Tracts
• Completed in 4-5
mos
Maturation of Visual System
Pre-natal
Post-natal
• Visual cortex
• 25-28 wks-starts
dendritic growth,
increasing synaptic
density, cortical
layers develop
• Visual cortex
• Doubles in density
first 2 years, adult
synaptic density and
functional maturity by
age 11
Eye Movement
• Able to fixate and make basic eye
movements by 2-3 months
• 2 years to obtain good control
• Up to 9 years to obtain complex control
Visual Acuity
• Newborn
• 20/200, sees best in 2-75 cm range
• 3 months
• 20/60
• 6 months
• 20/20
• 2 years
• Acute near vision-fine motor skills develop
Normal Age Related Changes
in Vision
Reduced Visual Acuity
• Static acuity
• Decreases to 20/30-20/40
• Prevalence 40% by age 70
• Dynamic acuity
• Decrease may be due to reduced OM
control
Loss of Accommodation
• A.k.a. presbyopia
• Result of compacting of protein fibers in
center of lens
• Lens thickens and loses flexibility
• Occurs gradually beginning in 40s
• Creates need for bifocal
Floaters
• Strands of protein which float in vitreous
• Float more easily in old eye because
vitreous is more fluid
• More noticeable in bright light
• Generally benign unless accompanied
by bright flashes of light or significant
increase in number
Dry Eyes
• Lacrimal glands do not make enough or
make poor quality tears
• More prevalent in women
• Can be exacerbated by medication
• Causes itchiness, burning, decreased
acuity
• Treated with artificial tears or surgery
Increased Need for Light
• Pupil diameter decreases
• A.k.a. senile miosis
• Lens thickens becoming more yellow
• Combined-these two conditions reduce
the amount of light coming into eye
• 80 yr old person needs 10x as much
light as an average 23 year old
Susceptibility to Glare
•
•
•
•
Lens and cornea become less smooth
Lens & vitreous develop protein strands
Combine to cause light scatter
Increased discomfort and disability
• Lose acuity under glare condition
• Also takes longer to recover from glare
Reduced Dark/Light Adaptation
• Takes longer to reform and store
pigments
• Never reach same level of dark
adaptation as younger person
• More difficult to go from bright to dark
than dark to bright
Reduced Contrast Sensitivity
• Caused by changes in color and density
of lens and decreased pupil aperture
• 75 year old needs 2x as much contrast
as younger person
• 90 year old needs 10x as much contrast
Reduced Color Perception
• Caused by yellowing of lens
• Decrease in sensitivity at violet end of
spectrum
• White objects may appear yellow
Reduced Visual Field
• Changes in facial structure
• Nose grows??
• Orbit loses fat and eye sinks in
Reduced Visual Attention
• Decline in ability to
• Attend to objects in complex, dynamic
arrays
• Simultaneously monitor central and
peripheral visual fields
• Diameter of visual field decreases
• 90 yr olds-40% have an attentional field of
less than 20 degrees
Pathology of the Visual
System
Anterior visual system has
three jobs to do
• Focus the image on the retina
• Capture the image (encode it)
• Transmit the image to the CNS
Sharp focusing of image on
retina depends on:
• Sufficient refraction of light rays entering
the eye
• Focal point established on the fovea
• Transparency of all intervening
structures between outside of eye and
retina
• Adequate illumination
Sufficient Refraction of Light
Rays entering the Eye
f
Emmetropia
Focal point is established on fovea of retina
Hyperopia
Myopia
Smoothness of Refracting
Surfaces
• Astigmatism
•
•
•
•
Cornea is spoon shaped or dimpled
Light rays are unevenly refracted
Can develop with trauma and age
Corrected for optically with cylinder
• Cataract
•
•
•
•
•
Dead cells deposited in lens, calcify
Begins in periphery, progressing to center
Surface becomes pitted
Causes light scatter and veiling glare
Eventually complete opacity
Closeness of Object
• As object comes closer, focal point on
retina is pushed back
F
E
Closeness of Object
• As object comes closer, focal point on
retina is pushed back
F
E
Accommodation
• 3 step process
• Convergence
• Lens thickens
• Pupil constricts to
reduce light scatter
• Controlled by CN III
• Affected by lens
• Presbyopia
• Aphakia
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
Control of Light Scatter
• Light rays are refracted more strongly in
periphery than center of lens
• Causes wild and scattered light rays
• Reduced by blocking peripheral rays
with pupillary constriction
• Increases acuity
• Pinpoint vision
Transparency of Intervening
Structures
• Any opacity in cornea, aqueous, lens,
vitreous will prevent image from
reaching retina
• Common conditions
• Corneal scarring
• Cataract
• Trauma-vitreous hemorrhage
• Also causes veiling glare
Adequate Illumination
• Retina must be adequately diffused with light
to capture an image
• Amount of light is controlled by pupil
• Any condition affecting responsiveness of
pupil will affect
• Tolerance of light
• Ability to rapidly adjust to changes in light
• Opacity in intervening structures also affects
amount of light entering eye
Ability of retina to capture image
• Retinal function can be affected by disease,
injury or congenital conditions
• Macular degeneration, diabetic retinopathy,
retinitis pigmentosa, retinal detachment
• Damaged retina creates blind spot in vision
• Known as a scotoma
• Performance limitations depend on area of
retina damaged
• Peripheral vs. central
Macular Scotoma
• Area of reduced light sensitivity within
central 20 degrees of the visual field
Macular Scotoma
• Occurs with retinal diseases
• Affects ability to
• See small details
• Discriminate contrast
• Discriminate color
• Primary pathology dealt with in patients with
low vision
• 83% of patients referred for low vision services
found to have dense macular scotomas regardless
of disease
Scanning Laser
Ophthalmoscope
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
Central Scotoma
• Scotoma impinges
on and involves the
fovea
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.
Para-central Scotoma
• Within the central 20
degrees of the field
but not involving the
fovea
Ring Scotoma
• Surrounds the fovea
on 4 sides
QuickTime™ and a
Photo CD Decompressor
are needed to use this pict ure
Scotomas may vary in density
• Dense
• No response to light
• Relative/Threshold
• Responds to light if
bright enough
QuickTime™ and a
Photo CD Decompressor
are needed to use this picture
Adaptation to Scotoma
• Scotoma creates a hole in visual field
• Deprives CNS of vision needed to identify
objects
• CNS adapts using various mechanisms
• Perceptual completion
• Metamorphopsia
• Development of PRL
Perceptual Completion
• Perception in which objects or a visual
scene appears complete despite
missing visual input
• Example of top down “cognitive
processing” where we see something
because we expect to see it
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
• Scotomas less than 5 degrees
• CNS can perceptually complete
• Example: own blind spot
• Person unaware of presence of scotoma
• Scotomas greater than 5 degrees
• CNS will attempt to perceptually complete but may
not be successful
• Person does not perceive black hole but instead a
blurriness or inability to bring object into focus
Metamorphopsia
• Scotoma is too big
to complete
perceptually
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
• Objects appear
warped or
misaligned
Preferred Retinal Locus (PRL)
• If scotoma covers fovea, CNS adopts
an eccentric retinal area to act as a
pseudo-fovea for visual tasks previously
completed by the fovea
• Develops within 24 hours of loss of
fovea
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.
40% place PRL above the
scotoma on the retina
(leaves lower portion of field clear)
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
35% place PRL to the right
(leaves left side of page clear)
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
20% place PRL to left
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
7% place PRL below
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
• Person may develop more than one PRL and
use a different one depending on task and
lighting conditions
• Ability to use PRL to direct eye movements is
more highly correlated to reading ability than
other visual function
• Although person develops PRL, he/she may
not be aware of it
• Important to assess ability to use PRL
Peripheral Visual Field Deficit
• Person also exercises perceptual
completion
• May be completely unaware of deficit
• Will not interfere with perception of
visual details (acuity)
• But will affect mobility
• Reduces detection of motion and form
Ability of optic nerve to transmit
visual input
• Can be damaged by disease, trauma
and congenital conditions
• Glaucoma
• Optic neuritis
• Head injury
• Can lose all or part of field
• Depending on location, extent of damage
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
Deficits in Posterior
Visual System
Visual Field Deficits
• Lesions along geniculocalcarine tracts or in
occipital lobe
• Most common cause in adult is stroke
• Posterior cerebral artery (PCA)
• Pure visual stroke (sometimes affects language)
• Middle cerebral artery (MCA)
• Mix of motor, sensory, visual, cognitive
• Lesion behind LGN will always cause
homonymous loss
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
Homonymous Hemianopsia
QuickT ime™ and a
P hoto CD Decompressor
are needed to use this pict ure
Hemianopsia with Macular
Sparring
Cortical Blindness
• Also called cortical visual impairment
• CVI
• Damage is so significant in occipital
lobe, CNS is not able to complete any
cortical processing of vision
Person loses:
Person loses:
• Object identification through visual
system
• Visual orientation to space
• Cognitive application of vision
Person retains
Person retains
• Subcortical processing of vision
• Navigational vision
• Vision for safety
• Other sensory processing
• Haptic discrimination
• Auditory discrimination
Alteration of Visual Attention
•
•
•
•
•
•
Difficulty arousing attention
Difficulty attending globally
Difficulty attending to details
Difficulty sustaining attention
Difficulty dividing/shifting attention
Asymmetrical attention
• Unilateral spatial neglect
Diminishment of Attention
• Has pervasive effect on cognition
• Person takes in information in incomplete
disorganized fashion
• CNS cannot properly analyze incoming
information
• Decision making is based in incomplete
and/or incorrect information
• Garbage in - garbage out
Medical ppt
http://hastaneciyiz.blogspot.com