Visual System
Lecture Outline
• Structures of the Eye
• Refraction and Image Formation
• Visual Acuity
• Autonomic Control of Pupil Diameter
• Clinical Correlations
•
Dr W Kolbinger, Visual System (2009)
1
Anatomic Considerations
The Ocular Fundus
Fovea
Optic
disc
Macula
The optic disc region itself only contains axons
of retinal ganglion cells, the output elements
of the retina, but it lacks photoreceptors. As a
consequence, the optic disc is responsible for
the blind spot, a region inside the boundaries
of the visual field, where we don’t receive
visual information.
3
Optics of the Eye
Convex Lens Focuses Light Rays
Concave Lens Diverges Light Rays
Measurement of the Refractive
Power of a Lens—“Diopter”
The refractive power in
diopters of a
convex lens is equal to 1 meter
divided by its focal
length.
Optics of the Eye
Cornea refractive power: 42 D
Flat lens refractive power: 13 D
Rounded lens refractive power: 26 D
Plasticity: 13 D
5
Accommodation
• Far Vision
Ciliary muscle relaxed
Suspensory ligaments tightened
Flat lens refractive power: 13 D
Focus on the Retina
Accommodation Adjusts the
Refractive Power of the Eye
6
Accommodation
• Near Vision
Ciliary muscle constricted
Suspensory ligaments floppy
Rounded lens refractive power: 26 D
Focus on the Retina
7
Presbyopia
• Near Vision
Flat lens
The variability of the refractive power
of the lens between far vision (13 D)
and near vision (26 D) is called
refractive plasticity.
Unfortunately,
the lens looses its elasticity
during aging, thereby
reducing the ability to focus
on near objects, a condition
called presbyopia.
Blurred picture on the Retina
8
Emmetropia (Normal Vision).
Cataracts
“Cataracts” are an especially common eye abnormality
that occurs mainly in older people. A cataract is a cloudy
or opaque area or areas in the lens
What is Visual
Acuity?
Visual acuity is the ability to distinguish between two nearby
points. Visual acuity is high when the two-point discrimination
threshold is low (high spatial resolution).
• Two point discrimination of the visual system
• Normal: an angle of 5 minutes of a degree
•
highly dependent on the densities of retinal photoreceptors
visual acuity also depends on a proper function of the optical apparatus of the eye,
including accommodation. When the optical apparatus fails to produce a focused (sharp)
picture on the retina, the objects in the visual field appear “blurry”.
5‘
Far vision
Near vision
10
Neurological Examination of Visual Acuity
• Eye charts and near cards
Patient
20
800
Normal
Distance
equivalents
20
20
11
RETINAReceptor and
Neural Function of the Retina
•
•
•
•
Dr W Kolbinger, The Retina (2009)
Photoreceptors and Phototransduction
Color Vision
Processing of Visual Information
Clinical Correlations
13
Layers of the Retina
Photoreceptor
layer
Photoreceptors
rhodopsin
Disks
Outer segment
-Visual pigment
Nucleus
Synaptic
ending
Rods are highly
sensitive to light
and enable us to
see under low
intensity light
conditions (at
night).
Inner segment
Synapses with bipolar
and horizontal cells
Rod
Cones(3) are less
sensitive to light.
enable
us to see colors.
glutamate
Cone
15
•Convergence is high in the rod
system. It is low in the cone system. As a
consequence, spatial resolution (visual acuity) is
better in bright light, when the cone system is
active.
•Rods and cones are not evenly distributed over
the whole retina and the fovea only contains
cones, but no rods. As a consequence, there is
no central vision under dim light conditions
• Night Blindness
The Visual Pigment
Rhodopsin is the visual
pigment of rods. It consists
of two components:
· Opsin, a protein
which is synthesized
in the photoreceptor
(cones have different
types of opsins).
· Retinal, a
chromophore, is the
light absorbing
compound or the
visual pigment. It is
derived from Vitamin
A and is the chromophore of the visual pigment in rods
and cones. Vitamin A is synthesized from beta-carotene
contained in our food.
Photoreception -The Dark Current
GDP
cGMP
Na+
,
G protein
(Transducin)
Visual pigment-Rhodopsin
Dr W Kolbinger, The Retina (2009)
cGMP-gated channel
cGMP phosphodiesterase
18
Phototransduction
GTP
cGMP
Na+
5’GMP
cGMP phosphodiesterase
Dr W Kolbinger, The Retina (2009)
19
Color Vision Is Based on Comparison of Activity of
Three Cone Types
Visible Light Is Part of the Electromagnetic Spectrum
The visible part of the spectrum is characterized by
wavelengths ranging from 400 to 700 nm (nanometers).
20
Monochromatic Light: Colors of a Rainbow
Dr W Kolbinger, The Retina (2009)
21
Three Types of Cones Have Different Spectral Sensitivities
The human retina contains
three types of cones:
· S (short wavelength
sensitive) cones, also
called “blue” cones,
with a maximum
sensitivity at 430 nm
· M (medium
wavelength
sensitive) cones, also
called “green” cones,
with a maximum
sensitivity of 530 nm
· L (long wavelength
sensitive) cones, also
called “red” cones, with a maximum sensitivity of 560 nm.
Log relative sensitivity
Stimulation of Cones by Monochromatic Light
450 nm
monochromatic
light
400
500
600
700
Wavelength (nm)
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Stimulation of Cones by Monochromatic Light
Log relative sensitivity
600 nm
monochromatic
light
400
500
600
700
Wavelength (nm)
24
Red-Green Color Blindness
Protanopia
Normal
Individuals affected by red-green color
blindness can no longer distinguish
certain red colors from certain green
colors.
Deuteranopia
25
pseudo-isochromatic color
plates like the one on the
right are presented to the
patient.
The numbers
embedded in the pattern of
colored dots can be
distinguished by individuals
with a fully intact color
vision.
Dichromats, who are weak
in red-green discrimination,
have difficulties in
identifying the numbers on all
plates
Visual Pathways
Lecture Outline
•
•
•
•
•
The Visual Field
Passage of Light
Neuronal Pathways
The Primary Visual Cortex
Parallel Pathways for Depth, Motion, Form and
Color
• Clinical Correlations
27
The Visual Field
Total amount of space we can see with
this eye, when the eye is fixed straight
ahead, pointing towards the center of the
visual field (point of fixation).
The extension of the visual field is
measured in degrees of maximum
deviation from this straight line in all
directions.
For right eye- left visual hemifield/nasal
- right visual hemifield/temporal
The vertical axis together with the
horizontal axis divide the visual field into
Four quadrants:superior left, superior right,
inferior left and inferior right
quadrant of the visual field.
Projections of the Visual Field on the Retina
The four quadrants of the visual
field are projected onto the retina.
The superior half of the visual field
is projected to the inferior half of
the retina, and vice versa.
The left half of the visual field is
projected on the right half of the
retina, and vice versa.
The nasal visual hemifield of the right eye  temporal hemiretina of the right eye.
temporal visual hemifield of the right eye nasal hemiretina of the right eye
visual field defects e.g “bitemporal hemianopia``
29
Chief Complaint: Headache and Nausea
History:
A 76 year old retired college dean presents with recurrent headaches over the past four
years. Her headaches have significantly increased in duration and intensity over the
past few months. She also reports episodes of nausea and vomiting. Her husband
adds that she has recently developed difficulties with comprehending spoken language.
The patient experiences uncoordinated movements of the right hand. She stopped
drinking and smoking cigarettes during her first pregnancy, when she was 27.
General Examination:
76 year old female in no acute distress. No significant cardiac, respiratory, or
abdominal abnormalities. Vital signs unremarkable.
Neurological Examination:
The patient can speak fluently, but the content is frequently incomprehensible to the
listener. Her ability to read and write is markedly reduced (dyslexia and dysgraphia
respectively.) The patient can not identify a hairbrush, but was able to demonstrate its
use. Numbness was detected in the right lower face and right hand. In addition, the
examination of the right hand revealed impaired 2-pt discrimination, joint position, and
fine touch.
Motor examination and reflex testing of the lower extremities were unremarkable.
Marked weakness of right arm flexion and extension was present, as well as elevated
right biceps and brachioradialis reflexes.
Brodmann’s Areas - Left Hemisphere.
Neuronal Pathways, From the
Retina Onwards
relay station between the retina and the
primary visual cortex.
 circadian clock
 afferent limb of the pupillary light reflex
contribute to eye movements
The Primary Visual Cortex V1
Occipital
pole
Area 17
34
The Primary Visual Cortex V1
Parieto-occipital
sulcus
Calcarine
sulcus
Area 17
35
Neuronal Pathways, From the Retina Onwards
• Fibers originating in nasal hemiretina
cross over at the optic chiasm, fibers
originating in the temporal hemiretina
don’t.
• LGN fibers carrying sensory
information of the superior half of the
visual field follow the temporal
radiation pathway, fibers originating in
the inferior half of the visual field
follow the parietal radiation pathway.
Example1- the axons originating in the
right LGN which are carrying
sensory information from the superior left
quadrant of the visual field, use the
temporal radiation and synapse in the
inferior portion (below the calcarine
sulcus) of the primary visual cortex (V1).
Example 2 - ?
Retinotopic Organization of V1
Visual field
left eye
Visual field
right eye
Left
hemisphere
Right
hemisphere
37
Neurological Examination of the Visual Fields
The visual pathways are commonly tested in neurological examinations and they
have high localizing value.
Loss of vision is clinically tested in each
quadrant of the visual field in a
“confrontation visual field test”. In this test,
each eye is tested separately by
having the patient look straight at the
examiners eye, while standing in double
arms length distance. While the examiner
occludes his left eye with one hand,
the patient occludes his right eye (and vice
versa). Then the examiner moves his
other hand, with one (or more) of his fingers
stretched out, gradually from the
periphery to the center of the visual field, to
determine where it is first seen.
Assuming the examiner has normal vision, the
patient should see the
appearance of the hand at the same time as
the examiner. He should also be
able to tell the number of fingers stretched.
Sparing of the Macula
When the macula is not included, Macular
sparing is often associated with vascular
lesions involving the
posterior cerebral artery or its branches,blood
supply of the occipital pole of the cerebbral
cortex (the area representing macular vision)
may stay intact, due to sufficient blood flow
from the middle cerebral artery.
Eye Movements
Lecture Outline
• Types of Eye Movements
• Extraocular Muscles, their Innervation and
Control
• Saccadic Eye Movements
• Clinical Correlations
40
Types of Eye Movements
• Conjugate
Eye Movements
• Saccadic eye movements (and gaze)
• Vestibulo-ocular reflex
• Optokinetic reflex (and smooth pursuit)
• Non-Conjugate Eye Movements
• Vergence (convergence and divergence)
41
Isolating Extraocular Muscle Function
42
Neurological Examination of Eye Movements
Extraocular movements are examined using the
“H-test”.
Cranial Nerve Nuclei and Control Units in
the Brainstem
Midbrain
CN III
CN IV
nucleus of MLF-control
of vertical eye
movements
III
III
IV
IV
Cerebellum
CN VI
Pons
VI
VI
pontine paramedian
reticular formation (PPRF)/
horizontal gaze center.
vestibulo-cerebellum
(flocculo-nodular
lobe)-optokinetic eye
movements
Medulla
44
Cortical Control Units
Frontal eye field
(Area 8)
planning and
initiation
of eye
movementssaccadic eye
movements
Parieto-occipital
eye field
dorsal (parietal)
pathway for motion (and
depth) led up exactly into
this area.-optokinetic
movements and smooth
pursuit
45
Saccadic Eye Movements to the Right
Frontal eye field
Right
Left
PPRF
Left MLF
46
Diplopia
Visual field
of the left eye
Visual field
of the
right eye
Fovea
Fovea
47
Internuclear Ophthalmoplegia
Internuclear
ophthalmoplegia is based
on a lesion of the medial
longitudinal fasciculus
(MLF), which prevents
adduction of the eye on the
side of the lesion during
attempted lateral gaze.
In the example on the right
shows a patient with a
lesion of the left MLF
(adduction of the left eye is
impaired).
Convergence does not involve the
MLF and is not affected by the
lesion. EXAMPLE- Multiple
Sclerosis.
Internuclear Ophthalmoplegia
Prevents adduction
of the right eye
X
MLF lesion on
the right
Right
Left
49
Chief Complaint: Nausea and Vomiting
History:
An 8 year old boy visited his pediatrician, and his mother reported that he was in
good health until about two weeks ago. He describes that his initial symptoms
include a mild bifrontal headache, which has become progressively worse. During
the last few days, he developed nausea and bouts of occasional projectile vomiting
accompanying the headaches. Furthermore, he recently noticed that he has
tremendous difficulty when walking down the stairs from his bedroom to the
kitchen and reported difficulties in sleeping. No family history of abnormal
development or mental retardation exists. He has two perfectly healthy younger
brothers.
General Examination:
This 8 year boy presents with pronounced pubic hair growth, a low pitched voice,
enlarged genitalia, and acne on his forehead. Cardiac and respiratory examinations
were unremarkable.
Neurological Examination:
Patient is alert and oriented x 3. No receptive or expressive aphasias were noted.
Pupillary light reflex was intact bilaterally. Both the left and right optic discs
appeared more pale than normal. Downward gaze (while the eyes were adducted)
was impaired bilaterally.
MLF lesion + PPRF lesion = 1 ½ Syndrome
Prevents adduction
of the right eye
X
X
PPRF lesion
on the right
Right
Prevents conjugate gaze
of both eyes to the right
X
MLF lesion on
the right
Left
51
The Basics of Hair Cell Morphology
Endolymph:
High in
Potassium
Apex of Hair Cell
with Cilia
Base of Hair Cell
with Synapse on
Afferent Fiber
52
Cells of the Retina
Photoreceptor
Horizontal cell
Bipolar cell
Amacrine cell
Ganglion cell
53
Some Bipolar Cells are Activated by Light, Others by Darkness
There are two basic types of retinal bipolar cells. Some bipolar cells are activated
(depolarized) when the light is ON. They are therefore called ON bipolar cells.
Other bipolar cells are activated (depolarized) during darkness, when the light is
OFF. They are therefore called OFF bipolar cells.
Light Depolarizes ON Center Bipolar Cells
Signconverting
synapse:
metabotropic
glutamate
receptor
Photoreceptor
hyperpolarizes
during light ON
On Center
Bipolar Cell
depolarizes
during light ON
54
Parallel Pathways
Specialized for Visual
Information of Depth, Motion, Form and Color
Dorsal (parietal)
pathway
Depth Motion
LGN
Where?
Magno
Parvo
Ventral (inferior temporal)
pathway
What?
Form Color
55
Light Hyperpolarizes OFF Center Bipolar Cells
Signconserving
synapse:
ionotropic
glutamate
receptor
Photoreceptor
hyperpolarizes
during light ON
OFF Center
Bipolar Cell
hyperpolarizes
during light ON
56