Download The eye

The eye
« Take Home Message » The Eye
• The wall of the eye has three layers: Outer layer of sclera and
cornea middle layer of choroid,
cornea,
choroid ciliary body and iris and
inner layer retina.
• The
Th eye is
i movedd by
b six
i muscles
l musculus
l rectus
t
superior/inferior/medialis/lateralis and musculus obliquus
s perior/inferior
superior/inferior.
• The blind spot corresponds to the area where the optic nerve
enters the
h eye.
Optics
Lens
(a) Emmetropia
(normal)
(b) Myopia
(nearsightedness)
Correction
(c) Hypertropia
(farsightedness)
Correction
Biconcave
(diverging)
lens
Biconvex
(converging)
lens
Snellen Tafel zur Bestimmung des Visus
Visus
i
= Distanz
i
des
d Beobachters
b h
zur Tafel
f l
/ Distanz bei der der kleinste lesbare
Optotyp 5 visuelle Arcminuten gross ist.
Normalwert: 1,0-1,6 (20 Jahre alt)
0,6-1,0 (60 Jahre alt)
G if
Greifvogel
l ~ 10,0
10 0
« Take Home Message » Optics
• The diffraction of light rays by the cornea and the lens create
an image on the retina
• If the optics of the eye is abnormal, then the image appears
bl
blurred:
d hyper-metropia,
h
t i myopia
i
• Corrections with lenses of abnormalities of image formation
• Visual acuity can be measured with the Snellen Chart.
Retina
Rod
Rod
Cone
Outer
nuclear
layer
Distal
Bipolar
cell
Outer plexiform
layer
Vertical
information
flow
Bipolar
cell
Horizontal cell
Lateral information flow
Amacrine
cell
Amacrine
cell
Proximal
Inner
nuclear
layer
Inner
plexiform
layer
Ganglion
cell layer
Ganglion
cell
Light
To optic nerve
Plexus: Geflecht
G
« Take Home Message » Retina
• The retina is a layer formed by different cells
• Vertical transfer of information flows from photoreceptor
cells (rods, cones) to bipolar cells and, then, ganglion cells
emit axons leaving the eye,
eye forming the optic nerve
• Horizontal transfer of information is mediated by connections
via
i the
th horizontal
h i t l cells
ll andd amacrine
i cells
ll
Photoreceptors
A
Discs
Cytoplasmic space
Outer
segment
Plasma
membrane
Outer
segment
Cilium
Mitochondria
Inner
segment
g
Inner
g
segment
Nucleus
Synaptic
terminal
Synaptic
terminal
Cone
Rod
B
Free-floating
discs
Folding of
outer cell
membrane
Folding of
outer cell
membrane
Rod
Connecting
cilium
Cone
« Take Home Message » Photoreceptors
• Transduction is the transformation of one form of energy into
another form of energy (e
(e.g.
g light into bioelectrical energy)
• In the eye, transduction takes place in the retina, and is
performed
f
d bby th
the photoreceptor
h t
t cells:
ll cones andd rods
d
• In the outer segment of cones and rods, photo-pigments are
modified
difi d chemically
h i ll by
b the
h presence off light:
li h in
i rods
d the
h
photo-pigment is rhodopsin
• Each rod has 1,000 disks, each with 10,000 rhodopsin
molecules
Photoreceptor distribution
« Take Home Message » Photoreceptor Distribution
• Cones are densest in the fovea (200,000 cones per sq.mm),
Rods in the periphery
• The central fovea covers about 5 degrees of visual angle, the
th b held
thumb
h ld att arms length
l th covers about
b t 1-2
1 2 ddegrees.
Dynamic range
« Take Home Message » Dynamic range
• The human visual system operates in a wide range of
luminance value spanning 14 log units.
units
• Cones (5-6 Million total number) important for color vision,
hi h acuity,
high
it llow sensitivity
iti it (daytime
(d ti vision),
i i ) weakly
kl
converging system of connections
• Rods
d ( 120 Million
illi totall number)
b ) are important
i
for
f gray scale
l
vision (in penumbra), low acuity, high sensitivity, highly
converging
i system off connections
i
• We distinguish scoptopic (rod only), photopic (cone only) and
mesopic (both receptors) vision.
Phototransduction
cGMP Phosphodiesterase
Outer segment Visual pigment G-Protein
(Rhodopsin) (Transducin)
membrane
Cytoplasm
Disc
Disc
interior
GTP
5' GMP cGMP
cGMP-gated
channel
Na+
Light
Extracellular
space
Rod outer segment
Na+
K+
Na+
K+
Membran
ane potential
Em
Time
Membrane pootential
Photons
Em
Time when photon is absorbed
Neurotransmitter
release (Glutamate)
« Take Home Message » Phototransduction
• At rest (in darkness)
darkness), cGMP maintains sodium channels open
and entrance of sodium thus depolarizes the photoreceptor at
a value of -40 mV
mV, a stable resting potential corresponding to
a so-called darkness current (entrance sodium, exit of
potassium)
• In presence of light, the photo-receptors are hyperpolarized
• The
h enzymatic
i cascade
d (including
(i l di transducine)
d i ) triggered
i
d by
b
light leads to a decrease of cGMP concentration in the
photoreceptors
h
• As a result, the release of neurotransmitter, that was relatively
high at rest (when the cell is depolarized), decreases
ON and OFF retinal ganglion cells
« Take Home Message » ON and OFF cells
• The receptive
p
field of retinal gganglion
g
cells is circular,,
comprising two antagonist zones: the center and the surround
On-center
center cells are activated when light is presented in the
• On
center and inhibited if the surround is illuminated
• The reverse is true for off-center
off center cells
• Surround suppression is the main function of the horizontal
cells.
ll
« Take Home Message » Adaptation
• The visual dynamic
y
range
g spans
p
14 log
g units.
• The amount of light the pupil admits into the eye varies over
a range of 16 to 1. Therefore the pupil makes only a very
limited contribution to adaptation.
• Retinal ganglion cells are sensitive to changes in
illumination, not absolute levels of illumination.
• The
Th visual
i l system
t adapts
d t to
t constant
t t illuminations,
ill i ti
andd if we
do not move our eyes we become blind to constant
ill i ti
illumination.
Retinal signalling
Sign inverting synapse
Receptor
all hyperpolarize to light
Horizontal Cell
all hyperpolarize to light
Bipolar Cell
some hyperpolarize and
some depolarize
Amacrine Cell
some give action potentials
Ganglion Cells
all give action potentials
« Take Home Message » Retinal signaling
• Depending
p
g on the receptors
p
on the bipolar
p
cell,, cone/rod
signals can conserve or invert the photoreceptor signal (sign
conservingg or sign
g inverting
g synapses).
y p )
• All photoreceptors and horizontal cells hyperpolarize to light.
• Bipolar cells can hyperhyper or depolarize to light.
light
• Some Amacrine and all Retinal ganglion cells emit action
potentials.
t ti l
Color vision
Light
Light
Light
B
G
R
red
orange
ll
yellow
S cone
M cone
L cone
green
violet
nal response
Retin
blue
100
Blue Green
cone cone
Red
cone
75
50
25
0 400
500
600
Wavelength
700
Monochromatic
Di h
Dichromatic
ti
C l bli
Color
blindness
d
1. Incidence:
males: 8/100 in whites, 5/100 in asians, 3/100 in africans
females: frequency 10 times less
2. Types:
protanopes:
p
p
lack L cones
deuteranopes: lack M cones
tritanopes: lack S cones, very rare (1/10000)
3. Color tests:
Ishihara plates
Farnsworth-Munsell Hue Test
Dynamic computer test
Ishihara plate #2
Those with normal colour
vision should read the
number 8. Those with redgreen colour vision
deficiencies should read the
number
b 33. T
Total
t l colour
l
blindness should not be able
to read any numeral.
« Take Home Message » Color Vision
• Cones are responsible
p
for color vision
• There are three main types of cones, each containing different
photopigments, absorbing preferentially different wave
lengths
• Only 1 of 8 cones is blue
blue. Red and green are equal.
equal
• Colour blindness occurs when particular cone types are
absent.
b t
Visual System
Fixationspunkt
Linkes
Gesichtsfeld
Rechtes
Gesichtsfeld
Rechtes Auge
Rechter optischer Nerv
Linkes Auge
R h optischer
Rechter
i h Trakt
T k
Linker optischer Nerv
Linker optischer Trakt
Optisches
Chiasma
« Take Home Message » Visual System
• The information originating from the temporal hemi-retina
remains on the same side
• On the other hand, the information originating from the nasal
h i i crosses midline
hemi-retina
idli (at
( the
h optic
i chiasma)
hi
)
• As a consequence, images in a given visual hemi-field are
represented in the opposite cerebral hemisphere
g
cells,, information is sent to the
• From the retinal gganglion
visual thalamus (lateral geniculate nucleus = LGN)
neurons
• Axons of the optic nerve make synapse on LGN neurons,
which send axons terminating in the primary visual cortex
Visual Lesions
Lesion linker
optischer Nerv
Lesion optisches
Chiasma
Lesion
li k
linker
optischer
Trakt
« Take Home Message » Visual Lesions
• A lesion at various locations along the visual pathways lead
to loss of vision in distinct zones of the visual field
(scotomas)
LGN
LGN
LGN rechts
6
5
4
3
2
1
links
6
5
4
3
2
1
Linke
nasale
Retina
Rechte
temporale
Retina
Rechte
nasale
Retina
Linke
temporale
Retina
« Take Home Message » LGN
• The LGN (lateral geniculate nucleus) is organized in six
concentric layers
• Each layer receives information from a single hemi-retina in
the order CIICIC (contralateral
(contralateral, ipsilateral)
ipsilateral).