Download The Physiology of Vision

Neurophysiology and vison
By Richard Libertini
Contents
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8)
Information processing and the eye
The eye and lens
The retina
Photoreceptors and phototransduction
Bipolar cells
Receptive fields
LG nucleus
Visual cortex (Basic)
Information is encoded and decoded
Information
Encoding
Decoding
Basic layout of information processing
Information
Light rays, photons
Encoding
Phototransduction- The conversion of light
waves into electrical signals in the retina via
rods and cones
Processing/computation
Information processing and
computation starts in the retina (e.g
on/off centres of retinal ganglion cells)
LG performs further processing such as
‘summing’ the signals from the
semifields as well as having more
on/off centres. In the visual cortex a
vast amount of processing occurs
LG
PVC
The Eye
BASIC FUNCTIONS OF THE EYE
1) Captures light from the outside world
2) The cornea and lens focuses light on to
the retina
3) The fovea is responsible for colour vison
and high visual acuity
4) The optic nerve/tracts send the
information to the visual cortex
Focusing of the image
The cornea is responsible for the
2/3 of the focusing power (43
dioptres)
The lens focuses and sharpens the image
Ciliary muscle contract/relax
adjusting the thickness and shape
of the lens
The retina
Light hits the retina at the back of the
eye, It s 200 um thick. The
photoreceptors are at the back of the
retina and convert light into an
electrochemical signal. Also the retina
is capable of complex processing of
visual information.
The backward set up of the retina
paradoxically allows for sharper
images. The retinal pigment
epithelium as the photoreceptors
undergoes constant renewal or their
membranes, also they capture lost
photons.
Photoreceptive cells -rods, cones and some
retinal ganglion cells
Photoreceptors- rods and cones
Rods
-only one type
-Monochromatic dark
adaptation vision
-outnumber cones 16:1
Cones
-Three subtypes
- Responsible for
colour vision
Fovea
The fovea is the central area of the primate retina,
300-700 um in diameter. A 1:1 relationship with
cones and ganglion cells. Inner layer neurons are
laterally displaced to minimize light scattering
Cones are at their greatest density
within the fovea and fall dramatically
with increasing distance from the
centre. Ganglion cells have small
receptive fields thus high resolution
vision.
Photoreceptor in more detail
Outer segment contains
around 1000 tightly packed
disks containing the
photopigment rhodopsin
Inner segment synthesizes
photopigments and is densely
packed with mitochondrion
Photoreceptors have a resting
membrane potential of around
-40mv. They do not create action
potential but a graded potential
Dark current and receptor potential
Na+ ions flow into the
photoreceptor via non
selective channel
The dark current is mainly carried
by the inward directed Na+ ions
and the outwardly directed K+ ions
Light indirectly regulates the Na+
channel. So in darkness the
channel is open. 90% of the dark
current is due to this. This
receptor potential in -40mv
This cause the
receptor potential
to be -40mv
This leads to an
increase in Ca++
channel open state
probability
This causes
neurotransmitter
release
Let there be light!!
Decrease inward flow
of NA+ and continued
efflux of K+ results in
hyperpolarization
K+ channel remains open
leading to increasing
intracellular – charge
(hyperpolaraization)
Light cause Na+ channel to
close via reduction in cGMP
Increased
intracellular negative
charge leads to a
decrease in Ca++
channel open state
probability
This leads to a
decrease in
neurotransmitter
release
Phototransduction 1
4) Phosphodiesterase
hydrolyses cGMP to 5’
GMP
5) Decreased leves of
cytoplasmic cGMP leads
to an increase in closed
states of cGMP gated ion
channels
6) Reduced efflux of Na+
leads to
hyperpolarization
1) Light cause a
conformational
change
rhodopsin
2) Transducin is
activated alpha
subunit decouples
3) Alpha subunit
stimulates
phosphodiesterase
Phototransduction 2
Rhodopsin is a combination of retinol and opsin
Light isomerizes 11- cis
retinal to all – trans retinal
This causes a
conformational
change of opsin that
is now called
metarhodopsin II
Phototransduction 3
Metarhodopsin II
Activates transducin
Metarhodopsin is then recycled
in the pigmented epithelium
Phototransduction 4
When transducing is activated the alpha sub
unit exchanges a bound GDP to a GTP
This then diffuses from the membrane and
activates phosphodiesterase
Phosphodiesterase then hydrolyzes cGMP
to 5’ GMP
Photoreceptor hyperpolarizes and
decreased glutamate is released
Bipolar cells responses to glutamate
On bipolar cell
Light decreases glutamate release
from photoreceptor
Glutamate usually inhibits On
centre bipolar cell. The cell loses
this inhibition
Bipolar cell depolarizes releasing
neurotransmitter
Bipolar cells continued
Off centre bipolar cells
Light decreases glutamate release
from photoreceptor
Off centre cell loses it’s excitation
from glutamate
Off centre bipolar cell
hyperpolarizes and does not
release neurotransmitter
Receptive fields
Receptive fields are volumes of
visual space. Light lands on the
retina and can alter the firing of
neurons. The organisation of
retinal ganglion fields computes
inputs form rods and cones and
provides a way of detecting
objects edges and contrast.
Receptive fields continued
On centre and off centre ganglion cells and
their responses to light by measuring action
potentials
This is achieved by the appropriate on/off
bipolar cell firing and lateral inhibition
Lateral inhibition
The light causes the centre photoreceptor to
release less glutamate and this causes the on
centre bipolar cell to fire
The surrounding cone is not hyperpolarized so it
releases glutamate that activates a horizontal cell
The horizontal cell releases GABA onto the
photoreceptor further hyperpolarizing it and inhibiting
glutamate release it.
This causes further activation of the on centre bipolar
cell
Lateral inhibition
As the horizontal cell releases GABA it
further hyperpolarizes the centre
photoreceptor
This causes further depolarization of
the on centre bipolar cell and
further hyperpolarization of the off
centre bipolar cell
Lateral geniculate nucleus
Visual pathways
This is the two stream hypothesis, the “what”
and the “where” stream
LG nucleus and visual cortex relations
LGN sends projections to the PVS. Then
further connections fro the PVC to V1 V2
occur. Content and colour are further
processed here.
Visual cortex
Any piece of primary visual cortex about 2 mm by 2
mm in surface area must have the machinery to
deal completely with some particular area of visual
field
Thank you