cap iv_curs 9_10_vision_2014_2015_.pdf
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I. General principles of sensory physiology
II. The somatosensory System
III. Chemical Senses
IV.VisionV. Hearing and Equilibrium
Lect. univ. dr. Loredana - Cristina MEREU
Laboratory of Biophysics & Med. Physics, Faculty of Physics,
'Alexandru Ioan Cuza' University of Iasi
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Studying vision provides the opportunity to explore
the brain at many different levels, from the physical andbiochemical mechanisms ofphototransduction to the
boundary betweenpsychology and physiology.
In many animals, primates in particular, more of
the brain is devoted to vision than to any other sensoryfunction.
VISION
This is perhaps because
of the extreme complexity ofthe task required of vision:to
classify and to interpret the
wide range of visual stimuli in
the physical world.
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Vision
Human visible light lies only
within the range ~380-750 nm;
The light is detected as photons* bythe retinal cells - rods & cones.
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At the highest levels of processing, the cerebral
cortex extracts from the world the diverse qualities
experienced asvisual perception: from motion, color,
texture, and depth to the grouping of objects, defined by
the combination of simple features.
The first steps in the process of seeing involve:
transmissionand refraction of light by the optics ofthe eye
the transduction of light energy into electrical
signals by photoreceptors
the refinement of these signals by synapticinteractions within the neural circuits of the retina.
Vision
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THE EYE AND THE RETINAThe Optics of the Eye Project an Inverted Visual Image on the Retina
The study of vision begins with the eye, whose
refractive properties are determined by the curvature of
the cornea and the lens behind it. These optical
elements act to focus an inverted image on theretina,
where the first stages of neural visual processingtake place.
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The Eye and the retina
The amount of light
that reaches the retina iscontrolled by theiris, whose
aperture is thepupil.
The iris, which is
situated between the corneaand the lens in the anterior
chamber of the eye,
contracts at high light levels
and expands in the dark.
The curvature of the cornea is fixed, but the
curvature of the lens is adjusted by smooth muscles that
flatten the lens when they relax, thus bringing moredistant objects into focus.
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DIOPTER = a unit of
measurement of the
refractive power of lenses
equal to the reciprocal of
the focal length
measured in meters.
The Eye and the retina
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Dynamic changes in the refractive power of the
lensare referred to asaccommodation.
The Eye and the retina
These changes result from the activity of theciliary
musclethat surrounds the lens. The lens is held in place
by radially arranged connective tissue bands (calledzonule fibers)that are attached to the ciliary muscle.
When viewingdistant objects, the lens
is made relatively thin
and flat and has the
least refractive power.For near vision,
the lens becomes thicker
and rounder and has the
most refractive power.
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The shape of the lens is thus determined by two
opposing forces:
the elasticity of the lens, which tends to keep it
rounded up (removed from the eye, the lens
becomes spheroidal)
the tension exerted by the zonule fibers, whichtends to flatten it.
The Eye and the retina
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When viewing distant objects, the force from the
zonule fibers is greater than the elasticity of the lens,
and the lens assumes the flatter shape appropriate fordistance viewing.
Focusing on closer objects requires relaxing the
tension in the zonule fibers, allowing the inherent
elasticity of the lens to increase its curvature. Thisrelaxation is accomplished by the sphincter-like
contraction of the ciliary muscle.
The Eye and the retina
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Because the ciliary muscle forms a ring around
the lens, when the muscle contracts, the attachment
points of the zonule fibers move toward the central axisof the eye, thus reducing the tension on the lens.
The Eye and the retina
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Unfortunately,
changes in theshape of the lens
are not always able
to produce a
focused image onthe retina, in which
case a sharp image
can be focused only
with the help of
additional corrective
lenses
(see annex 1!).
The Eye and the retina
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(D) Changes in the ability of the lens to round up (accommodate)
with age. The graph also shows how the near point (the closest
point to the eye that can be brought into focus) changes.
One of the many consequences of aging is that
thelens loses its elasticity; as a result, the maximum
curvature the lens can achieve when the ciliary musclecontracts is gradually reduced.
Accommodation,
which is an opticalmeasurement of
the refractive power
of the lens, is given
in diopters.
The Eye and the retina
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Spatial Orientation and the Visual FieldThe visual field is that area in space perceived
when the eyes are in a fixed, static position looking
straight ahead. The monocular visual field - is thatarea of space visible to one eye and is subdivided into
two halves,the hemifields:
1. A horizontal line drawn from 0 to 180 through center
of the field definesthe superior & inferior hemifields.
2. A vertical line drawn
from 90 to 270 through
center point defines theleft & right hemifields,
which are often termed
the nasal and temporal
hemifields.
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Spatial Orientation and the Visual Field
The monocular visual field may be further subdivided
into quadrants:
the superior and inferior nasal quadrants
the superior and inferior temporal quadrants
Contains a blind
spot - a small area
in which objects
cannot be viewed,which is located
within the temporalhemifield.
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Spatial Orientation and the Visual Field
The monocular visual field is determined with
one eye covered.
l d h l ld
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Spatial Orientation and the Visual Field
As our eyes are angled
slightly toward the nose, the
monocular visual fields of the
left and right eyes overlap toform the binocular visual
field (colored red).
The area of overlap of the visual field of one eye
with that of the opposite eye is called the binocular
field. All areas of the binocular visual field are seen byboth eyes.
S i l O i i d h Vi l Fi ld
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Spatial Orientation and the Visual Field
Objects within the binocular visual field are visible
to each eye, albeit from different angles.
RETINA
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RETINA - a Three-layered Structure with Five Types of Neurons
The vertebrate retina is oriented within the eye so
that light must travel through the entire thickness of the
neuropil to reach the photoreceptors.
The retina is composed of five principal layers:
three layers of cell bodies separated by two layers of
neural processes, dendrites and axons.
RETINA h h l d h f
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RETINA -The Retina Is a Three-layered Structure with Five Types of Neurons
Of the three cell layers, the first is farthest from the
center of the eye and thus is calledthe outer nuclear
layer.It contains the cell bodies of thephotoreceptors,therods and cones.
The next cell layer isthe
inner nuclear layer, whichcontains the cell bodies of the
interneurons of the retina,
both excitatory and inhibitory.
These include: horizontal cells,
bipolar cells,
and
amacrine cells.
RETINA Th R i I Th l d S i h Fi T f N
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RETINA -The Retina Is a Three-layered Structure with Five Types of Neurons
Finally, the ganglion cell layer is home to the
retinal neuronswhose axons formthe optic nerve, the
sole pathway from the retina to the rest of the CNS
Interposed between the
cell body layers are two layers
of cell processes: inner plexiform layer
and
outer plexiform layer
, which are the sites of all
interactions between the
neurons of the retina.
RETINA Th R ti I Th l d St t ith Fi T f N
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R, rod;C, cone;
H, horizontal cell;
FMB, flat midget bipolar;
IMB, invaginating midget
bipolar;
IDB, invaginating diffuse
bipolar;RB, rod bipolar;
A, amacrine cell;
P, parasol cell;
MG, midget ganglion cell.
RETINA -The Retina Is a Three-layered Structure with Five Types of Neurons
Summary diagram of the cell types and connectionsin the primate retina.
RETINA
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There are five types of
neurons in the retina: photoreceptors
bipolar cells
ganglion cells
horizontal cells amacrine cells
A directthree-neuron chain - photoreceptor cell
to bipolar cell to ganglion cell - is the major route of
information flow from photoreceptors to the optic nerve.
RETINA
RETINA visual receptive field
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As defined by H. K. Hartline in 1938, a visual receptive
field is the region of the retina which must be
illuminated in order to obtain a response in any given
fiber. In this case, fiber refers to the axon of a retinal
neuron, but any visual neuron, from a photoreceptor to
a visual cortical neuron, has a receptive field.
The definition was later extended to include not only
the region of the retina that excited a neuron, but also
the specific properties of the stimulus that evoked the
strongest response.
Visual neurons can respond preferentially to the turning
on or turning off of a light stimulus - termedon and -
offresponses - or to more complex features, such as
color or the direction of motion. Any of these
preferences can be expressed as attributes of the
receptive field.
RETINA visual receptive field
RETINA Photoreceptors
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The two major types of photoreceptors in the
vertebrate eye are therods and cones. Both types of
photoreceptor havean outer segmentthat contains themolecular machinery for phototransduction, an inner
segment that contains densely packed mitochondria,a
cell bodythat contains the nucleus and other important
organelles,and a
terminal
process
that
releases
neurotransm
itter.
RETINA -Photoreceptors
RETINA Ph t t
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RETINA -Photoreceptors
RETINA Photoreceptors- Functional Specialization of the Rod and Cone Systems
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RETINA Photoreceptors- Functional Specialization of the Rod and Cone Systems
The two types of photoreceptors, rods and cones,
are distinguished by:
shape(from which they derive their names), the type ofphotopigmentthey contain,
distribution across theretina,
and pattern ofsynaptic connections.
These properties reflect
the fact thatthe rod and cone
systems (the receptors andtheir connections within the
retina) are specialized for
different aspects of vision.
RETINA -Photoreceptors
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Rods and cones work
together to allow the visual
system to operate over a wide
range of luminance
conditions.
RETINA Photoreceptors
RETINA -Photoreceptors
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Under scotopic conditions
when luminance levels are very low
(e.g., starlight), only the rods are
active.
As luminance levels increase
to mesopic conditions (e.g.,moonlight), both therods and cones
contribute to vision.
As luminance levels increase
further yet, to photopic condition(e.g., sunlight), rod responses
saturate and only the cones
contribute to vision.
RETINA Photoreceptors
RETINA Photoreceptors- Functional Specialization of the Rod and Cone Systems
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The rod system has very low spatial resolution
but is extremely sensitive to light; it is therefore
specialized forsensitivityat the expense of resolution.Conversely, the cone system has very high
spatial resolution but is relatively insensitive to light; it is
therefore specialized for acuity at the expense of
sensitivity. The properties of the cone system also allowhumans and many other animals to see color.
The range of luminance values over which the visual system operates.
RETINA Photoreceptors Functional Specialization of the Rod and Cone Systems
RETINA Photoreceptors- Functional Specialization of the Rod and Cone Systems
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At the lowest levels of light, only the rods are
activated. Such rod-mediated perception is called
scotopic vision.Although cones begin to contribute tovisual perception at about the level of starlight, spatial
discrimination at this light level is still very poor. As
illumination increases, cones become more and more
dominant in determining what is seen, and they are themajor determinant of perception under relatively bright
conditions such as normal indoor lighting or sunlight.
RETINA Photoreceptors Functional Specialization of the Rod and Cone Systems
RETINA Photoreceptors- Functional Specialization of the Rod and Cone Systems
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The contributions of rods to vision drops out nearly
entirely in so called photopic vision because their
response to light saturatesthat is, the membranepotential of individual rods no longer varies as a function
of illumination because all of the membrane channels
are closed.
Mesopic vision occurs in levels of light at whichboth rods and cones contribute at twilight, for
example.
RETINA Photoreceptors Functional Specialization of the Rod and Cone Systems
RETINA Photoreceptors- Functional Specialization of the Rod and Cone Systems
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From these considerations
it should be clear that most of
what we think of as normal seeing is mediated by the
cone system, and that loss of
cone function is devastating, as
occurs in elderly individualssuffering from macular
degeneration.
oto ecepto s u ct o a Spec a at o o t e od a d Co e Syste s
People who have lost cone
function are legally blind,whereas those who have lost rod
function only experience difficulty
seeing at low levels of
illumination (night blindness).
RETINA Photoreceptors- Functional Specialization of the Rod and Cone Systems
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Differences in the transduction mechanisms
utilized by the two receptor types is a major factor in the
ability of rods and cones to respond to differentranges of light intensity.
For example, rods produce a reliable response to
a single photon of light, whereas more than 100 photons
are required to produce a comparable response in acone.
It is not, however, that cones fail to effectively
capture photons. Rather, the change in current
produced by single photon capture in cones is
comparatively small and difficult to distinguish from
noise.
p p y
RETINA Photoreceptors- Functional Specialization of the Rod and Cone Systems
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Another difference is that the response of an
individual cone does not saturate at high levels of
steady illumination, as does the rod response.Although both rods and cones adapt to operate
over a range of luminance values, the adaptation
mechanisms of the cones are more effective.
This difference in adaptation is apparent in thetime course of the response of rods and cones to light
flashes.
The response of a cone, even to a bright light flash
that produces the maximum change in photoreceptor
current, recovers in about 200 milliseconds, more than
four times faster than rod recovery.
p p y
RETINA Photoreceptors- anatomical distribution of the Rod and Cone Systems
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The distribution of rods and cones across the
surface of the retina also has important consequences
for vision. Despite the fact that perception in typical
daytime light levels is dominated by cone-mediatedvision, the total number of rods in the human retina,
As a result, the
density of rods is much
greater than cones
throughout most of the
retina.
p y
(about 90 million) far
exceeds the number ofcones (roughly 4.5
million).
RETINA Photoreceptors- anatomical distribution of the Rod and Cone Systems
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In the fovea, cone density increases almost 200-
fold, reaching, at its center, the highest receptor packing
density anywhere in the retina.This high density is achieved by decreasing the
diameter of the cone outer segments such that foveal
cones resemble rods in their appearance. The
increased density of cones in the fovea is accompaniedby a sharp decline in the density of rods. In fact, the
central 300 mof the fovea, called the foveola,is totally
rod-free.
p y
RETINA Photoreceptors- anatomical distribution of the Rod and Cone Systems
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Because the center of our retina (the fovea)
contains very few, if any rods, we havea foveal blind
spotunder very dim conditions.
Color Vision
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Color Vision
Perceiving color allows humans (and many other
animals) to discriminate objects on the basis of the
distribution of the wavelengths of light that they reflect tothe eye.
Color vision is the ability to detect differences in
the wavelengths of light.
The human has a
trichromatic visual system,
whereby visible colors can becreated by a mixture of red,
green and blue lights.
Color Vision
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Color Vision
Unlike rods, which contain a single photopigment,
there are three types of cones that differ in the
photopigment they contain. Each of thesephotopigments has a different sensitivity to light of
different wavelengths, and for this reason are referred to
as:
blue, green,
redor, more appropriately:
short (S), medium (M),
long (L)
,wavelength cones - terms
that more or less describe their spectral sensitivities.
Color Vision
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Individual cones provide color information for the
wavelength of light that excites them best . In fact,
individual cones, like rods, are entirely color blind in thattheir response is simply a reflection of the number of
photons they capture, regardless of the wavelength of
the photon (or, more properly, its vibrational energy).
The light absorption spectra ofthe four photopigments in the
normal human retina. The
solid curves indicate the three
kinds of cone opsins; thedashed curve shows rod
rhodopsin for comparison. Absorbance is defined as the log value
of the intensity of incident light divided
by intensity of transmitted light.
Co o s o
Color Vision
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The most common form of color blindness
results in a confusion of red and green shades (i.e., red-
green color blindness).Most cases of color blindness result from an
absent or defective gene responsible for producing the
red or green photopigment:
protanopia- the lack ofredand
deuteranopia- the lack ofgreen.
Color Vision
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As these genes are located on the X chromosome,
color blindness is more common in males than in
females.
Th h i t J h D lt l bli d H th ht it b blColor Blindness: John Daltons Experiment from the Grave
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The chemist John Dalton was color-blind. He thought it probable
that the vitreous humor of his eyes (the fluid that fills the eyeball behind
the lens) was tinted blue, unlike the colorless fluid of normal eyes. He
proposed that after his death, his eyes should be dissected and the
color of the vitreous humor determined. His wish was honored.The day after Daltons death in July 1844, Joseph Ransome
dissected his eyes and found the vitreous humor to be perfectly
colorless. Ransome, like many scientists, was reluctant to throw
samples away. He placed Daltons eyes in a jar of preservative, where
they stayed for a century and a half.
Then, in the mid-1990s, molecular biologists in England took
small samples of Daltons retinas and extracted DNA. Using the known
gene sequences for the opsins of the red and green photopigments,
they amplified the relevant sequences and determined that Dalton hadthe opsin gene for the red photopigment but lacked the opsin gene for
the green photopigment. Dalton was a green dichromat.
Daltons e es.
p
So, 150 years after his death, the experiment
Dalton startedby hypothesizing about the cause
of his color blindnesswas finally finished.
RETINA Bipolar and horizontal cells
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The bipolar and
horizontal cells
respond to theglutamate released
by the
photoreceptor cells.
p
Within the outer plexiform layer of the retina,
approximately 125 million photoreceptor cells synapse
with approximately10 million bipolar cells. A smallernumber of horizontal cells also synapse with the
photoreceptor cells within the outer plexiform layer of theretina.
Bipolar cells do notRETINA Bipolar cells
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pgenerate action potentials and respond to
the release of glutamate from
photoreceptors with graded potentials
(i.e., by hyperpolarizing or depolarizing).
Theoffbipolar cells aredepolarized by glutamate
and function to detect dark objects in a lighter
background
The on bipolar cells are hyperpolarized byglutamate and function to detect light objects in a
darkerbackground.
The functional importance of the on and the off
athwa s can best be understood in terms of contrast.
There are at least two types of bipolar cells based
on their responses to glutamate and with different
functional properties:
RETINA Bipolar cells
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The stimulus condition that produces a
depolarizing response from a bipolar cell is used to
name the bipolar cell type.
An off bipolar cell depolarizes when the
photoreceptors that synapse with it are in the dark.
An on bipolar cell depolarizes when the
photoreceptors that synapse with it are in the light.
RETINA Bipolar cells
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In contrast, light
directed onimmediately
surrounding
receptors produce
the oppositeresponse.
Bipolar cells haveconcentric receptive fields.
Light directed on the photoreceptor(s) that synapse
with a bipolar cell produces a response from thebipolar cell called the center response.
RETINA Bipolar cells
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When both the center and surrounding receptor
cells are illuminated with light, the on bipolar cell
response to stimulation of the center receptors is
reduced by stimulation of the surround receptors.
Consequently, the
strongest on bipolar cell
response is producedwhen the stimulus is a
light spot encircled by a
dark ring.
For the off bipolarcell, a dark spot
encircled by a light ring
produces maximal
de olarization.
RETINA Horizontal cells
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Within the outer plexiform layer , the
photoreceptor cells make both presynaptic and
postsynaptic contact with horizontal cells.
RETINA Horizontal cells
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The horizontal cellshave large receptive fields
involving presynaptic
(axonal) contact with a small
group of photoreceptors andpostsynaptic (dendritic)
contact with a larger group of
surrounding photoreceptor
cells.
RETINA Horizontal cells
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By controlling the responses of their centerphotoreceptors (based on the responses of the
surrounding photoreceptors), the horizontal cells
indirectly produce the bipolar cell receptive field
surround effect.The surround effect produced by the horizontal cell
is weaker than the center effect. The surround effect,
produced by the horizontal cells,enhances brightness
contrasts to produce sharper images, to make an
object appear brighter or darker depending on the
background and to maintain these contrasts under
different illumination levels.
RETINA Ganglion cells
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Within the inner plexiform layer, the axon terminals
of bipolar cells (the 2 visual afferents) synapse on the
dendritic processes of amacrine cells and ganglion cells.
Most bipolar cells releaseglutamate, which is excitatory
to most ganglion cells (i.e., depolarizes ganglion cells).
It is the axons of the retinalganglion cells (the 3 visual
afferents) that exit the eye to form
the optic nerve and deliver visual
information to the lateralgeniculate nucleus of the thalamus
and to other diencephalic and
midbrain structures.
RETINA Ganglion cells
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Because they must carry visual information some
distance from the eye, they posses voltage-gatedsodium channels in their axonal membranes and
generate action potentials when they are depolarized by
the glutamate released by the bipolar cells.
The retinal ganglion
cells are the final retinal
elements in the direct
pathway from the eye to the
brain.
Ganglion cells also have either on or off
responses in the center of their receptive fi elds,
according to which class of bipolar cells provide their
input.
RETINA Ganglion cells
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Theoffbipolar cell will depolarize when it is dark
on its center cones and will therefore release glutamate
when it is dark on the center of its receptive field.
This will result in the
depolarization of the retinal
ganglion cells with which theoff
bipolar synapses and in theproduction of action potentials
(i.e., discharges) by these
ganglion cells. Consequently,
the retinal ganglion cells thatsynapse with off bipolar cells
will haveoff-center/on-surround
receptive fields and are called
offganglion cells.
Th bi l ll ill d l i h th i li ht
RETINA Ganglion cells
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Theonbipolar cell will depolarize when there is light on
its center cones and will therefore release glutamate
when it is light on the center of its receptive field.
This will result in the
depolarization of the retinal
ganglion cells with which the
on bipolar synapses and inthe production of action
potentials(discharges)by these
ganglion cells. Consequently,
the retinal ganglion cells thatsynapse with on bipolar cells
will have on-center/off
surround receptive fields and
are calledonganglion cells.
The selective response of on and off center bipolar cells to lightRETINA Ganglion cells
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The selective response of on- and off- center bipolar cells to light
increments and decrements is explained by the fact that they express
different types of glutamate receptors.Off-center bipolar cells
have ionotropic receptors (AMPAand kainate) that cause the cells
to depolarize in response to
glutamate released from
photoreceptor terminals.
In contrast, on-centerbipolar cells express a G-
protein-coupled metabotropic
glutamate receptor (mGluR6).
Glutamate receptors. Once released from the presynaptic
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terminal, glutamate diffuses across the cleft and binds onto
receptors located on the dendrites of the postsynaptic cell(s).
Two classesof glutamate
receptors
have been
identified:
(1)ionotropic glutamate receptors (AMPA and kainate), whichdirectly gate ion channels.
(2) metabotropic glutamate receptors (G-protein-coupled
metabotropic glutamate Receptor - mGluR6), which may be
coupled to an ion channel or other cellular functions via an
intracellular second messenger cascade.
Th f on center d off center ti l li ll
RETINA Ganglion cells
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The responses of on-centerand off-centerretinal ganglion cells
to stimulation of different regions of their receptive fields.
The recepti e field properties ca se ganglion cellsRETINA Ganglion cells
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The receptive field properties cause ganglion cells
to fire differently depending on whether the surround of
the equiluminant target is dark or light.
Two photometrically identical
(equiluminant) gray squares
appear differently bright as a
function of the background in
which they are presented.
In short, the receptive
fields of the bipolar cells with
which the retinal ganglioncell synapses determine the
receptive field configuration
of a retinal ganglion cell.
Th i l li ll id i f i
RETINA Ganglion cells
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The retinal ganglion cells provide information
important for detecting the shape and movement of
objects.In the primate eye, there aretwo major types of
retinal ganglion cells that process information aboutdifferent stimulus properties:
Type M cells -the slowly adapting response of the Type P
retinal ganglion cell is best suited for signaling the
presence, color and duration of a visual stimulus and ispoor for signaling stimulus movement.
Type P cells - the rapidly adapting responses of Type M
ganglion cells are best suited for signaling temporal
variations in, and the movement of, a stimulus.The axons of the M and P retinal ganglion cells travel
in the retina optic nerve fiber layer to the optic disc where
they exit the eye. Most of the axons travel to and terminate
in the lateral geniculate nucleus of the thalamus.
RETINA Amacrine cells
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Amacrine cells synapse with bipolar cells and
ganglion cells and are similar to horizontal cells in
providing lateral connections between similar types ofneurons (e.g., they may connect bipolar cells to other
bipolar cells or may synapse with other amacrine cells.
RETINA Amacrine cells
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They differ from horizontal cells, however, in also
providing vertical links between bipolar and
ganglion cells.
There are 20 or more types of amacrine cells
based on their morphology and neurochemistry.
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Phototransduction
Th i d t i th
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The opsin determines the
wavelength specificity of the
photopigment and has the ability to
interact with G proteins.
Rods contain a single
photopigment,rhodopsin whereas
cones contain one of three cone
opsins.
When the retinal moiety in the rhodopsin moleculePhototransduction
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When the retinal moiety in the rhodopsin molecule
absorbs a photon, its configuration changes from the11-
cisisomer toall-trans retinal.
This change then
triggers a series of
alterations in the protein
component of the molecule.The changes lead, in
turn, to the activation of an
intracellular messenger
called transducin, whichactivates a
phosphodiesterase that
hydrolyzes cGMP.
Likely structure of rhodopsin complexed with thePhototransduction
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The chromophore 11-cis retinal (blue),
attached to Lys256 of the seventh helix,
lies near the center of the bilayer.
Cytosolic loops that interact with the Gprotein transducin are shown in orange.
The three subunits of transducin
(green) are shown in their likely
arrangement.
Likely structure of rhodopsin complexed with the
G protein transducin. Rhodopsin (red) has seven
transmembrane helices embedded in the disk
membranes of rod outer segments and is oriented withits carboxyl terminus on the cytosolic side and its amino
terminus inside the disk.
Phototransduction
Lik l t t f h d i l d ith th
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Likely structure of rhodopsin complexed with the
G protein transducin.
Phototransduction
O h t i b b d b th h t i t i
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Once a photon is absorbed by the photopigment in
the receptor disks,a cascade of events(see annex 2!)
occurs that ultimately affects the membrane potential of
the photoreceptor.
Rhodopsin = retinal(~vit. A)+ scotopsin (a transmembrane protein)
f
Phototransduction
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This cascade of events begins with theactivation
of the G protein (transducin) that then activates a
phosphodiesterase that hydrolyzes cGMP (cyclicguanosine monophosphate) and reduces the
intracellular concentration of cGMP.Because the outer
membrane of aphotoreceptor contains
many cGMP-gated
cation channels, a
decrease in theintracellular concentration
of cGMP will cause the
photoreceptor to
hyperpolarize.
http://pubs.rsc.org/en/content/articlehtml/2010/pp/b9pp00134dPhototransduction
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Phototransduction
Importantly,h t t d
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photoreceptors do
not produce action
potentials, but
rather have graded
potentials that are
modulated around a
mean level.
Phototransduction
A i t ll l di f i l
Phototransduction
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An intracellular recording from a single cone
stimulated with different amounts of light (the cone has
been taken from the turtle retina, which accounts for therelatively long time course of the response). Each trace
represents the response to a brief flash that was varied
in intensity.
The hyperpolarizing response is characteristic of
vertebrate photoreceptors; some invertebrate
photoreceptors depolarize in response to light.
At the highestlight levels, the
response amplitude
saturates (at about65 mV).
Absorption of light by the photopigment in the
Phototransduction
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Absorption of light by the photopigment in the
outer segment of the photoreceptors initiatesa cascade
of events (see annex 2!) that changes the membrane
potential of the receptor, and therefore the amount of
neurotransmitter released by the photoreceptor
synapsesonto the cells they contact.
Phototransduction
The synapses between photoreceptor
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y p p p
terminals and bipolar cells (and horizontal cells) occur
in the outer plexiform layer; more specifically, the cell
bodies of photoreceptors make up the outer nuclearlayer, whereas the cell bodies of bipolar cells lie in the
inner nuclear layer.
The short axonal processes of
bipolar cells make synapticcontacts in turn on the dendritic
processes ofganglion cells in the
inner plexiform layer.
The much larger axons of the
ganglion cellsform the optic nerve
and carry information about retinal
stimulationto the rest of the CNS.
Phototransduction
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In most sensory systems, activation of a receptor
by the appropriate stimulus causes the cell membrane to
depolarize, ultimately stimulating an action potential andtransmitter release onto the neurons it contacts.
In the retina, however, photoreceptors do not
exhibit action potentials; rather, light activation causesa graded change in membrane potential and a
corresponding change in the rate of transmitter release
onto postsynaptic neurons.
Perhaps even more surprising is thatshining lighton a photoreceptor, either a rod or a cone, leads to
membrane hyperpolarization rather than
depolarization.
Phototransduction
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In the dark, the receptor is in adepolarized state,
with a membrane potential of roughly 40 mV(including
those portions of the cell that releasetransmitters).Progressive increases in the intensity of
illumination cause the potential across the receptor
membrane to become more negative, a response that
saturates when the membrane potential reaches about
65 mV.
Although the sign of the potential change may
seem odd, the only logical requirement for subsequentvisual processing is a consistentrelationship between
luminance changes and the rate of transmitter
releasefrom the photoreceptor terminals.
Phototransduction
Light increments lead to hyperpolarization and
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Light increments lead tohyperpolarization and
a reduction in neurotransmitter release, whereas
light decrements lead to depolarization and an
increase in transmitterrelease.
As in other nerve
cells, transmitter
release from the
synaptic terminals of
the photoreceptor is
dependent onvoltage-sensitive Ca2+
channels in the
terminal membrane.
Phototransduction
Th i th d k h h t t
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Thus, in the dark, when photoreceptors are
relatively depolarized, the number of open Ca2+
channelsin the synaptic terminal ishigh, and the rateof transmitter release is correspondingly great;
In the light, whenreceptors are
hyperpolarized, the number
of open Ca2+ channels is
reduced, and the rate oftransmitter release is also
reduced.
Phototransduction
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The relativelydepolarized stateof photoreceptors
in the dark depends onthe presence of ion channels
in theouter segment membrane that permitNa+
andCa2+ ionsto flow into the cell, thus reducing the degree
of inside negativity.
The probability of these channels in the outer
segment being open or closed is regulated in turn by the
levels of the nucleotide cyclic guanosine
monophosphate (cGMP) - as in many other secondmessenger systems.
Phototransduction
Light-induced hyperpolarization of rod cells
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Light-induced hyperpolarization of rod cells.
The membrane potential is reduced by the flow of Na+
and Ca2+ into the cell through cGMP gated cation
channels in the plasma membrane of the outer segment.
When rhodopsin absorbs light, it triggers degradation of
cGMP (green dots) in the outer segment, causing
closure of the cation channel.
Without cation
influx through this
channel, the cell
becomes
hyperpolarized.
Phototransduction
Cyclic GMP- gated channels in the outer
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Cyclic GMP- gated channels in the outer
segment membrane are responsible for the light-induced
changes in the electrical activity of photoreceptors.
.In darkness, high levels
of cGMP in the outer segment
keep thechannels open.
In the light, however,cGMP levels drop and some of
thechannels close, leading to
hyperpolarization of the outer
segment membrane, and
ultimately the reduction of
transmitter release at the
photoreceptor synapse.
In the dark, cGMP levels in the outer segment are high;Phototransduction
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, g g ;
this molecule binds to the Na+ - permeable channels in the
membrane, keeping them open and allowing sodium (and
other cations) to enter, thusdepolarizing the cell.
the same scheme applies to cones
Exposure to light
leads to a decrease
in cGMP levels, a
closing of the
channels, and
receptor
hyperpolarization.
The hydrolysis by phosphodiesterase at thePhototransduction
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disk membrane lowers the concentration of cGMP
throughout the outer segment, and thus reduces the
number of cGMP molecules that are available forbinding to the channels in the surface of the outer
segment membrane, leading tochannel closure.
Phototransduction
Transduction of stimulus energy into neural activity
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by photoreceptors requires intracellular second
messengers.
1.The outer segment of both photoreceptors contains the photopigment rhodopsin,
which changes configuration when it absorbs light. 2. Stimulation of the chromophore by
light reduces the concentration of cGMP in the cytoplasm. This hyperpolarizes the
photoreceptor by closing cation channels, decreasing the transmitter released by the
photoreceptor terminals in the inner segment. 3. Receptor currents evoked by light
flashes.
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Circuitry
responsible
forgenerating
receptive
field center
responses
of retinal
ganglioncells.
Photoreceptor synapses with off-center bipolar cells are called sign-conserving (+), since the
sign of the change in membrane potential of the bipolar cell (depolarization or hyperpolarization)
is the same as that in the photoreceptor.
Photoreceptor synapses with on center bipolar cells are called sign-inverting (-) because thechange in the membrane potential of the bipolar cell is the opposite of that in the photoreceptor.
Phototransduction
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One of the important features of this complex
biochemical cascade initiated by photon capture is that
it provides enormoussignal amplification.
It has been estimated that a single light-
activated rhodopsin molecule can activate 800
transducin molecules, roughly eight percent of thetransducin molecules on the disk surface.
Although each transducin molecule activates
only one phosphodiesterase molecule, each of these
is in turn capable of catalyzing the breakdown of asmany as six cGMP molecules.
As a result, the absorption ofa single photonby aPhototransduction
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, p g p y
rhodopsin molecule results in the closure of
approximately 200 ion channels, or about 2% of the
number of channels in each rod that are open in thedark. This number of channel closures causes a net
change in the membrane potential of about1 mV.
Equally important is the fact that the magnitude of
this amplification varies with the prevailing levels of
illumination, a phenomenon known aslight adaptation.
At low levels of illumination, photoreceptors are
the most sensitive to light. As levels of illuminationincrease, sensitivity decreases, preventing the receptors
from saturating and thereby greatly extending the range
of light intensities over which they operate.
Phototransduction
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Consistent with its status as a full-fledged part of
the CNS, the retina comprises complex neural circuitry
that converts the graded electrical activity of
photoreceptors into action potentials that travel to
the brain via axons in the optic nerve.
Despite its
peripheral location, the
retina or neural portion
of the eye, is actually part
of the central nervous
system.
Central Visual Pathways
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Information supplied by the retina initiates
interactions between multiple subdivisions of the brainthat eventually lead to conscious perception of the visual
scene, at the same time stimulating more conventional
reflexes such as:
adjusting the size of the pupil,
directing the eyes to targets of interest,
and
regulating homeostatic behaviors that are tied to
the day/night cycle.
Central Visual Pathways
The parallel processing of different categories of
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The parallel processing of different categories of
visual information continues in cortical pathways that
extend beyond primary visual cortex, supplying a varietyof visual areas in theoccipital, parietal, and temporal
lobes.
Visual areas in the temporal lobe are primarily
involved in object recognition, whereas those in theparietal lobeare concerned withmotion.
Normal vision
depends on theintegration of information
in all these cortical areas
Ganglion cell axons exit the retina through a
Central Visual Pathways
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Ganglion cell axons exit the retina through a
circular region in its nasal part called theoptic disk(or
optic papilla), where they bundle together to form theoptic nerve.
This region of the
retina contains nophotoreceptors and,
because it is insensitive
to light, produces the
perceptualphenomenonknown as
the blind spot.
Axons in theoptic nerverun a straight course toCentral Visual Pathways
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Central projections
of retinal ganglion
cells. For clarity, onlythe crossing axons
of the right eye are
shown
the optic chiasm at the base of the diencephalon. In
humans, about 60% of these fibers cross in the chiasm,
while the other 40% continue toward the thalamus andmidbrain targets on the same side. Once past the
chiasm, the ganglion cell axons on each side form the
optic tract.
Distinct populations of retinal ganglion cells send
Central Visual Pathways
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Distinct populations of retinal ganglion cells send
their axons to a number of central visual structures that
serve different functions.The most important projections are:
to the pretectum for mediating the pupillary light
reflex,
to thehypothalamus for the regulation ofcircadianrhythms,
to the superior colliculus for the regulation of eye
and headmovements, to the lateral geniculate nucleus for mediating
vision andvisual perception(most important of all).
Central Visual Pathways
The circuitry responsible for the pupil lary l ight reflex.
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This pathway includes bilateral projections from
the retina to the pretectum and projections from thepretectum to the Edinger-Westphal nucleus. Neurons in
the Edinger-Westphal nucleus terminate in the ciliary
ganglion, and neurons in the ciliary ganglion innervate
the pupillary constrictor muscles.
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Visuotopic organization of the striate cortex in the right
i it l l b i id itt l i (A) Th i
Central Visual Pathways
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occipital lobe, as seen in mid-sagittal view. (A) The primary
visual cortex occupies a large part of the occipital lobe. The area
of central vision (the fovea) is represented over adisproportionately large part of the caudal portion of the lobe,
whereas peripheral vision is represented more anteriorly. (B)
Photomicrograph of a coronal section of the human striate cortex.
The primary visual cortical receiving area is in theVisual Cortical Areas
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p y g
occipital lobe. Nearly the entire caudal half of the
cerebral cortex is dedicated to processing visual
information.
(A) A lateral view of the left cerebral hemisphere. (B) A view of the medial
surface of the right hemisphere.
The primary motor cortex (i.e., the precentral gyrus), and the primary
somatosensory receiving area (i.e., the postcentral gyrus) are represented in
red and blue, respectively.
The flow of visual information from the primary
Visual Cortical Areas
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p y
visual cortex to other cortical areas depends on the type
of information being processed.
Information used to locate objects and detect their
motion is sent to more superior cortex (a.k.a. the dorsal
stream). Information necessary to detect, identify and
use color and shape information is sent to inferiorcortical areas (a.k.a., the ventral stream).
Visual Association Cortex.
Visual Cortical Areas
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The responses of a "shape-
form" type primary visual cortexneuron is recorded while a light bar
is flashed on and off the screen.
For each of the frames, the
light bar has a different orientation.
The neuron displays a
preference (i.e., produces a
maximal response) for a light barcentered and parallel to the long
axis of the receptive field.
The responses of a "motion sensitive" primary
Visual Cortical Areas
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The responses of a motion sensitive primary
visual cortex neuron recorded in response to movement
of a light bar across the neuron's receptive field.The neuron responds vigorously to movement in
one direction (i.e., from left to right) and poorly to
movement in the opposite direction (i.e., from right to
left). Consequently, this neuron exhibits directionalsensitivity.
Stereoscopy depends on matching informationVisual Cortical Areas
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An autostereogram.
The hidden figure
(three geometrical
forms) emerges by
diverging the eyes in
this case
seen by the two eyes without any prior recognition of
what object(s) such matching might generate.
Looking at a plane more distant than the plane ofthe surface causes divergence; looking at a plane in
front of the picture causes the eyes to converge).
NEUROSCIENCE: Third Edit ion, Dale Purves et al., 2004 Sinauer
A i t I
References:
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Associates, Inc.
Fundamental neuroscience /by Larry Squire et al.3rd ed. 2008,
Elsevier Inc.
Lehninger_Biochemistry_4e_2005_Acrobat_60
EBooks - Chemistry - Biochemistry Garrett And Grisham 2Nd Ed
Coding of Sensory Information, Esther P. Gardner John H. Martin;
http://homepage.psy.utexas.edu/homepage/class/psy394U/hayhoe/Intr
oSensoryMotorSystems/week3/Kandel%20Ch%2021,%2022,%2023.pdfhttp://cnx.org/content/m46577/latest/?collection=col11496/latest
http://neuroscience.uth.tmc.edu/s2/chapter09.html
ht tp://downloadpdfz.com/ppt/what-is-adaptation-of-sensory-receptors
http://classes.midlandstech.edu/carterp/Courses/bio110/chap09/chap09.htm
http://webvision.med.utah.edu/book/part-v-phototransduction-in-rods-
and-cones/glutamate-and-glutamate-receptors-in-the-vertebrate-retina/
http://highered.mheducation.com/sites/dl/free/0072437316/120060/rave
nanimation.html
a. If the eyeball is too long the flat lens focuses distant objects in front of
Annex 1 - Vision Problems
http://homepage.psy.utexas.edu/homepage/class/psy394U/hayhoe/IntroSensoryMotorSystems/week3/Kandel%20Ch%2021,%2022,%2023.pdfhttp://homepage.psy.utexas.edu/homepage/class/psy394U/hayhoe/IntroSensoryMotorSystems/week3/Kandel%20Ch%2021,%2022,%2023.pdfhttp://homepage.psy.utexas.edu/homepage/class/psy394U/hayhoe/IntroSensoryMotorSystems/week3/Kandel%20Ch%2021,%2022,%2023.pdfhttp://homepage.psy.utexas.edu/homepage/class/psy394U/hayhoe/IntroSensoryMotorSystems/week3/Kandel%20Ch%2021,%2022,%2023.pdfhttp://cnx.org/content/m46577/latest/?collection=col11496/latesthttp://cnx.org/content/m46577/latest/?collection=col11496/latesthttp://cnx.org/content/m46577/latest/?collection=col11496/latesthttp://neuroscience.uth.tmc.edu/s2/chapter09.htmlhttp://neuroscience.uth.tmc.edu/s2/chapter09.htmlhttp://neuroscience.uth.tmc.edu/s2/chapter09.htmlhttp://downloadpdfz.com/ppt/what-is-adaptation-of-sensory-receptorshttp://downloadpdfz.com/ppt/what-is-adaptation-of-sensory-receptorshttp://downloadpdfz.com/ppt/what-is-adaptation-of-sensory-receptorshttp://classes.midlandstech.edu/carterp/Courses/bio110/chap09/chap09.htmhttp://classes.midlandstech.edu/carterp/Courses/bio110/chap09/chap09.htmhttp://classes.midlandstech.edu/carterp/Courses/bio110/chap09/chap09.htmhttp://classes.midlandstech.edu/carterp/Courses/bio110/chap09/chap09.htmhttp://webvision.med.utah.edu/book/part-v-phototransduction-in-rods-and-cones/glutamate-and-glutamate-receptors-in-the-vertebrate-retina/http://webvision.med.utah.edu/book/part-v-phototransduction-in-rods-and-cones/glutamate-and-glutamate-receptors-in-the-vertebrate-retina/http://webvision.med.utah.edu/book/part-v-phototransduction-in-rods-and-cones/glutamate-and-glutamate-receptors-in-the-vertebrate-retina/http://webvision.med.utah.edu/book/part-v-phototransduction-in-rods-and-cones/glutamate-and-glutamate-receptors-in-the-vertebrate-retina/http://highered.mheducation.com/sites/dl/free/0072437316/120060/ravenanimation.htmlhttp://highered.mheducation.com/sites/dl/free/0072437316/120060/ravenanimation.htmlhttp://highered.mheducation.com/sites/dl/free/0072437316/120060/ravenanimation.htmlhttp://highered.mheducation.com/sites/dl/free/0072437316/120060/ravenanimation.htmlhttp://highered.mheducation.com/sites/dl/free/0072437316/120060/ravenanimation.htmlhttp://webvision.med.utah.edu/book/part-v-phototransduction-in-rods-and-cones/glutamate-and-glutamate-receptors-in-the-vertebrate-retina/http://classes.midlandstech.edu/carterp/Courses/bio110/chap09/chap09.htmhttp://downloadpdfz.com/ppt/what-is-adaptation-of-sensory-receptorshttp://neuroscience.uth.tmc.edu/s2/chapter09.htmlhttp://cnx.org/content/m46577/latest/?collection=col11496/latesthttp://homepage.psy.utexas.edu/homepage/class/psy394U/hayhoe/IntroSensoryMotorSystems/week3/Kandel%20Ch%2021,%2022,%2023.pdf -
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a. If the eyeball is too long the flat lens focuses distant objects in front of
the retina. Since light rays reflected from closer objects diverge more, and the
focal distance is longer, the focal point moves back to the retina without the lens
having to accommodate, and near vision is OK, but distant vision is blurred.You cant flatten the lens any flatter than it already is, so youre stuck. This is
known as myopia, or nearsightedness. It can be corrected by placing a
concave lens in front of the eye, which diverges the light rays a bit before they
enter the eye. This increases the focal length and allows the relaxed lens to
focus precisely on the retina.
b. If the eyeball is too short, the focal point from distant objects is behind
the retina, but the lens can round up to move the focal point forward, like
accommodating for near vision, and distant objects appear to be in focus. The
problem comes when objects close to the eye cause the focal length to be
longer, and the lens, which is already rounded up, cant round up any more.This causes close objects to be blurred and is known as hyperopia, or
farsightedness. Hyperopia can be corrected by placing a convex lens in front of
the eye, which converges the light rays a bit before they enter the eye. This
decreases the focal distance so the lens can focus distant objects without
rounding up and can round up enough to focus near objects.
c. A normal part of the aging process is loss of elasticity by the lens,
which inhibits its ability to round up and focus on close objects This age
Annex 1 - Vision Problems
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which inhibits its ability to round up and focus on close objects. This age-
related farsightedness in an eye with a perfectly good shape is called
presbyopia(old vision) and usually begins to be noticed around 40 years of
age. Presbyopia can also be treated with convex lenses, but since the focallength is normal this correction will cause distant vision to be blurred, so people
commonly wear half glasses in order to be able to look over them at distant
objects and peer down through them at close objects. This makes negotiating
stairs a challenge, especially if someone was myopic to begin with and must
then wear bifocals (Think about it).
d.Astigmatismresults from the surface of the lens or cornea being uneven,
which causes light to be focused on the retina in lines rather than as a single
point.
e.Cataractsare clouding of the lens due to damage from things like ultraviolet
rays, cigarette smoke, and other toxic things. The lens eventually becomes
so clouded that a person with cataracts is functionally blind even though the
photoreceptors are fine. To correct cataracts the lens can be removed and
replaced with an artificial lens. Obviously the artificial lens cant
accommodate for close vision so it has to be preset for one or the other and
supplemented with contacts or glasses.
Annex 2
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Rhodopsin light
Rh*
T
T + T
GDP GTP
about 109 rhodopsinmolecules and about108 G-protein molecules
per rod outer segment (ROS)
photoactivated rhodopsin
(Rh*) forms in about 1 ms
and serially activates 100 to
1000 Transducin molecules
per second
Transducin is a
heterotrimeric
G protein
specific to vision
Rhodopsin is a G-Protein coupled receptor
Rhodopsin lightAnnex 2
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Rh*
T-GDP T -GTP + T
PDE PDE*
cGMP GMP
CNG Channels: OPEN CLOSED
Effector
Next question:
How are the activated
Shut-offAnnex 2
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Rhodopsin light
Rh*
T-GDP T -GTP + T
PDE PDE*
cGMP GMP
CNG Channels: OPEN CLOSED
How are the activated
intermediates shut off?
Transducin is inactivated
by the intrinsic GTPas
activity which
hydrolyzes GTP to GDP
The intrinsic
GTPase activity of
the subunit
is regulated by a
GAP (GTPase
accelerating protein)
RGS-9
Annex 2
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Signaling
cascade
Alberts et al, Mol Bio of the Cell
Converts a microscopic stimulus
activation of a single molecule
-into a macroscopicresponse
light
Phototransduction Cascadeas an enzymatic amplifier
Annex 2
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Rhodopsin Rh*
cGMP GMP
1st Stage of amplification:
200 - 1000 G* per Rh*
2nd Stage amplification:
each PDE* hydrolyzes
~ 100 cGMP molecules.
G
-GDP G*
-GTP + G
Phosphodiesterase (PDE) PDE*
Total gain:
2 x 105 106
cGMP / Rh*
channel closure
GC*GTP
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