cap IV_curs 9_10_vision_2014_2015_.pdf

7/25/2019 cap IV_curs 9_10_vision_2014_2015_.pdf

1/111

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


2/111

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.


3/111

Vision

Human visible light lies only

within the range ~380-750 nm;

The light is detected as photons* bythe retinal cells - rods & cones.


4/111

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


5/111

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.


6/111

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.


7/111

DIOPTER = a unit of

measurement of the

refractive power of lenses

equal to the reciprocal of

the focal length

measured in meters.



8/111

Dynamic changes in the refractive power of the

lensare referred to asaccommodation.


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.


9/111

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.



10/111

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.



11/111

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.



12/111

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!).



13/111

(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.



14/111

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.


15/111

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.


16/111


The monocular visual field is determined with

one eye covered.

l d h l ld


17/111


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


18/111


Objects within the binocular visual field are visible

to each eye, albeit from different angles.

RETINA


19/111

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


20/111

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


21/111


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


22/111

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.


Summary diagram of the cell types and connectionsin the primate retina.

RETINA


23/111

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


24/111

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


25/111

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


26/111


RETINA Photoreceptors- Functional Specialization of the Rod and Cone Systems


27/111


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.



28/111

Rods and cones work

together to allow the visual

system to operate over a wide

range of luminance

conditions.




29/111

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.




30/111

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



31/111

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.




32/111

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.




33/111

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).



34/111

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



35/111

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


36/111

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).



37/111

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



38/111

Because the center of our retina (the fovea)

contains very few, if any rods, we havea foveal blind

spotunder very dim conditions.

Color Vision


39/111

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


40/111

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


41/111

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


42/111

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


43/111

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


44/111

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


45/111

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


46/111

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


47/111

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.



48/111

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.



49/111

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


50/111

Within the outer plexiform layer , the

photoreceptor cells make both presynaptic and

postsynaptic contact with horizontal cells.



51/111

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.



52/111

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


53/111

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.



54/111

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.



55/111

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



56/111

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


57/111

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


58/111

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



59/111

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


60/111

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



61/111

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


62/111

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


63/111

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.


64/111


65/111

Phototransduction

Th i d t i th


66/111

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


67/111

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


68/111

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


69/111

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


70/111

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


71/111

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
http://pubs.rsc.org/en/content/articlehtml/2010/pp/b9pp00134dhttp://pubs.rsc.org/en/content/articlehtml/2010/pp/b9pp00134d


72/111

Phototransduction

Importantly,h t t d


73/111

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


74/111

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


75/111

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


76/111

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


77/111

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


78/111

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


79/111

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


80/111

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


81/111

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


82/111

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


83/111

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


84/111

, 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


85/111

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


86/111

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.


87/111

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


88/111

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


89/111

, 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


90/111

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


91/111

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.


The parallel processing of different categories of


92/111

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



93/111

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


94/111

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



95/111

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).


The circuitry responsible for the pupil lary l ight reflex.


96/111

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.


97/111

Visuotopic organization of the striate cortex in the right

i it l l b i id itt l i (A) Th i



98/111

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


99/111

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


100/111

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.



101/111

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



102/111

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


103/111

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:


104/111

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


105/111

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


106/111

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


107/111

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


108/111

Rh*

T-GDP T -GTP + T

PDE PDE*

cGMP GMP

CNG Channels: OPEN CLOSED

Effector

Next question:

How are the activated

Shut-offAnnex 2


109/111

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


110/111

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


111/111

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

cap IV_curs 9_10_vision_2014_2015_.pdf

Documents

Transcript of cap IV_curs 9_10_vision_2014_2015_.pdf