The Photochemistry of Vision

5/28/2018 The Photochemistry of Vision

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The Physiology of Vision


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Objectives

Review anatomy and histology of light receptors

Compare and contrast the rods and Cones

Describe the photochemistry of vision

Describe the receptor potential of rods and cones

Describe dark and light adaptation Explain color vision and color blindness

List the cells of the neural pathway

Follow the neural pathway up to the visual cortex

Resources:

Guytons Textbook of Physiology


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Retina

Light sensitive portion of the eye

Contains conesfor color vision

Containsrods for night vision Contains neural architecture

Light must pass through the neural elements to

strike the light sensitive rods and cones


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(1) pigmented layer, (2) layer of rods and

cones projecting to the pigment, (3) outer

nuclear layer containing the cell bodies of therods and cones, (4) outer plexiform layer, (5)

inner nuclear layer, (6) inner plexiform layer,

(7) ganglionic layer, (8) layer of optic nerve

fibers, and (9) inner limiting membrane.


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The Fovea

A small area at the center of the retina about 1 sqmillimeter

The center of this area, the central fovea, containsonly cones

These cones have a special structure

Aid in detecting detail

In the central fovea the neuronal cells and blood

vessels are displaced to each side so that the light canstrike the cones directly.

This is the area of greatest visual acuity


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Fovea

Minute area(1sq. mm) in

the centre of retina.

100% conescapable

of detecting acute and

detailed vision.

Also has blood vessels,

ganglion cells, INL &

plexiform layer displaced

to side. Light passes unimpeded for detailed

vision.


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Pigment Layer of Retina

Pigment layer of the retina is very important

Contains the black pigment melanin

Prevents light reflection in the globe of the eye

Without the pigment there would be diffusescattering of light rather than the normal contrast

between dark and light.

This is what happens in albinos

poor visual acuity because of the scattering oflight


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photoreceptors

There are 2 types of photoreceptors in the

retina: rods and cones.

The rodsare most sensitive to light anddark changes, shape and movement andcontain only one type of light-sensitive

pigment.

Rods are more numerous than cones in

the periphery of the retina.

120 million rods in the human retina.


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Cones

The cones are not as sensitive to light as

the rods.

Cones are most sensitive to one of three

different colors (green, red or blue).

You cannot see color very well in dark

places as cones work in bright light.


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Functional segments of rods and

cones

Outer segment:

Has light sensitive photopigment.

Large no.(1000) of discs which are foldsof cell membrane.

Photopigment is present as

transmembrane proteins & form 40% ofouter segment mass.


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Inner segment:

Contains cytoplasm and organelles.

Most importantmitochondria whichprovide energy.

Nucleus:

Synaptic body:Connects to subsequent horizontal and

amacrine cells.


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Rods Cones

High sensitivity;specialized for nightvision

More photopigment High amplification; single

photon detection

Slow response, longintegration time

More sensitive to scatteredlight

Lower sensitivity;specialized for dayvision

Less photopigment Less amplification

Fast response, shortintegration time

More sensitive to directaxial rays


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Rods Cones

low acuity; highly

convergent retinal

pathways, not present in

central fovea

achromatic; one type of

rod pigment

high acuity; lessconvergent retinal

pathways, concentrated incentral fovea

chromatic; three types ofcones, each with adifferent pigment that issensitive to a different partof the visible spectrum

(Red, Green and Blue)


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of Rodsones


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Structure of the Rods and Cones


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Retinal Rhodopsin visual cycle

Both rods & cones have chemicals which on

exposure to light decompose and excite nerve

fibres leading from the eye.

In rods this light sensitive pigment isRHODOPSIN.

In cones, it is colour/ cone pigment that may

be;

Erythrolabe ( red sensitive)

Chlorolabe ( green sensitive)

Cyanolabe ( blue sensitive)


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Visual cycle

Rhodopsin or visual purplepresent in

the outer segment of retina.

It is a combination of proteinscotopsin

& carotenoid pigmentRetinal/

Retinene.

Retinal is 11-cis retinal type which is the

only form that binds with scotopsin to

produce rhodopsin.


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Rhodopsin Activation

Light falling onrhodopsin cause it to

decompose in a fractionof a second & changefrom cis form to trans

form ( same chemicalstructure but different

conformationalstructure).

The immediate product

thatBathorhodopsinis changes into.Lumirhodopsin

In microseconds itdecays into

Metarhodopsin I


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In milliseconds itchanges to

II andMetarhodopsinlater more slowly into

scotopsin and alltrans retinal.

Metarhodopsin II isactivatedcalled

as itRhodopsincauses electrical

changes in the rodsthat transmit visualsignals to the CNS.


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Vitamin A & its role in rhodopsin

formation

All trans retinal can be converted to 11-cis

retinal by an alternate route.

This is to convert all trans retinal to all trans-

retinol which is a form of vitamin A.Then all trans retinol is changed to 11-cis

retinol under influence of isomerase.

Finally 11-cis retinol is converted to 11-cisretinal.


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Re- formation of Rhodopsin

All trans retinal is

converted to 11-cis

retinal.

This requires energy &

enzyme Retinal

Isomerase.

11-cis retinal

automatically combines

with scotopsin to re-formrhodopsina process

occuring in darkness.


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Nyctalopia/ Night Blindness

This occurs in severe vitamin A deficiency.

Amount of retinal and subsequent rhodopsin areseverely decreased.

It is called night blindness as the amount of light

at night is too little to allow adequate vision invitamin A deficient people.

Person must be on vit. A deficient diet formonths to develop nyctalopia as large amounts

stored in liver.It can be corrected by a single injection of vit. A

in less than an hour.


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Photochemistry

of Vision


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Rod receptor potential

hyperThere israther thanpolarization

depolarization.

There is increased

negativity of intra rodmembrane potential.

When rhodopsin

decomposes it decreases

the rod membrane

conductance for Na+ ions

in the outer segment of

rod.


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Inner segmentcontinually pumps

Na+ from inside to theoutside of rodsove

potential.

Outer segment is very

leaky to Na+ in darkstate.

So much of +ve Naleaks back into inside

of rod to neutralizemost of the negativity.


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So in darkness there

is decreased

negativity of -40 mVrather than -70-80 mV

in sensory receptors.

When rhodopsin inouter segment is

exposed to light &

decomposes, it

decreases the outersegment membrane

conduction for Na+


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But the inner segment continues to pump

out Na+.

So more Na+ leaks out than move back

in.

There is increased negativity inside rod

membrane.

Greater the light falls on rods greater is

the intra rod membrane negativityhyper

polarization.( -70-80 mV)


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S f t i


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Sequence of events in

Phototransduction

Incident light( photon of light activates

electron)

Structural change in retinene photo

pigment.

Conformational change in photo

pigment.

.TransducinActivation of G protein

Activated transducin activates

.Phosphodiestrase


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Decreased intracellular cGMP. cGMP is

destroyed by hydrolysis, before which it was

bound with Na channel protin of outer segmentto splint it in open state.

Closure of sodium channel.

Hyper polarization.Decreased release of synaptic transmitter.

Response in bipolar & other neural elements

Rhodopsin kinase (in a sec) inactivates activatedrhodopsin and entire cascade reverses back to

normal.


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D ti d S iti it f th


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Duration and Sensitivity of theReceptor Potential

A single pulse of light causes activation ofthe rod receptor potential for more than asecond.

In the cones these changes occur 4 X faster.

Receptor potential is proportional to thelogarithm of the light intensity.

very important for discrimination ofthe light intensity


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Dark and Light Adaptation

In light conditions

1. most of the rhodopsin has been reduced to retinal.

2.much of the retinal of both the rods and the

cones will have been converted into vitamin A.Because of these two effects, theconcentrations of the photosensitive chemicals

remaining in the rods and cones areconsiderably reduced, and the sensitivity of theeye to light is correspondingly reduced. This is

called light adaptationt

the eye


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In dark conditions1retinal is converted back to rhodopsin.

2.vitamin A is converted back into retinal toincrease light-sensitive pigments,( the finallimit being determined by the amount ofopsins in the rods and cones to combine

with the retinal). This is called darkadaptation

Opening and closing of the pupil also contributesto adaptation because it can adjust the amount

entering light.


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the sensitivity of the retina is very low on first entering

the darkness, but within 1 minute, the sensitivity

increased 10-fold-

(that is, the retina can respond to light of one tenth the

previously required intensity.)

At the end of 20 minutes, the sensitivity has increased

about 6000-fold,and at the end of 40 minutes, about 25,000-fold.


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Other Mechanisms of Light and Dark Adaptation

change in pupillary size,This can cause adaptation of

approximately 30-fold within a fraction of a second

because of changes in the amount of light allowed

through the pupillary opening

neural adaptation,involving the neurons in thesuccessive stages of the visual chain in the retina itself

and in the brain

degree of adaptation fewfold,but occurs in a fraction of

a second

Importance of Dark and Light


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Importance of Dark and Light

Adaptation

Between the limits of maximal darkadaptation and maximal light adaptation, theeye can change its sensitivity to light as muchas 500,000 to 1 million times, the sensitivityautomatically adjusting to changes inillumination.

Because registration of images by the retinarequires detection of both dark and light spotsin the image, it is essential that the sensitivityof the retina always be adjusted so that thereceptors respond to the lighter areas but notto the darker areas.

It


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example of maladjustment of retinal adaptation

Enter the sun from a movie theater, even the dark spotsappear bright leaving little contrast.

Enter darkness from light, the light spots are not lightenough to register.

the intensity of sunlight is about 10 billion times

that of starlight, yet the eye can function both inbright sunlight after light adaptation and instarlight after dark adaptation.

Sensitivity and Acuity


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Sensitivity and Acuity

Rods and cones synapse with bipolar cells

Bipolar cells synapse with ganglion cells

Ganglion cells synapse with neurone fibres

At the fovea each cone synapses individually with a ganglion cell

This gives good Acuity (resolution).Many Rods synapse with one bipolar neuroneRETINAL

CONVERGENCE

Dim light results in small amount of neurotransmitter release

Individually, this would be insufficient to over come the threshold of

the bipolar cell, but the total amount of transmitter from several rods is

sufficient.

This gives less acuitybut bettersensitivity


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Photochemistry of Vision

Rods and cones contain chemicals that decompose on

exposure to light.

This excites the nerve fibers leading from the eye.

The membranes of the outer-segment of the rods containrhodopsinor visual purple.

Rhodopsin is a combination of a protein calledscotopsin

and a pigment, retinal.

The retinal is in the cisconfiguration. Only the cisconfiguration can bind with scotopsin to form

rhodopsin.


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Light and Rhodopsin

When light is absorbed by rhodopsin it immediately beginsto decompose.

Decomposition is the result of photoactivation of electronsin the retinal portionof rhodopsin which leads to a change

from the cis formof the retinal to the trans formof themolecule.

Transretinal has the same chemical structure but is astraight molecule rather than an angulated molecule.

This configuration does not fit with the binding site onthe scotopsin and the retinal begins to split away.

In the process of splitting away a number ofintermediary compounds are formed.


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Mechanism for Light to Decrease


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Mechanism for Light to Decrease

Sodium Conductance

cGMP is responsible for keeping Na+channel in the outer

segment of the rods open.

Light activated rhodopsin (metarhodopsin II) activates a G-

protein, transducin.

Transducin activates cGMPphosphodiesterasewhich

destroys cGMP.

Rhodopsin kinase deactivates the activated rhodopsin

(which began the cascade) and cGMP is regenerated re-

opening the Na+ channels.


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Role of Vitamin A

Vitamin A is the precursor of all-trans-

retinal, the pigment portion of rhodopsin.

Lack of vitamin A causes a decrease in

retinal.

This results in a decreased production of

rhodopsin and a lower sensitivity of the

retina to light or night blindness.

Th R d R t P t ti l


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The Rod Receptor Potential

Normally about -40 mV

Normally the outer segment of the rod is very

permeable to Na+ions.

In the dark an inward current (the dark current)carried by the Na+ ions flows into the outer

segment of the rod.

The current flows out of the cell, through the

efflux of K+, ions in the inner segment of the rod.


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The Dark Current

In the dark an inward current(the dark current) carried by

the Na+ ions flows into theouter segment of the rod.


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Rod Receptor Potential (Contd)

When rhodopsin decomposes it causes ahyperpolarizationof the rod by decreasing Na+

permeability of the outer segment.

The Na

+

pump in the inner segment keepspumping Na+out of the cell causing the membranepotential to become more negative(hyperpolarization).

The greater the amount of light the greater theelectronegativity.

Th D k C


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When rhodopsin decomposes in

response to light it causes a

hyperpolarizationof the rod by

decreasing Na+ permeability of the

outer segment.

The Dark Current


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Color Vision

Color vision is the result of activation of cones. 3 types of cones:

blue cone

green cone

red cone

The pigment portion of the photosensitivemolecule is the same as in the rods, the protein

portion is different for the pigment molecule ineach of the cones.

Makes each cone receptive to a particularwavelength of light


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the three types of cones show peakabsorbencies at light wavelengths of

445, 535, and 570 nanometers,


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E h C i R ti t P ti l


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Each Cone is Receptive to a ParticularWavelength of Light


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orange light with a wavelength of 580nanometers

stimulates the red cones 99 (99 percent of the peak stimulation at optimum

wavelength);

stimulates the green cones 42

, blue cones not at all

.

, the ratios of stimulation are 99:42:0.


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set of ratios-0:0:97-is interpreted by the nervoussystem as blue.

ratios of 83:83:0 are interpreted as yellow

, and 31:67:36 as green

. Perception of White Light

About equal stimulation of all the red, green, andblue cones gives one the sensation of seeing

white.

Color Blindness


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Color Blindness

Lack of a particular type of cone

Genetic disorder passed along on the Xchromosome

Occurs almost exclusively in males About 8% of women are color blindness carriers

Most color blindness results from lack of the redor green cones

Lack of a red cone,protanope. Lack of a green cone, deuteranope.


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Neural Organization of the Retina

Direction of

light

Si l T i i i th R ti


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Signal Transmission in the Retina

Transmission of signals in the retina is by

electrotonic conduction.

Allows graded response proportional to light

intensity. The only cells that have action potentials are

ganglion cells.

send signals all the way to the brain

Lateral Inhibition to Enhance


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Lateral Inhibition to EnhanceVisual Contrast

Horizontal cells connect laterally between the rodsand cones and the bipolar cells

Output of horizontal cells is always inhibitory

Prevents the lateral spread of light excitation onthe retina

Have an excitatory centerand an inhibitorysurround

Essential for transmitting contrast borders in thevisual image


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Lateralinhibition,

the

function

of

horizontal

cells


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Function of Amacrine Cells

About 30 different types Some involved in the direct pathway from rods to

bipolar to amacrine to ganglion cells

Some amacrine cells respond strongly to the onset

of the visual signal, some to the extinguishment of

the signal

Some respond to movement of the light signal

across the retina

Amacrine cells are a type of interneuron that

Aid in the beginning of visual signal analysis.

R d C d G li C ll


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Rods, Cones and Ganglion Cells

Each retina has 100 million rods and 3 million

cones and 1.6 million ganglion cells.

60 rods and 2 cones for each ganglion cell

At the central fovea there are no rods and the ratioof cones to ganglion cells is 1:1.

May explain the high degree of visual acuity in the

central retina


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Three Types of Ganglion Cells

W cells(40%) receive most of their excitation fromrod cells.

sensitive to directional movement in the visual

field

X cells(55%) small receptive field, discrete retinal

locations, may be responsible for the transmission of

the visual image itself, always receives input from at

least one cone, may be responsible for color

transmission.

Y cells(5%) large receptive field respond to

instantaneous changes in the visual field.

E it ti f G li C ll


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Excitation of Ganglion Cells

Spontaneously active with continuous action

potentials

Visual signals are superimposed on this

background

Many excited by changes in light intensity

Respond to contrast borders, this is the way the

pattern of the scene is transmitted to the brain

Eye Movements are Controlled by


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Medial and lateral

recti move eyes side

to side

Superior and inferior

recti move eyes up

and down

Superior and inferior

obliques rotate the

eyes

Eye Movements are Controlled by

3 Separate Pairs of Muscles.

Figure 51-7; Guyton & Hall

Neural Pathways for Control


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Neural Pathways for Controlof Eye Movement

Fixation movements of the eyes controlled by two

neuronal mechanisms, voluntaryand involuntary.

Voluntary fixation movements controlled by an area in the

premotor cortex.

Involuntary fixationmechanism causes eyes to lock on

object of attention found with the voluntary fixation

mechanism.

Controlled by secondary visual areas of the occipital cortex.

Results from negative feedback mechanism controlled at the

level of thesuperior colliculusthat prevents objects of

attention from leaving the foveal portion of the retina.


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Neural Pathways for Control of Eye Movement


Saccadic Eye Movements


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Saccadic Eye Movements

When the visual scene is moving (turning thehead), the eyes fix on one highlight after

another in the visual field jumping at a rate of2 to 3 jumps/sec. These jumps are called

saccades, and the movements are calledopticokinetic movements.

Saccades occur very rapidly (only 10% of thetime is spent making saccades).

Vision is suppressed during a saccadicmovement.


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The Autonomic Nerves to the Eyes

The eye is innervated by both parasympatheticand sympathetic neurons.

Parasympathetic fibers arise in the Edinger-Westphal nucleus, pass in the 3rd cranial nerve

to the ciliary ganglion.Postganglionic fibers excite the ciliary muscle and

sphincter of the iris.

Sympathetic fibers originate in theintermediolateral horn cells of the superior

cervical ganglion.

Postganglionic fibers spread along the corotidartery and eventually innervate the radial fibers

of the iris.


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Autonomic Pathways to the Eye


Control of Accommodation


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Co o o cco oda o(Focusing the Eyes)

results from contraction or relaxation of the

ciliary muscle

regulated by negative feedback

mechanism that automatically adjust thefocal power of the lens for highest degree

of visual acuity within about 1 sec

exact mechanism is not known

Control of Pupillary Diameter


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Control of Pupillary Diameter

miosis: decreasing of pupillary aperture

due to stimulation of parasympathetic

nerves that excite the pupillary sphincter

musclemydriasis: dilation of pupillary aperture due

to stimulation of sympathetic nerves that

excite the radial fibers of the iris


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Pupillary Light Reflex

When the amount of light entering the eyes

increases, the pupils constrict.

Functions to help the eye adapt extremely

rapidly to changing light conditions.Light excites fibers going to pretectalnuclei.

From pretectal nuclei fibers pass to Edinger-

Westphal nucleus and back throughparasympatheticnerves to constrictiris

sphincter.


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Visual field of

left eye

Temporal

half

Visual field of

right eye

Temporal

half

Nasal

half

Midbrain

Left eye

Temporal

retina

Optic

radiations

Left eye and its pathways

Primary visual area of cerebral

cortex (area 17) in occipital lobe

Lateral geniculate nucleus

of the thalamus

Optic

radiations

Midbrain

Temporal

retina

Nasal

retina

Right eye

Right eye and its pathways

Nasal

half

Nasal retina

1 1

Visual field of

left eye

Temporal

half

Visual field of

right eye

Temporal

half

Nasal

half

Midbrain

Left eye

Temporal

retina

Optic

radiations





of the thalamus

Optic

radiations

Midbrain

Temporal

retina

Nasal

retina

Right eye


Nasal

half

Nasal retina

1 1

22

Visual field of

left eye

Temporal

half

Visual field of

right eye

Temporal

half

Nasal

half

Midbrain

Left eye

Temporal

retina

Optic

radiations





of the thalamus

Optic

radiations

Midbrain

Temporal

retina

Nasal

retina

Right eye


Nasal

half

Nasal retina

1 1

22

3

3

Visual field of

left eye

Temporal

half

Visual field of

right eye

Temporal

half

Nasal

half

Midbrain

Left eye

Temporal

retina

Optic

radiations


Optictract




of the thalamus

Optic

radiations

Midbrain

Temporal

retina

Nasal

retina

Right eye


Nasal

half

Nasal retina

1 1

2244

3

3

Visual field of

left eye

Temporal

half

Visual field of

right eye

Temporal

half

Nasal

half

Midbrain

Left eye

Temporal

retina

Optic

radiations


Optictract




of the thalamus

Optic

radiations

Midbrain

Temporal

retina

Nasal

retina

Right eye


Nasal

half

Nasal retina

1 1

2244

5 5

3

3

Visual field of

left eye

Temporal

half

Visual field of

right eye

Temporal

half

Nasal

half

Midbrain

Left eye

Temporal

retina

Optic

radiations


Optictract




of the thalamus

Optic

radiations

Midbrain

Temporal

retina

Nasal

retina

Right eye


Nasal

half

Nasal retina

1 1

2

3

24 34

5 5

66

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The Photochemistry of Vision

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