august 8, 2015retina1 physiology of the retina receptor and neural function of retina

April 19, 2023 Retina 1

Physiology of the RetinaPhysiology of the Retina

Receptor and neural function of retina


Functional AnatomyFunctional Anatomy

• Retina is the light sensitive part of the eye

• contain photoreceptors– Cones• Responsible for color vision

– Rods • Responsible for

– black & white vision

– Vision in the dark

Structure of retinaStructure of retina

• Organized in 10 layers• Inside these layers lies

– Photoreceptor cells

– Horizontal cells

– Bipolar cells

– Amacrine cells

– Ganglionic cells

• These cells synapse with each other



Functional AnatomyFunctional Anatomy

• Rods and cones synapse with bipolar cells

• Bipolar cells synapse with ganglion cells

• Ganglion cells converge and leave the eye as optic nerve

• Horizontal cells connect receptor cells

• Amacrine cells connect ganglion cells to one another as well as to bipolar cells


Pigment epithelium

Rods and Cones

Horizontal cells

Bipolar cells

Amacrine cells

Ganglion cells

Optic nerve


Distribution of ReceptorsDistribution of Receptors• Rods are responsible for dim light (scotopic

vision)• Cones are responsible for daylight (photopic)

vision• Cones are found in greatest number at the

optical axis– At fovea there are no rods

• Rods extend at the periphery– Where there are no cones


Distribution of ReceptorsDistribution of Receptors

• In each human eye there are– 6 million cones– 120 million rods – 1.2 million fibres in each optic nerve

• There is overall convergence– Receptor cells on ganglion cells– 105 receptor cells to 1 ganglion cell

April 19, 2023 Retina 8Fovea

Rods

Cones

Nasal Temporal

Fovea

Concentration of rods

Concentration of cones


Fovea Fovea • Minute area in the center of retina

• In this region – Ganglion cells, blood vessels, inner nuclear

layer– Have all been displaced to one side– Light falls directly on the cones

• Acuity of vision is very high– That is responsible for acute and detailed

vision


The Pigment Layer of RetinaThe Pigment Layer of Retina

• Cells contain black pigment- melanin– Prevent light rays reflection– Responsible for clear vision– Prevents light reflection thruogh out the

globe of the eye ball

• Layer contain stores of vitamin A– An important precursor of chromophore

Photochemistry of visionPhotochemistry of vision

• Light sensitive photochemical inrods –RhodopsinCones-colour pigment

• Differs in spectral sensitivity



Structure of ReceptorsStructure of Receptors

• Photo-receptors convert light energy into action potential

• Rods are slender elongated structures– Diameter =1 µm

– Length = 40 µm

• Outer segment of rods is specialized for photo-reception

Ou

ter

segm

en

tIn

ner

se

gm

en

t

Rods



• Contains stacks of about 1000 discs– Closed and flattened sacks

(160 angstroms)

• Densely packed with photo-sensitive pigment

• Discs are formed by inner segment– Migrate to outer segment

Ou

ter

segm

en

tIn

ner

se

gm

en

t

Rods



• Cones have thick inner segment and conical outer segment

• The saccules formed in outer segment– By in-folding of the

cell membrane

Ou

ter

seg

men

tIn

ner

se

gm

en

t

Cones


Photo Pigments Photo Pigments

• Consist of – Opsin - a Glycoprotein– Retnene (retinal)• Carotenoid pigment

• Aldehyde of vit. A1

• Responsible for capture of light

• Is the same in all photo pigments


Photo Pigments Photo Pigments

• Rods have rhodopsin– Scotopsin + retinal

• Cones have iodopsin– Photopsin + retinal


Formation and Breakdown of Formation and Breakdown of Rhodopsin Rhodopsin

Retinol (Vit. A1)

All – trans- retinal

11- cis- retinal

Rhodopsin

Scotopsin

Dehydrogenase

Isomerase

Dark phase

Light phase


Excitation of ReceptorExcitation of Receptor

• Membrane of receptors have cation specific channels

• That are open in the dark

• Na+ ions flows into the outer segment– Following the

Electrochemical gradient

Na+

Na+

Na+

K+

Na+

Neurotransmitter

Dark Phase

+-



• This gradient is maintained by N+/K+ pump at inner segment

• EFFECT OF LIGHT– blocks the channels

– Sodium influx decreases

– Membrane hyperpolarizes

– Decrease in neurotransmitter release

Na+

Na+

Na+

K+

Na+

Neurotransmitter

Dark Phase

+-



• Cyclic GMP is responsible for keeping the channels open

• Conc of cGMP in the cytoplasm of receptor is high in the dark

• In the presence of light– Excited rhodopsin

– Activates TRANSDUCIN

Na+

Na+

Na+

K+

Na+

Neurotransmitter

Dark Phase

+-



• The transducin then activates cGMP phosphodiesterase

• Catalyses the breakdown of cGMP to 5’GMP

• The fall in conc of cGMP leads to closure of Na+ channels

• There is hyper-polarization

• Decrease in release of transmitter

Na+

Na+

Na+

K+

Na+

Neurotransmitter

Dark Phase

+-


Excitation of ReceptorsExcitation of Receptors

• In the dark– Release of

neurotransmitter is greatest

– Leads to inhibition of bipolar cells

• Ganglion cells – Not excited by

bipolar cells

Rod

Horizontal cell

NT -

NT -

NT +

G cell

Amacrine cell

Light ray

Optic nerve



• In the presence of light– Rods and cones

hyperpolarize

• Decrease release of transmitter Inhibition of the

bipolar cells

Rod

Horizontal cell

NT -

NT -

NT +

G cell

Amacrine cell

Light ray

Optic nerve



• Bipolar cell release of transmitter

• Excitation of ganglion cells

• Increase in AP conducted

Rod

Horizontal cell

NT -

NT -

NT +

G cell

Amacrine cell

Light ray

Optic nerve



• The horizontal cell– Depolarized by light

• Release inhibitory neurotransmitter– Lateral inhibition

Rod

Horizontal cell

NT -

NT -

NT +

G cell

Amacrine cell

Light ray

Optic nerve


Dark & Light AdaptationDark & Light Adaptation

• The sensitivity of photoreceptors– Depends on conc of photo pigments

• A slight change causes – Great change in retinal sensitivity


Light AdaptationLight Adaptation

• Exposure to light– Photo pigment is broken down• Opsin• Retinal vitamin A

In conc of photo pigment (photosensitive chemicals- rods/cones) Sensitivity of retina to light

• This is known as – Light adaptation


Dark AdaptationDark Adaptation

• In the dark– Vitamin A retinal– Retinal combine with opsin

• Light sensitive pigment

• The conc of photo pigment – Greatly increases

• Photo receptor – Become highly sensitive to light

• This is dark adaptation


Dark & Light AdaptationDark & Light Adaptation

Dark adaptation

Light adaptation

Exposure to light or darkness (minutes)

Reti

nal

sen

siti

vity

Dark & light adaptationDark & light adaptation

• Other mechanisms

2.Change in pupilary size

3.Neural adaptationBipolar cellsHorizontal cellsAmacrine cellsGanglionic cells

• Initial increased light intensity; all become intense, then decrease with time

Retina 30


Ganglion CellsGanglion Cells

• Each retina has– 120 million rods– 6 million cones– 1.2 million ganglion cells

• Hence many rods & cones – Converge on each ganglion cell



• At fovea centralis– 1 cone connect to 1 ganglion cell– High degree of visual acuity

• At periphery– About 200 rods– Converge on a single ganglion cell



• Signals from rods– Summate to sensitivity Intensity of stimulation to peripheral

ganglion cells


Types of Ganglion CellsTypes of Ganglion Cells

1. Large cells– Magnocellular cells (M cells or Y cells)– Large in diameter ( 35 m)– Axons transmit at 50 m/sec– Respond to rapid change in visual image • Rapid movement

• Rapid change in light intensity

• Project to layer 1,2 of LGB



2. Small ganglion cells– Parvocellular , “P” cells, “X”-cells– 10 – 15 m in diameter– Transmit at 14 m/sec

• Responsible for – Transmission of fine details of visual image

such as Colour, texture & shape of objects

• Project to layers– 3, 4, 5, 6 of lateral geniculate body (LGB)


• 3. W cells– < 10 m in diameter– Transmit at 8 m/sec

• Responsible for – Detecting directional movements in the field

of vision– Crude rod vision under dark condition



Pathway to CortexPathway to Cortex

• Retinaoptic nerve

• Optic chiasma– Cross over

• Optic tract– optic nerve –nasal side & optic nerve temporal side

• Lateral geniculate body (LGB) of thalamusOptic radiation

• Primary Visual cortex


Temporal

Nasal

Retina rt eye

Optic nerve

Optic chiasma

Optic radiation

Lateral geniculate body

Optic tract

Visual cortex

Pathway to cortexPathway to cortex

• Other fibers from optic tract

• Suprachiasmatic nucleus-hypothalamus– Control circadian rhythms

– Synchronize various physiologic body changes with night and day

• Pretectal nuclei-midbrain– Elicit reflex movements of the eyes

– To focus on the objects of importance and to activate the pupilary light reflex


Pathway to cortexPathway to cortex

• Superior colliculus– To control rapid directional movements of

the two eyes

• Ventral lateral geniculate nucleus of the thalamus– Control some of the behavioral body

movements


Function of dorsal lateral Function of dorsal lateral geniculate nucleus of thalamusgeniculate nucleus of thalamus

1. Relay visual information to the brain From optic tract to visual cortex

2. Gate the transmission of signals to the visual cortex

Nucleus receives gating control signals from Corticofugal fibers-from primary visual cortex Reticular areas of mesencephalon

• Both are inhibitory signals


Organization and function of Organization and function of the visual cortexthe visual cortex

• Primary visual cortex– All visual signals terminate in the visual lobe– Analyses visual details and color

• Secondary visual areas of cortex– Visual association areas• Responsible for analysis of visual meanings



Lesions of Optic Pathways Lesions of Optic Pathways

• Lesion of optic nerve causes – Blindness in that eye

• Lesion at the optic chiasma (central) causes– Blindness in the opposite visual fields– Bitemporal hemianopsia• Heteronymous hemianopsia


Lesions of Optic PathwaysLesions of Optic Pathways

• Lesion of optic tract causes– Blindness in half of visual fields• Homonymous hemianopsia


Temporal

Nasal

Retina rt eye

Optic nerve

Optic chiasma

Optic radiation

Lateral geniculate body

Optic tract

Visual cortex

1 2

3

Lesion at 1

Blindness in the eye

Lesion at 2 bitemporal hemianopsia

Lesion at 3 homonymous hemianopsia


Colour VisionColour Vision

• Primary colours– Blue, green, red

• Human eye can detect – All gradation of calours when– Red, blue & green are • Mixed in different combination



• Young Helmhotz theory

• Young proposed that– Colour vision was mediated by– 3 fundamental receptors for – The 3 fundamental colours• Blue, green, red



• 3 types of cones– Blue absorbing cone– Green absorbing cone– Red absorbing cone



• 3 types of colour photo-pigments– Cyanolabe• Blue sensitive photo-pigment

– Chrolabe • Green sensitive photo-pigment

– Erythrolabe • Red sensitive photo-pigment



• The three types of cones

• The blue absorbing cone– Wave length from

• 370 – 510 nm

• Maximum at 445 nm

• The green absorbing cone

400 500 600 700

25

100

75

50

% o

f L

igh

t ab

sorp

tion

Wave length () nm

Blue (445)

green

(535)

Red (5

75)



• The green absorbing cone– Wave length from

• 450 – 630 nm• Maximum at 535 nm

• The red absorbing cone– Wave length from

• 470 – 700 nm• Maximum at 575 nm

400 500 600 700

25

100

75

50

% o

f L

igh

t ab

sorp

tion

Wave length () nm

Blue (445)

green

(535)

Red (5

75)



• Sensation of given colour determined by – Relative frequency of

impulses• From each of the three

types of cones

400 500 600 700

25

100

75

50

% o

f L

igh

t ab

sorp

tion

Wave length () nm

Blue (445)

green

(535)

Red (5

75)



• A light in the red – green spectral band– Will stimulate red &

green cones

• The sensation of red or green will depend on– Particular ratio of

response in the two types of cones

400 500 600 700

25

100

75

50

% o

f L

igh

t ab

sorp

tion

Wave length () nm

Blue (445)

green

(535)

Red (5

75)



• A light in the red – green spectral band– At wavelength of 610 nm

• Stimulate – Red cone 85%

– Green cone 15%

– Blue cone 0%

• This will be interpreted by the brain as red colour

400 500 600 700

25

100

75

50

% o

f L

igh

t ab

sorp

tion

Wave length () nm

Blue (445)

green

(535)

Red (5

75)



• A light in the red – green spectral band– At wavelength of 550

nm

• Stimulate – Red cone 90%

– Green cone 90%

– Blue cone 0%400 500 600 700

25

100

75

50

% o

f L

igh

t ab

sorp

tion

Wave length () nm

Blue (445)

green

(535)

Red (5

75)



• Both the red cone and the green cone – Same number of AP

to the brain– While no AP from the

blue cone

• This will be interpreted by the brain as yellow colour

400 500 600 700

25

100

75

50

% o

f L

igh

t ab

sorp

tion

Wave length () nm

Blue (445)

green

(535)

Red (5

75)



• The sensation of any colour

• Determined by – The relative

frequency of impulses

– Reaching the brain from • Each of the 3 types of

cones400 500 600 700

25

100

75

50

% o

f L

igh

t ab

sorp

tion

Wave length () nm

Blue (445)

green

(535)

Red (5

75)


Colour BlindnessColour Blindness

• Normal colour vision– Tri-chromatic

• Loss of any cone function– Leads to colour vision abnormalities

• Dichromatic vision– Unable to perceive

• Green or red colours

– Blue colour blindness • Very rare


Colour BlindnessColour Blindness

• Protanopia– Red cone non functioning

• Deuteranopia– Green cone non functioning

• Tritanopia– Blue cone non functioning


Inheritance of Colour Inheritance of Colour BlindnessBlindness

• Colour blindness– Genetically transmitted– Sex linked, recessive– X chromosome linked

• Colour blindness– Will not appear as long as – Another X chromosome carries the gene


Inheritance of Colour Inheritance of Colour BlindnessBlindness

XY XX

XYXYXXXX

Colour blind Normal

Carrier Normal Normal Carrier

XX XY

XX XY XX XY

Carrier Colour blind

Carrier Normal

Normal Normal

august 8, 2015retina1 physiology of the retina receptor and neural function of retina

Documents