April 19, 2023 Retina 1
Physiology of the RetinaPhysiology of the Retina
Receptor and neural function of retina
April 19, 2023 Retina 2
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
April 19, 2023 Retina 3
April 19, 2023 Retina 4
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
April 19, 2023 Retina 5
Pigment epithelium
Rods and Cones
Horizontal cells
Bipolar cells
Amacrine cells
Ganglion cells
Optic nerve
April 19, 2023 Retina 6
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
April 19, 2023 Retina 7
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
April 19, 2023 Retina 9
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
April 19, 2023 Retina 10
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
April 19, 2023 Retina 11
April 19, 2023 Retina 12
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
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se
gm
en
t
Rods
April 19, 2023 Retina 13
Structure of ReceptorsStructure of Receptors
• 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
April 19, 2023 Retina 14
Structure of ReceptorsStructure of Receptors
• 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
April 19, 2023 Retina 15
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
April 19, 2023 Retina 16
Photo Pigments Photo Pigments
• Rods have rhodopsin– Scotopsin + retinal
• Cones have iodopsin– Photopsin + retinal
April 19, 2023 Retina 17
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
April 19, 2023 Retina 18
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
+-
April 19, 2023 Retina 19
Excitation of ReceptorExcitation of Receptor
• 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
+-
April 19, 2023 Retina 20
Excitation of ReceptorExcitation of Receptor
• 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
+-
April 19, 2023 Retina 21
Excitation of ReceptorExcitation of Receptor
• 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
+-
April 19, 2023 Retina 22
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
April 19, 2023 Retina 23
Excitation of ReceptorsExcitation of Receptors
• 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
April 19, 2023 Retina 24
Excitation of ReceptorsExcitation of Receptors
• 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
April 19, 2023 Retina 25
Excitation of ReceptorsExcitation of Receptors
• 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
April 19, 2023 Retina 26
Dark & Light AdaptationDark & Light Adaptation
• The sensitivity of photoreceptors– Depends on conc of photo pigments
• A slight change causes – Great change in retinal sensitivity
April 19, 2023 Retina 27
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
April 19, 2023 Retina 28
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
April 19, 2023 Retina 29
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
April 19, 2023 Retina 31
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
April 19, 2023 Retina 32
Ganglion CellsGanglion Cells
• 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
April 19, 2023 Retina 33
Ganglion CellsGanglion Cells
• Signals from rods– Summate to sensitivity Intensity of stimulation to peripheral
ganglion cells
April 19, 2023 Retina 34
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
April 19, 2023 Retina 35
Types of Ganglion CellsTypes of Ganglion Cells
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)
Types of Ganglion CellsTypes of Ganglion Cells
• 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
April 19, 2023 Retina 36
April 19, 2023 Retina 37
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
April 19, 2023 Retina 38
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
April 19, 2023 Retina 39
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
April 19, 2023 Retina 40
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
April 19, 2023 Retina 41
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
April 19, 2023 Retina 42
April 19, 2023 Retina 47
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
April 19, 2023 Retina 48
Lesions of Optic PathwaysLesions of Optic Pathways
• Lesion of optic tract causes– Blindness in half of visual fields• Homonymous hemianopsia
April 19, 2023 Retina 49
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
April 19, 2023 Retina 50
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
April 19, 2023 Retina 51
Colour VisionColour Vision
• Young Helmhotz theory
• Young proposed that– Colour vision was mediated by– 3 fundamental receptors for – The 3 fundamental colours• Blue, green, red
April 19, 2023 Retina 52
Colour VisionColour Vision
• 3 types of cones– Blue absorbing cone– Green absorbing cone– Red absorbing cone
April 19, 2023 Retina 53
Colour VisionColour Vision
• 3 types of colour photo-pigments– Cyanolabe• Blue sensitive photo-pigment
– Chrolabe • Green sensitive photo-pigment
– Erythrolabe • Red sensitive photo-pigment
April 19, 2023 Retina 54
Colour VisionColour Vision
• 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)
April 19, 2023 Retina 55
Colour VisionColour Vision
• 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)
April 19, 2023 Retina 56
Colour VisionColour Vision
• 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)
April 19, 2023 Retina 57
Colour VisionColour Vision
• 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)
April 19, 2023 Retina 58
Colour VisionColour Vision
• 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)
April 19, 2023 Retina 59
Colour VisionColour Vision
• 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)
April 19, 2023 Retina 60
Colour VisionColour Vision
• 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)
April 19, 2023 Retina 61
Colour VisionColour Vision
• 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)
April 19, 2023 Retina 62
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
April 19, 2023 Retina 63
Colour BlindnessColour Blindness
• Protanopia– Red cone non functioning
• Deuteranopia– Green cone non functioning
• Tritanopia– Blue cone non functioning
April 19, 2023 Retina 64
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
April 19, 2023 Retina 65
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