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Physiology of the Retina

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Presentation on theme: "Physiology of the Retina"— Presentation transcript:

1 Physiology of the Retina
Receptor and neural function of retina April 19, 2017 Retina

2 Functional 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 April 19, 2017 Retina

3 Structure 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, 2017 Retina

4 Functional 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, 2017 Retina

5 Pigment epithelium Rods and Cones Horizontal cells Bipolar cells
Amacrine cells Ganglion cells Optic nerve April 19, 2017 Retina

6 Distribution 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, 2017 Retina

7 Distribution 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, 2017 Retina

8 Concentration of cones
Concentration of rods Fovea Concentration of cones Rods Cones Nasal Temporal April 19, 2017 Retina Fovea

9 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, 2017 Retina

10 The 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 April 19, 2017 Retina

11 Photochemistry of vision
Light sensitive photochemical in rods –Rhodopsin Cones-colour pigment Differs in spectral sensitivity April 19, 2017 Retina

12 Structure 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 Outer segment Inner segment Rods April 19, 2017 Retina

13 Structure 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 Outer segment Inner segment Rods April 19, 2017 Retina

14 Structure of Receptors
Cones have thick inner segment and conical outer segment The saccules formed in outer segment By in-folding of the cell membrane Outer segment Inner segment Cones April 19, 2017 Retina

15 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, 2017 Retina

16 Photo Pigments Rods have rhodopsin Cones have iodopsin
Scotopsin + retinal Cones have iodopsin Photopsin + retinal April 19, 2017 Retina

17 Formation and Breakdown of Rhodopsin
Retinol (Vit. A1) Dark phase Dehydrogenase All – trans- retinal Isomerase 11- cis- retinal Light phase Scotopsin Rhodopsin April 19, 2017 Retina

18 Excitation of Receptor
Dark Phase 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+ - + K+ Na+ Na+ Neurotransmitter April 19, 2017 Retina

19 Excitation of Receptor
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+ - + K+ Na+ Na+ Neurotransmitter April 19, 2017 Retina

20 Excitation of Receptor
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+ - + K+ Na+ Na+ Neurotransmitter April 19, 2017 Retina

21 Excitation of Receptor
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+ - + K+ Na+ Na+ Neurotransmitter April 19, 2017 Retina

22 Excitation of Receptors
Light ray Rod In the dark Release of neurotransmitter is greatest Leads to inhibition of bipolar cells Ganglion cells Not excited by bipolar cells Horizontal cell NT - NT - Amacrine cell NT + G cell Optic nerve April 19, 2017 Retina

23 Excitation of Receptors
Light ray Rod In the presence of light Rods and cones hyperpolarize Decrease release of transmitter Inhibition of the bipolar cells Horizontal cell NT - NT - Amacrine cell NT + G cell Optic nerve April 19, 2017 Retina

24 Excitation of Receptors
Light ray Rod Bipolar cell  release of transmitter Excitation of ganglion cells Increase in AP conducted Horizontal cell NT - NT - Amacrine cell NT + G cell Optic nerve April 19, 2017 Retina

25 Excitation of Receptors
Light ray Rod The horizontal cell Depolarized by light Release inhibitory neurotransmitter Lateral inhibition Horizontal cell NT - NT - Amacrine cell NT + G cell Optic nerve April 19, 2017 Retina

26 Dark & Light Adaptation
The sensitivity of photoreceptors Depends on conc of photo pigments A slight change causes Great change in retinal sensitivity April 19, 2017 Retina

27 Light 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, 2017 Retina

28 Dark Adaptation In the dark The conc of photo pigment Photo receptor
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, 2017 Retina

29 Dark & Light Adaptation
Dark adaptation Retinal sensitivity Light adaptation Exposure to light or darkness (minutes) April 19, 2017 Retina

30 Dark & light adaptation
Other mechanisms Change in pupilary size Neural adaptation Bipolar cells Horizontal cells Amacrine cells Ganglionic cells Initial increased light intensity; all become intense, then decrease with time Retina

31 Ganglion Cells Each retina has Hence many rods & cones
120 million rods 6 million cones 1.2 million ganglion cells Hence many rods & cones Converge on each ganglion cell April 19, 2017 Retina

32 Ganglion Cells At fovea centralis At periphery
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, 2017 Retina

33 Ganglion Cells Signals from rods Summate to  sensitivity
 Intensity of stimulation to peripheral ganglion cells April 19, 2017 Retina

34 Types of Ganglion Cells
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, 2017 Retina

35 Types of Ganglion Cells
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) April 19, 2017 Retina

36 Types 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, 2017 Retina

37 Pathway to Cortex Retina Optic chiasma Optic tract
optic nerve Optic chiasma Cross over Optic tract optic nerve –nasal side & optic nerve temporal side Lateral geniculate body (LGB) of thalamus Optic radiation Primary Visual cortex April 19, 2017 Retina

38 Temporal Nasal Retina rt eye Optic nerve Optic chiasma Optic tract Lateral geniculate body Optic radiation Visual cortex April 19, 2017 Retina

39 Pathway 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, 2017 Retina

40 Pathway 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, 2017 Retina

41 Function of dorsal lateral geniculate nucleus of thalamus
Relay visual information to the brain From optic tract to visual cortex 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, 2017 Retina

42 Organization and function of the 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, 2017 Retina

43 Lateral Geniculate Body
Has six layers Layer 1 & 2 Have large cell Magnocellular “M” cells Layer 3,4,5,6 Have small cells Parvocellular “P” cells 6 5 4 2 3 1 April 19, 2017 Retina

44 Lateral Geniculate Body
Layers 1, 4, 6 Receive inputs from Contra lateral side Layers 2, 3, 5 Ipsilateral side 6 5 4 2 3 1 April 19, 2017 Retina

45 Lateral Geniculate Body
Visual cortex Large ganglion cells Add responses from different cones Project to magnocellular portion of LGB Carry signals for detection of Movements, depth, flicker Retina LGB Large ganglion cells Project to layer 1,2 Small ganglion cells Project to layer 3, 4, 5, 6 April 19, 2017 Retina

46 Lateral Geniculate Body
Visual cortex Small ganglion cells Project to parvocellular portion of LGB Carryes signals for Color vision Texture, shape of objects Fine details Retina LGB Large ganglion cells Project to layer 1,2 Small ganglion cells Project to layer 3, 4, 5, 6 April 19, 2017 Retina

47 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, 2017 Retina

48 Lesions of Optic Pathways
Lesion of optic tract causes Blindness in half of visual fields Homonymous hemianopsia April 19, 2017 Retina

49 Temporal Nasal Lesion at 1 Retina rt eye Blindness in the eye 2 Optic nerve 1 Optic chiasma Lesion at 2 bitemporal hemianopsia 3 Optic tract Lateral geniculate body Lesion at 3 homonymous hemianopsia Optic radiation Visual cortex April 19, 2017 Retina

50 Colour Vision Primary colours Human eye can detect Blue, green, red
All gradation of calours when Red, blue & green are Mixed in different combination April 19, 2017 Retina

51 Colour 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, 2017 Retina

52 Colour Vision 3 types of cones Blue absorbing cone
Green absorbing cone Red absorbing cone April 19, 2017 Retina

53 Colour Vision 3 types of colour photo-pigments Cyanolabe Chrolabe
Blue sensitive photo-pigment Chrolabe Green sensitive photo-pigment Erythrolabe Red sensitive photo-pigment April 19, 2017 Retina

54 Colour Vision The three types of cones The blue absorbing cone
green (535) Blue (445) Red (575) The three types of cones The blue absorbing cone Wave length from 370 – 510 nm Maximum at 445 nm The green absorbing cone 100 75 % of Light absorption 50 25 400 500 600 700 Wave length () nm April 19, 2017 Retina

55 Colour Vision The green absorbing cone The red absorbing cone
Blue (445) Red (575) The green absorbing cone Wave length from 450 – 630 nm Maximum at 535 nm The red absorbing cone 470 – 700 nm Maximum at 575 nm 100 75 % of Light absorption 50 25 400 500 600 700 Wave length () nm April 19, 2017 Retina

56 Colour Vision Sensation of given colour determined by
green (535) Blue (445) Red (575) Sensation of given colour determined by Relative frequency of impulses From each of the three types of cones 100 75 % of Light absorption 50 25 400 500 600 700 Wave length () nm April 19, 2017 Retina

57 Colour Vision A light in the red – green spectral band
Blue (445) Red (575) 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 100 75 % of Light absorption 50 25 400 500 600 700 Wave length () nm April 19, 2017 Retina

58 Colour Vision A light in the red – green spectral band Stimulate
Blue (445) Red (575) 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 100 75 % of Light absorption 50 25 400 500 600 700 Wave length () nm April 19, 2017 Retina

59 Colour Vision A light in the red – green spectral band Stimulate
Blue (445) Red (575) A light in the red – green spectral band At wavelength of 550 nm Stimulate Red cone 90% Green cone 90% Blue cone 0% 100 75 % of Light absorption 50 25 400 500 600 700 Wave length () nm April 19, 2017 Retina

60 Colour Vision Both the red cone and the green cone
Blue (445) Red (575) 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 100 75 % of Light absorption 50 25 400 500 600 700 Wave length () nm April 19, 2017 Retina

61 Colour Vision The sensation of any colour Determined by
green (535) Blue (445) Red (575) The sensation of any colour Determined by The relative frequency of impulses Reaching the brain from Each of the 3 types of cones 100 75 % of Light absorption 50 25 400 500 600 700 Wave length () nm April 19, 2017 Retina

62 Colour Blindness Normal colour vision Loss of any cone function
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, 2017 Retina

63 Colour Blindness Protanopia Deuteranopia Tritanopia
Red cone non functioning Deuteranopia Green cone non functioning Tritanopia Blue cone non functioning April 19, 2017 Retina

64 Inheritance of Colour Blindness
Genetically transmitted Sex linked, recessive X chromosome linked Will not appear as long as Another X chromosome carries the gene April 19, 2017 Retina

65 Inheritance of Colour Blindness
Normal XY XX Carrier Normal XX XX XY XY XX XY Carrier Carrier Normal Normal XX XY XX XY Carrier Colour blind Normal Normal April 19, 2017 Retina

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