Lecture 6- Vision I: The Retina

Question 1

Describe the anatomy of the eye:

Answer 1

-cornea right at the front= clear surface, the light passes through -lens bends the light and this is focused on the retina -retina= 5/6 of the back of the eyeball -eyeball like an onion= outer layer= there for keeping strength, -middle layer= lot of blood vessels for nutrition -and the inside layer= retina= constitutes all of the nerves that allow us to see, -optic nerve= contains the axons of the ganglion cells as they go off to the brain

Question 2

What does the retina look like normally?

Answer 2

-when you look into it with an opthalamoscope(expensive torch) -whitish thing= where the axons go into the brain pinky thing= fovea (also called macula)

Question 3

What does the fovea (macula) do?

Answer 3

-this is the most important of the eye, allows you to see centrally, allows you to read, see faces, and see dead centre, all the rest is for peripheral

Question 4

What are the optical factors affecting visual acuity?

Answer 4

1. Pupil size 2.Clarity of optical media- cataracts, corneal opacities 3.Refractive errors-myopia, hypermetropia, astigmatism and presbyopia

Question 5

How does pupil size affect visual acuity?

Answer 5

-the smaller the pupil size, the clearer the vision -if a large pupil can’t see as well at night

Question 6

How does the clarity of optical media affect visual acuity?

Answer 6

-how the light passes through the eye: if any of the structures that the light must pass through are effected (scarred) then this will effect the baility to see

Question 7

How do refractive errors affect visual acuity?

Answer 7

-if the size of the eyeball is not the right length= then the focusing doesn’t work and vision= blurry -if the eyeball is mismatch (size-wise) = refractive error

Question 8

What is myopia?

Answer 8

-shortsighted= the eyeball is too long and the light focuses it is too early, in front of the back of the eye= shortsigted

Question 9

What is hypermetropia?

Answer 9

-Hypermetropia= the eyeball is too small=the light is focused behind the back of the eye, not focused as much on the retina

Question 10

What is astigmatism?

Answer 10

-Astigmatism:surface of the eye is squashed a bit (like rugby ball) as the light goes through the cornea it creates two focuses, one might be clear in one domain and the other domain it’s completely blurry

Question 11

What is presbyopia?

Answer 11

-feel like arms not long enough= the process that allows us to read, to focus on sth closer becomes impaired and eventually may need glasses

Question 12

Why are the photoreceptors of the retina at the bottom?

Answer 12

cross section through the back of the eyeball -series of neuron layers -follow the light- it has to go through all the layers of the retina before it hits the photoreceptors -they are the light detectors, they respond to light and create the neural signal that allows us to see -why does it seem back to front? -because of the cells that sit underneath the retina: retinal pigmental epithelial cells, very important to keep the retinal cells alive, if it were switched around then the pigmented cells wouldn’t let light through= too dense -the retinal pigmented cells= provide nutrition to retinal cells

Question 13

What are the characteristics of rods?

Answer 13

-night vision (scotopic) -very sensitive -only one type -black and white, no colour vision -100million =95% of all photoreceptors -absent from the fovea

Question 14

What are the characteristics of cones?

Answer 14

-day vision (photopic) -less sensitive -three types (red, blue and green) -allow colour vision -5 million (only about 5% of the photoreceptors) -densest in the fovea

Question 15

What is the ratio of rods to cones?

Answer 15

20:1

Question 16

How do we optimize our vision?

Answer 16

• to resolve fine detail: at the fovea the density of cones and rods changes -green line: cones are extremely dense in the middle of the fovea -whereas rods are the lowest at the middle of the fovea -ability to see fine detail and acuity during the day is completely determined by the number of cones we have in our retina -0 is the fovea

Question 17

What is the structure of the photoreceptors?

Answer 17

• rods have a cell body, axon going off to the synapse and the other side of the axon going deep into the eyeball and it has the region= called outer segment (the red hats in the pic), this contains the proteins that are sensitive to light, in rods photopigment in there called Rhodopsin -cones have cone-opsin

Question 18

What are the opsins and where are they?

Answer 18

-Opsins are proteins, that bind to vitamin A (technical name for vit A= Retinal) -in the outer segment of the cone and rod,

Question 19

What is phototransduction?

Answer 19

-process by which photoreceptors respond to light -retinal is normally present in the opsins in a kinked form= cis -then light enters and the molecule becomes straight= trans -this conformational process in the retinal stimulates the response to that

Question 20

What happens to the photoreceptors in terms of membrane potential when they are hit by light?

Answer 20

-in terms of neurons, photoreceptors hyperpolarised by light, they are not like othe rneurons in the brain, (like when you touch sth the sensory neuron depolarised), in retina it is the reverse HYPERPOLARISATION

Question 21

Do neurons of the retina fire action potentials?

Answer 21

-neurons of the retina with the exception of the ganglion cells do not fire action potential, the way they respond to a stimulus is to change their membrane potential and based on how that changes determines how much neurotransmitter gets released -so if the potential goes down= less glutamate if up= more glutamate released= it is not an all or nothing reaction like in other neurons -continuous release of neurotransmitter -goes up if depolarised, down if hyperpolarised -respond with graded potentials

Question 22

What neurotransmitter is used by the photoreceptors?

Answer 22

-glutamate -the main excitatory neurotransmitter of the brain

Question 23

How does a photoreceptor function in the dark?

Answer 23

-in the dark the photoreceptors will be depolarised -in the outer segment we have sodium channels,they are always open in the dark, and they are gated by cyclic GMP, -Na channels held open by cGMP binding to those channels, -in the dark constant stream of Na into the cell and then out and in… and this constant flow of sodium causes the depolarisation that we see in the dark

Question 24

What happens to a photoreceptor when it is hit by light?

Answer 24

light: -Na channels close, no influx of sodium, the cell is hyperpolarised, -light hits the receptor and cascade is initiated breaks down the cGMP -light hits the photoreceptor, and activates rhodopsin (via retinal) initiates a cascade activates a G protein= G protein cascade, that leads to closure of Na channels due to breakdown of cGMP, the enzyme that does that is called:PDE (phosphodiestarase) -the cascade: Rhodopsin-Trasducin-PDE-breaks down cGMP

Question 25

How is the information carried through the retina?

Answer 25

-retina has a circuit that goes through the retina, involves photoreceptors, second order neurons(bipolar cells),and they synapse on third order neurons (ganglion cells= the output neurons of the retina), what the brain sees is what the ganglion cells tell it -we need complex task to be performed by the visual system (colour, motion etc.) thus there has to be modulation of the pathway = done in two places 1. outer retina=horizontal cells modify the singal as it passes through 2.Amacrine cells= inner retina main circuit is the PH-BC-GC

Question 26

What are the characteristics of the bipolar cells?

Answer 26

-bipolar= have two processes that extend from the cell body -found in the second layer of the retina= the inner nuclear layer the blue dots are the cell bodies, have dendrites connecting to the photoreceptors and receive info from there and an axon terminal that communicates with ganglion cells -many different types, -9 connect to cones(cone-bipolar cells), and 1 type connects to rods (rod-bipolar cells) -important for visual acuity, spatial vision and colour vision

Question 27

What are the general characteristics of the ganglion cells?

Answer 27

about 22 different types - some respond to light falling on the retina by depolarising= ON ganglion cells -others reeact by hyperpolarising= OFF ganglion cells =M-motion ganglion cells, carry info to brain about that =P-colour and fine detail info sent to brain (very numerous) -release glutamate - -AP! only cell type that fire AP in the retina

Question 28

What are the properties of receptive fields of ganglion cells?

Answer 28

-the ganglion cell reaction to light on the retina:- pink pic’= recpetive field of one ganglion cell,put electrode in and shine light on retina, in central part (b)= lot of AP, as you’d expect, when you move to the peripheral part of the receptive field= a-fewer APs, d- different again, -the ganglion cells do respond to light on the receptive field but it depends where you shine the light in the receptive field, the central response is the opposite to the response in the periphery normally sematosensory= touch sth and the neuron will depolarise, doesn’t matter where in the receptive field you touch it

Question 29

What is a receptive field of a ganglion cell?

Answer 29

the area of retina that when stimulated with light changes the cell’s membrane

Question 30

How can ganglion cell response change over time?

Answer 30

-Transient: sudden burst of APs at the onset of the stimulus (ie transient) -Sustained: continuous APs during stimulation

Question 31

What sort of information do ganglion cells convey?

Answer 31

-ganglion cell response can ben increase or decrease in firing, and sustained or transient response -20 different types and each of these are breaking down what you see into one piece of information, edges, black and white edges, movement, colour of the object -cells that decosntruct what we see -in mammalian system= at least 13 different channels of information -the brain has to put it all back together -the visual information is passed to higher centres in parallel -the ganglion cells are sending the brain information about the edges (the picture of engine= only edges!) have on and off ganglion cells, send the same info but rverse (black on white and the otehr: white on black) -they are breaking down what we see to a crude level

Question 32

What are the characteristics of the horizontal cells?

Answer 32

-how do you get the breakdown of the info= that is where the lateral cells are really important(the breakdown shown in ganglion cells) -horizontal cells extend horizontally in the retina and they act together across the retina, receive input from photoreceptors and output to photoreceptors -receive from some photoreceptor and talking back to other photoreceptors -inhibitory neurons, -create the surround property of the ganglion cells (where central response opposite to peripheral)they do it by talking back to photoreceptors -respond to light by hyperpolarising -use inhibitory neurotransmitter GABA

Question 33

What are the characteristics of the amacrine cells?

Answer 33

-inner retina modifiers -they only have dendrites, receive input from bipolar cells and other amcrine cells and feed of to bipolar cells and ganglion cells -modulating info in in inner retina -many different types -axonless cells -important for lateral inhibition -for the most part considered inhibitory cells (release inhibitory neurotransmitters= glycine and GABA)

Question 34

What’s wrong with Joan?

Answer 34

-45 yrs old, trouble seeing at night, trips over things, has had many car accidents, can read OK -joan’retina appears:black clot= in the peripheral retina= indicates disease of retina, this black stuff is pigment that migrated from underneath the retina into the retina, her macula is fine (that is why she doesn’t complain and thinks she can see normally), she has problem with everything in the periphery, she has tunnel vision but is not aware -disease where her rods have died. -inherited - rhodopsin coded abnormally, there ar 150 mutations in rhodposin alone -genetic defect in rhodopsin involved in phototransduction -tunnel vision -this means they can’t respond to light and die, they die from the periphery into the center, gradually -ongoing disease, causes complete blindness -target of bionic eye= could fix it. to make ganglion cells respond to light -this is the device= on the left, 24 elctrodes so like 24 photoreceptors, so very crude