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neurophysiology > Lecture 20 - Vision 2 > Flashcards

Lecture 20 - Vision 2 Flashcards

1
Q

Eye function and its components

A

The eye detects light and processes visual information
Creates visual perceptions and guides behaviour

The eye consists of two basic components

  1. Optical component – collects and focuses light onto the plane of the retina
  2. Neural component – converts light energy into patterned changes of membrane potential that the brain can decode to create visual perceptions
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2
Q

The retina is part of which components

A

the neural component

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3
Q

Vitreous humour and retina

A

Vitreous humour is in front and light goes through all the layers and goes to the back of the retina

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4
Q

Choroid and retina

A

Choroid - pigmented epithelium: heavily pigmented cells with black pigment to absorb all of the light that comes into the retina

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5
Q

Photoreceptors of the eyes =

A

rods and cones

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6
Q

Cells of the retina

A

Horizontal cell, bipolar cell, amacrine cell, ganglion cells, rods and cones

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7
Q

Horizontal cell

A

Interneuron
Dendrites run horizontally along the top of the photoreceptors
Crucial in mediating the input between photoreceptors and bipolar cells

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8
Q

Bipolar cell

A

Interneurons

Bipolar because 2 connections

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9
Q

Amacrine cell

A

Interneurons
Dendrites are playing with the bipolar cells and the retinal ganglion cells
Mediate/modulate the input between the bipolar cells and the retinal ganglion cells

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10
Q

Ganglion cells

A

Ganglion cell (axons become the optic nerve)

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11
Q

Flow of information in the retina cells

A

Information coming from the photoreceptors and going through the bipolar cells which is being modulated by the horizontal cells and information is then going from the bipolar cells to the retinal ganglion cells and it is being modified by the amacrine cells. These cells do not communicate through action potentials, they instead communicate with each other with graded changes in membrane potential (local change in excitability, local release of neurotransmitter on synapse)

LOOK AT PRELAB AND EDIT THIS WITH WHAT THE ONLY ONE IS THAT COMMUNICATES WITH APS I THINK IT IS JUST THE GANGLION CELLS

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12
Q

Light sensitive cells (photoreceptors) are

A

furthest from incoming light

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13
Q

Information flows between photoreceptors and ganglion cells via

A

interneurons

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14
Q

Rods

A

120 million per retina
Function in low light (night vision)
Not colour sensitive (not responsive to colour)
Responsible for monochromatic, dark adapted vision

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15
Q

Cones

A

8 million per retina
Require relatively high light levels (day and twilight vision)
Responsive to colour
Have photopigments
3 types, sensitive to red, green and blue light

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16
Q

Structure of photoreceptors

A

Photopigments sit in the outer segments of our rods and cones
Outer segment of rods - rods have discs of membrane, stacked on top of each other, increases the surface area of the membrane which is what makes it so sensitive to light which is why it can sense these low light levels
Outer segment of cones - smaller, no free floating discs like in rods, have infoldings/invaginations of the membrane so have a much smaller surface area so require much more light to function

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17
Q

Outer segment of rods

A

Outer segment of rods - rods have discs of membrane, stacked on top of each other, increases the surface area of the membrane which is what makes it so sensitive to light which is why it can sense these low light levels

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18
Q

Outer segment of cones

A

Outer segment of cones - smaller, no free floating discs like in rods, have infoldings/invaginations of the membrane so have a much smaller surface area so require much more light to function

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19
Q

Phototransduction is effectively

A

light to action potentials

20
Q

What makes photoreceptors light sensitive?

A

the presence of photopigments

21
Q

Photo pigmment has two components

A

A membrane spanning protein called an “opsin”

A chromophore called Retinal, a Vitamin A derivative

22
Q

Opsin in rods

A

Rods have Rhodopsin

23
Q

Opsin in cones

A

Cones have either S(blue), M(green), or L(red) Photopsin

Short wavelength, medium wavelength and long wavelength photopsin

24
Q

Retinal in rods and cones

A 
A chromophore called Retinal, a Vitamin A derivative
Night blindness (eyes unable to adapt to low-light conditions) (when deficient) 
Eat carrots to prevent deficiency
25
Q

There is more or less ______ being released depending on the amount of light

A

neurotransmitter

26
Q

In the dark what is the phototransduction pathway?

A

In absence of light, Retinal is non-activated (inactive11-cis isoform)
Intracelluar [cGMP] is high
cGMP-gated channels open
Much Na influx (dark current)

Photoreceptor depolarised (~ -35mV)
Depolarisation causes opening of voltage gated Ca2+ channels and Ca2+ comes in and causes the release of the neurotransmitter glutamate

Lots of glutamate released onto bipolar cells

27
Q

In the light what is the phototransduction pathway?

A

Light energy (photons)

Retinal changed to active all-trans isoform
All of our properties of vision are solely down to the conversion of retinal from 11-cis to trans, it is the only light dependent step in the whole process, everything else happens because of this change

Trans-retinal activates a G protein (transducin)

Transducin activates cGMP phosphodiesterase, breaking down cGMP
Breaking down cGMP into GMP which reduces intracellular concentration of cGMP so gated channels can no longer open, not Na+ coming in so leads to hyper polarisation of the photoreceptor therefore Ca2+ channels do not open so Ca2+ does come in so no release of glutamate

Less cGMP, cGMP-gated channels close

Photoreceptor hyperpolarised (~ -60mV)

Less glutamate release

28
Q

Colour vision

A

Photoreceptors contain photopigments
Cones each have one of 3 types of photopsin, most sensitive to either short wavelengths (S-photopsin, blue), medium wavelengths (M-photopsin, green), or long wavelengths (L- photopsin, red)
Light of specific wavelengths is reflected from coloured objects.
Perception of colour created by relative activation of 3 cone types.
Under optimal conditions, humans can perceive about a million colours.
Perception of yellow for example = get relative activation of our green and red cones at different levels and this relative activation leads to our perception of yellow

29
Q

Colour blindness

A

Can be inherited (congenital) … or acquired (due to disease)

Diseases that damage the optic nerve or the retina

Glaucoma, increase in pressure in the anterior chamber (aqueous humour)
Increasing pressure can lead to damage to the optic nerve

Diabetes
Sugar build up can damage the retina and lead to colour blindness

Alzheimer’s and Parkinson’s diseases

Congenital (inherited) forms affect 8 % of males and 0.5% of females (although varies with ethnicities)
Genes encoding production of M & L opsins (ie green & red sensitivity) are on the X chromosome
Recessive X chromosome disorder
Red and green colour blindness is the most common as a result of their sensitivities being coded for on the X chromosomes
A variety of forms exist, not just the relatively common R/G form

30
Q

At night….

A

At night, intracellular cGMP levels are high BECAUSE cGMP-gated channels are open (both statements are correct and not causally related)

31
Q

Retinal processing

A

Once light is detected, get change in neurotransmitter release which can release onto bipolar cells and horizontal cells
Change in neurotransmitters can affect a huge network of interneurons and this is the first stage in retinal processing

Info from photoreceptors and interneurons is combined, so that the output from ganglion cells depend critically on the spatial and temporal pattern of light stimulation on the retina.
Output from retinal ganglion cells depends on the exact pattern of light and movement of light that falls on the retina in the parts of the retina where these ganglion cells are connected

The inputs to each ganglion cell arise from neighbouring photoreceptors in a circumscribed (limited) area of the retina, the cell’s RECEPTIVE FIELD
Ganglion cells are connected to a series of interneurons and through these interneurons to the photoreceptors and it is the photoreceptors in a specific part of the retina that influence the excitability of that ganglion cell that sits in front of them
Think of it like this…. One retinal ganglion cell is responsible for one pixel and when you put all the retinal ganglion cells together (put all the pixels together) you get an image

32
Q

Receptive field

A

The inputs to each ganglion cell arise from neighbouring photoreceptors in a circumscribed (limited) area of the retina, the cell’s RECEPTIVE FIELD
Ganglion cells are connected to a series of interneurons and through these interneurons to the photoreceptors and it is the photoreceptors in a specific part of the retina that influence the excitability of that ganglion cell that sits in front of them
Think of it like this…. One retinal ganglion cell is responsible for one pixel and when you put all the retinal ganglion cells together (put all the pixels together) you get an image

33
Q

Ganglion cell receptive field

A

Also called centre surround receptive fields

Are roughly circular
Behind each retinal ganglion cell is a group of interneurons and photoreceptors that fall in a pretty circular manner around it

Are divided into two parts
A circular central zone (the “centre”)
An annulus around the centre (the “surround”)

Ganglion cells respond optimally to differential illumination of the centre and the surround (they are contrast detectors)
Neither respond to diffuse illumination

Small receptive fields = can differentiate

34
Q

On centre field

A

activated by light falling on the centre

35
Q

off center field

A

activated by light falling on the surriound (annular illumination_

36
Q

Visual acuity

A

Receptive fields correlate with visual acuity

2 point discrimination in visual terms

Limited by resolving power of retina
Highest acuity in fovea (small receptive fields)
Fovea = just cone cells, no rods, best vision here
Retinal ganglion cells at the fovea have very small receptive fields

Cones 2μm diameter (therefore poor acuity in low light)

Acuity lower away from fovea which means the periphery of the retina (larger receptive fields)
Therefore cannot discriminate between points of light as well

Low convergence ganglion cells
Small receptive fields, high acuity
Cones, fovea

High convergence ganglion cells
Large receptive fields, low acuity
Rods and cones, periphery
Note = Larger the dendritic tree, larger the receptive field, lower the visual acuity

37
Q

Where is the highest visual acuity?

A

Fovea

38
Q

Low convergence ganglion cells

A

Low convergence ganglion cells
Small receptive fields, high acuity
Cones, fovea

39
Q

High convergence ganglion cells

A

High convergence ganglion cells
Large receptive fields, low acuity
Rods and cones, periphery
Note = Larger the dendritic tree, larger the receptive field, lower the visual acuity

40
Q

Larger the dendritic tree

A

the larger the receptive field therefore the lower the visual acuity

41
Q

Define in one sentence a retinal ganglion cells receptive field

A

Retinal ganglion cells receive input from a number of photoreceptors and interneurons
known as the ganglion cell’s receptive field

42
Q

Receptive field comprises of the

A

The receptive field comprises the centre and the surround

Differential light patterns falling on the centre and the surround excite the ganglion cell

43
Q

The bigger the receptive field …

A

the lower the visual acuity

44
Q

cGMP production from GMP catalysed by

A

guanylyl cyclase - maintains high intracellular cGMP levels in the dark

45
Q

Blind spot

A

Optic disk is where the retinal ganglion cells leave the retina so have no visual activity i.e. it is our blind spot

neurophysiology (29 decks)

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