Lecture 20 - Vision 2 Flashcards
Eye function and its components
The eye detects light and processes visual information
Creates visual perceptions and guides behaviour
The eye consists of two basic components
- Optical component – collects and focuses light onto the plane of the retina
- Neural component – converts light energy into patterned changes of membrane potential that the brain can decode to create visual perceptions
The retina is part of which components
the neural component
Vitreous humour and retina
Vitreous humour is in front and light goes through all the layers and goes to the back of the retina
Choroid and retina
Choroid - pigmented epithelium: heavily pigmented cells with black pigment to absorb all of the light that comes into the retina
Photoreceptors of the eyes =
rods and cones
Cells of the retina
Horizontal cell, bipolar cell, amacrine cell, ganglion cells, rods and cones
Horizontal cell
Interneuron
Dendrites run horizontally along the top of the photoreceptors
Crucial in mediating the input between photoreceptors and bipolar cells
Bipolar cell
Interneurons
Bipolar because 2 connections
Amacrine cell
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
Ganglion cells
Ganglion cell (axons become the optic nerve)
Flow of information in the retina cells
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
Light sensitive cells (photoreceptors) are
furthest from incoming light
Information flows between photoreceptors and ganglion cells via
interneurons
Rods
120 million per retina
Function in low light (night vision)
Not colour sensitive (not responsive to colour)
Responsible for monochromatic, dark adapted vision
Cones
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
Structure of photoreceptors
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
Outer segment of rods
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
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
Phototransduction is effectively
light to action potentials
What makes photoreceptors light sensitive?
the presence of photopigments
Photo pigmment has two components
A membrane spanning protein called an “opsin”
A chromophore called Retinal, a Vitamin A derivative
Opsin in rods
Rods have Rhodopsin
Opsin in cones
Cones have either S(blue), M(green), or L(red) Photopsin
Short wavelength, medium wavelength and long wavelength photopsin
Retinal in rods and cones
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
There is more or less ______ being released depending on the amount of light
neurotransmitter
In the dark what is the phototransduction pathway?
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
In the light what is the phototransduction pathway?
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
Colour vision
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
Colour blindness
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
At night….
At night, intracellular cGMP levels are high BECAUSE cGMP-gated channels are open (both statements are correct and not causally related)
Retinal processing
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
Receptive field
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
Ganglion cell receptive field
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
On centre field
activated by light falling on the centre
off center field
activated by light falling on the surriound (annular illumination_
Visual acuity
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
Where is the highest visual acuity?
Fovea
Low convergence ganglion cells
Low convergence ganglion cells
Small receptive fields, high acuity
Cones, fovea
High convergence ganglion cells
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
Larger the dendritic tree
the larger the receptive field therefore the lower the visual acuity
Define in one sentence a retinal ganglion cells receptive field
Retinal ganglion cells receive input from a number of photoreceptors and interneurons
known as the ganglion cell’s receptive field
Receptive field comprises of the
The receptive field comprises the centre and the surround
Differential light patterns falling on the centre and the surround excite the ganglion cell
The bigger the receptive field …
the lower the visual acuity
cGMP production from GMP catalysed by
guanylyl cyclase - maintains high intracellular cGMP levels in the dark
Blind spot
Optic disk is where the retinal ganglion cells leave the retina so have no visual activity i.e. it is our blind spot
