Vision Flashcards
How does the eye maintain a stable shape?
Our eye has got a non-stretchy outside layer
At the front, the collagen fibers and cells that make up that outer layer, align in a way that makes that structure appear transparent
That transparent bit is the cornea
Intra-ocular pressure is created by the production of aqueous humor, which is produced by the ciliary body, flows outwards and is absorbed at the angle of the eye
Non-stretchy sclera plus intra-ocular pressure keep eye rigid, back surface smooth and stable, and distances between optics and retina correct
Behind the lens you have a jelly like structure known as the vitreous humour which is hydrated by the aqueous humour
In older age the vitreous humour starts to clump and shrink leaving watery patches
What are the two structures in the eye that bend the light rays?
The cornea:
As the light passes from the air to the water via this curved interface the light rays are bent inwards
Behind the cornea you have the lens:
A structure which is transparent and can change shape
By changing shape it can change the focus of the eye
The lens is suspended by a ring of suspensory ligaments from the ciliary body and that can change the shape of the lens because it has a ring of muscles
When the ring of muscle contracts, its diameter becomes smaller and when that happens the lens becomes fatter
That is how you adjust the focus of the lens for close vision and vice versa
The iris is a ring of muscle that makes up the coloured part of the eye
The function of the iris is to produce this aperture in the middle of the pupil
The retina adjusts your eyes for brightness
You pupil maintains the smallest aperture it can for the illumination condition; the smaller the aperture the better the focus
What is the purpose of the iris?
The eye captures an image of the world it needs the ability to focus the image
Iris controls how much light enters the eye via the pupil
If light passes through the edges of the iris the rays wont come to a single point so the image wont be focused
Cornea is primarily responsible for focusing the light rays
Lens provides additional, variable “fine” focus
What is the primary visual pathway?
Those ganglion cells axons projects down the optic nerve to the optic chiasm
At the optic chiasm the ones at the nasal half of the retina swap sides while the ones that came from the temporal half of the retina, stay on the same side
They project back to the lateral geniculate nucleus which is the specific nucleus in the thalamus that serves the visual system
Cells in the LGN send their axons through a region of white matter known as the optic radiation back to the occipital cortex where you find the primary visual area
Some of these axons make branches which head down towards the brainstem where they innervate a number of nuclei involved in subconscious things like subconscious control of your eye movements
What is the structure of the cone photoreceptor?
It has an inner segment, which is the bit where the nucleus and mitochondria etc. are located
It has an ‘axon’ but its more a neurite- it doesn’t fire action potentials
It has a synaptic terminal which releases glutamate depending on the depolarisation which here is a graded potential produced by the transduction operators
The transduction operators is up here in the outer segment
The outer segment is a bag containing tightly packed layers of phospholipid layers
The layers are holding the chromophore, which is light-sensitive and the layers are holding it perpendicular to the light path
What is the resting membrane potential like in cones?
All the nerve cells are leaking potassium all the time and that produces a negative internal potential
In these cells the resting membrane potential is likely -45mV, seeing how the normal is -60 that means they are depolarised even at rest
This is brought about because in the outer segments there are sodium channels which are open by default
If the light striking the outer segment gets brighter, some of those channels close which allows the cell to get more negative, so it hyperpolarises
This hyperpolarisation prevents some of the release of glutamate
And vice versa
SO LIGHT IS HYPERPOLARISING
How does the initiation of the light response happen?
Transduction- initiation of the light response
The sodium channel is held open by an intracellular messenger called cyclic GMP
So long as there’s enough cGMP around that channel is going to be open and leaking sodium
Photopigment is made up of opsin which is a protein and a molecule called retinal
The retinal is the chromophore- the light sensitive bit
When light strikes the molecule it causes the cis bond in the retinal to rupture and when it reforms it takes on the more stable trans configuration
So now opsin would be linked to trans retinal as an activates photopigment
How is the transduction biochemical cascade amplified?
This activated photopigment activates a G protein (or multiple)
Each of those G proteins activate an enzyme which destroy cGMP which causes the remaining cGMP to diffuse allowing it to move away from the sodium channels
How is the transduction response terminated?
Termination is done by a whole cascade of biochemicals including the removal of transretinal, which stops the cascade and allows a second enzyme to replenish the cGMP levels
Another molecule of retinal is attached
What is the peripheral retina like? (i.e. why is it blurry)
In peripheral retina you have huge gaps in the sampling array
This converges
The part of the visual world that focused on this little bit of the retina that is directly linked to that ganglion cell is called the ganglion cells receptive field centre
The bigger the receptor field is the less fine detail you will see because of the gaps and convergence
In peripheral retina the light is scattered before it hits so there’s no points of smaller gaps in sampling array
Why is central vision so clear, structure-wise?
The central retina is centred around the fovea centralis
The retina has the photoreceptors at the top and the ganglion cells down
Right in the very centre, the bit you point at when you look at something is called the foveal pit
This is a region where the photoreceptors are uncovered so there’s no retina between it and the light path which is why there’s not image blur and excellent sampling array
The cones are special ultra thin cones so more can be fit in
Only red and green cones because those are associated with the circuitry that pulls out fine detail
The ganglion cells that serve this part get input from one cone each so there is no covergence
How do retinal ganglion cells report changes in illumination from one location to another?
Retinal ganglion cells report changes in illumination from one location to another
It does that with lateral inhibition
In the central retina the ganglion cell is receiving input from a single cone as well as inhibitory interneurons which are picking up information from the whole pool of cones around the cone
If the light decreases over the central cone, it will depolarise this cone, depolarise the bipolar cell and it will depolarise the ganglion cell
If light decreases over the cone next to the original, it will depolarise the cone, it will depolarise the inhibitory interneuron which will inhibit the bipolar cell in the ganglion cell
So what happens over the entire field will produce a mixture of excitation and inhibition which largely cancels out
What’s the difference between off and on centre retinal ganglion cells?
Half of all retinal ganglion cells respond to increase in brightness and half to decreases
Off centre:
central photoreceptor depolarised (red) by decreased illumination
bipolar and ganglion cells depolarised by excitatory synapses
On centre, there is an inverting synapse in the pathway:
central photoreceptor hyperpolarised (blue) by increased illumination
bipolar cell depolarised by inverting synapse, excites ganglion cell
What’s the difference between parvocellular and magnocellular ganglion cells?
Retinal ganglion cells can be divided into different classes Two ganglion cells from the same location in the retina Parvocellular: small field with strong surround fine resolution accurately follows changes in light needs stable image Magnocellular: large field with weak surround coarse resolution transient responses to change responds well to fast movement
What’s the difference between parvocellular and bistratified?
Parvocellular:
selective inputs from “red” or “green” photoreceptors
by comparing these responses they can encode wavelength
RED vs GREEN
Bistratified:
selective inputs from “blue” or “red+green” photoreceptors
by comparing these responses they can encode wavelength
BLUE vs YELLOW
