W9 - Neuro: Vision Flashcards
How does the eye work as a camera?
The two eyes lead back to the optic chiasm.
There is a temporal and nasal bone.
The temporal part of the vision focuses on the nasal part of the retina vise versa, because they invert the image.
Our eyes have a non-stretchy outside layer.
The sclera is the white part of the eye.
The transparent part is the cornea.
If you poke the side of the eye, it dimples, because of the pressure generated by the aqueous humour. That is produced by the ciliary body and it is eventually reabsorbed through the angle of the eye - a very slow flow. It is only enough pressure to make the eye rigid.
Behind the lens there is a jelly like structure - vitreous. It is hydrated by the aqueous. At old age, this structure starts to clump, leaving watery patches and starts to pull away from the back of the eye so shrinks down creating fuzzy floaters in the vision.
We need to adjust to light, and 2 structures help with this.
1) Cornea = most powerful, a front curved structure. As the light passes from air to the water, the light bends inwards, so that’s part of the optics of the eye.
2) Behind the cornea are the lens = it is transparent and can change shapes. When it changes shape, it changes the focus of your eye.
The lens is suspended by a ring of suspensory ligaments from the cilliary body. This is what changes the lens shape. The muscles in the ring contracts making the diameter smaller, the lens become flatter for distance vision.
The retinas adjust the brightness levels.
The pupil maintains the smallest aperture it can for the illumination conditions. The smaller the aperture, the better the focus will be. The diverging light rays are bent to bring to them a single point of focus from an image. The iris controls how much light enters the eye via the pupils. So the pupil only opens up when it has to in order to let in enough light to give a bright enough image, which it will only do in dimmer conditions. This is why when we look at something close, the pupil automatically closes down to give better focus.
At the back of the eye, there is the retina.
The innermost is the neural retina and behind it the retinal pigment epithelium, which is important support for the photoreceptors by chemical support. Also holds the retina in place and prevents it from peeling away. The retinal pigment epithelium was developed embryonically from the neural tube as well as the retina - they are CNS structures. The optic nerve is a CNS tract made of oligodendrocytes. First symptom of multiple sclerosis is usually problems with the optic nerve. The neural retina has the photoreceptors and the afference (retinal ganglion cells that run across the surface of retina then from optic nerve).
What is the primary visual pathway?
The ganglion cell axons project down the optic nerve to the optic chiasm. The nasal half of the retina swap sides, the ones from the temporal half stays on the same side. They project back to the lateral geniculate nucleus. Cells in the lateral geniculate sends axons through white matter (optic radiation) back to the occipital cortex.
Some of the branches head down towards the brainstem where they innovate a number of different nuclei involved in the subconscious. Like controlling eye movement or pupils.
What is phototransduction?
Rod photoreceptors; night vision
Cone photoreceptors; day vision
Twilight = both active
Now with artificial light, only time rods are used is when lights are off.
What is the cone photoreceptor?
The inner segment is where it keeps its nucleus, protein making machinery etc. Does not have voltage gated channels needed for action potentials. It does have a synaptic terminal at the end which releases glutamate, the fast excitatory neurotransmitter via depolarisation. This is a graded potential being produced by the transduction operators. That is located on the outer segments. This is a bag filled with tightly packed layers of phospholipid membrane in the cones. This holds chromophores, the light sensitive centres is holding it in neat layers perpendicular to the light path - efficient trapping of light.
All nerve cells are leaking potassium all the time. It is negative inside compared to the outside. In these cells, the resting potential is more like -40 to 45 millivolts, quite depolarised naturally. In the outer segment, there are sodium channels, which are open by default.If the light gets brighter, some of those channels close and it prevents some of the release of glutamate. If it gets darker, more channels open depolarising the cell and causing it to release more glutamate.
cGMP opens the Na+ channels, depolarising to an extent. The photopigment is made up of opsin and retinal (chromophore). It is made of a carbon ring and a carbon tail. All the structures are trans except for the C in the 11th position, which is cis. The cis is less stable. So when light strikes, it forms a rupture in this unstable molecule and it reforms to a more stable trans configuration. This makes the opsin an activated photopigment. A single activated molecule can activate many G-proteins, which goes onto activate an enzyme. And the enzymes destroy cGMP. You get a fall in the level of cGMP intracellularly and some of the cGMP diffuse away from its channels and allows them to close.
We do not want this to linger, or we will be seeing after images. We want each proton to produce a very transient response. The G proteins inactivate quite quickly.The key is to stop this activated photopigment from activating any more G proteins.
What is retinal processing?
You can only see in detail with the very centre of your visual field. The brain is able to reconstruct images from quick movements.
What is loss of peripheral vision?
egs. glaucoma, retinitis pigmentosa
If you have a disease attacking the peripheral retina, you are going to lose visual field , but you’ll keep the vision in the middle for a long time until late in the disease.
What is loss of central vision?
eg. age-related macular degeneration.
Something like age related macular degeneration though it goes for the central retina.
This means loss of ability to see details.
Everything else intact will still be registered blind.
They can navigate through the world, but will not be able to recognise faces and will not be able to read.
What does the cross section through peripheral retina consist of?
In peripheral retina, anywhere outside the specialised bit, we have x10 rods as cones. Cones are separated by pools of rods. There are also the ganglion cells receiving input from photoreceptors via bipolar cells and the bipolar cells are picking up info from a pool of photoreceptors, not just a single one. The part of the visual world that’s focused on the little bit of retina directly linked to the ganglion cell, is called the ganglion cells receptive field centre. The bigger that receptive field centre is, the less fine detail you’ll see.
The light has to pass through dense capillary beds, nuclei, and organelles of cells. So in peripheral retina, the light is scattered before it hits the outer segments so the focus is never good.
Why is the central vision very special?
The fovea centralis is the bit in the middle if you look at from the back of the eye, all the blood vessels head towards but doesn’t touch. It would have photoreceptors at the top and ganglion cells at the bottom. Right at the centre when we look at something, we have the foveal pit - photoreceptors are uncovered. There is no retina sitting between them and the light path. Only red and green photoreceptors.
The ganglion only get signals from single cone each, so there is no convergence. The signals remain uncontaminated all the way back to the primary visual cortex.
The fovea is specialised for high resolution:-
* good focus – overlying layers are absent
* only cone photoreceptors, primarily red and green
* which are narrow and closely packed
* the signals from the photoreceptors are kept
separate throughout the primary visual pathway
Photoreceptors report changes in illumination
from one from one moment moment to another
Why is the peripheral vision important?
Majority of the retina serves only coarse vision:-
* the visual image is optically blurred.
* the cone photoreceptors are large and widely
spaced (separated by larger number of rods).
* the signals from many cones converge onto single ganglion cells.
Why is the peripheral vision important?
Majority of the retina serves only coarse vision:-
* the visual image is optically blurred.
* the cone photoreceptors are large and widely
spaced (separated by larger number of rods).
* the signals from many cones converge onto single ganglion cells.
How does the primary visual pathway work?
Image is inverted by the optics left side of the image
= right side of both retinae
Image is mapped on to LGN and cortex with expanded central region right side of both retinae
= right side of brain
Axons from nasal retina swap sides
Axons form a “retinotopic map” in LGN and cortex, with the maps for the two eyes in
register
How is the retinal ganglion cell set to look at the relative brightness?
Retinal ganglion cells report changes in illumination
from one from one location to another.
Photoreceptors
Inhibitory interneurones
Ganglion cell
The example with the rows of cubes with the same colours appearing as different colours:
The retina is not telling the brain anything about the shading. It’s talking about the edges, where there is contrast, where light shines onto a receptor field centre will be different from that shining onto the surrounding. The sharp edges say it is brighter than the sides that is darker, making a mistake in reconstructing the image.
How do retinal ganglion cells respond to increase in brightness?
Half of all retinal ganglion cells respond to increases in brightness.
“Off” centre
* central photoreceptor depolarised (red) by decreased illumination
* bipolar and ganglion cells depolarised by excitatory synapses
“on”centre
* central photoreceptor hyperpolarised (blue) by increased illumination
* bipolar cell depolarised by inverting synapse, excites ganglion cell
What are the two classes of 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
