NEURO: Vision Flashcards
What side of the retina do the temporal and nasal visual fields focus on?
The temporal visual field focuses on the nasal part of the retina.
The nasal visual field focuses on the temporal part of the retina.
This is because the optics of the eye invert the image.
What gives the eye its shape?
Outer fibrous layer:
- sclera
- cornea
Inner:
- aqueous humour
- vitreous humour
Sclera: eye shape
white non-stretchy layer creating the anchoring point for ocular muscles, keeping the eye rigid
Cornea: eye shape
collagen fibres align in a way to make the outer fibrous layer transparent
Aqueous humour: eye shape
- fluid
- produced by the ciliary body
- flows outwards (front of the eye) and is eventually reabsorbed through the angle of the eye
-the balance between production and reabsorption which generates intraocular pressure keeping the sclera rigid
Vitreous humour: eye shape
jelly-like fluid behind the lens, hydrated by aqueous humour, that maintains shape of eye and keeps it transparent
What gives the eye the ability to focus on an image?
Optics:
- cornea
- lens
- ciliary body
- iris
Cornea: focus
bends light rays to bring them back to a single point (focusing of the image)
Lens: focus
the transparent structure that changes shape to provide the variable fine focus of the eye
*suspended by a ring of suspensory ligaments from the ciliary body
Ciliary Body: Focus
contains a ring of muscle which changes shape of lens by contracting or relaxing:
- contracts to fatten lens for close vision (decreased diameter)
- relaxes to flatten lens for distance vision (increased diameter)
Iris: focus
ring of muscle which creates a coloured part of the eye
controls diameter of the aperture (pupil), controlling how many light eyes the eye via the pupil
-the smaller the aperture, the better the focus
When does the pupil dilate?
dim light to let enough light to give a bright enough image
What structure transmits light information to the brain?
Retina
>Neural retina: contains photoreceptors and afferent retinal ganglion cells which have the axons that run across the surface of the retina and form the optic nerve
> Retinal pigment epithelium: provides a lot of biochemical support for the photoreceptors and also holds the retina in place, preventing it from peeling away
> Optic nerve: central nervous system tract myelinated by oligodendrocytes
How does the eye transmit image/light to the visual cortex?
Primary Visual Pathway:
1) The retinal ganglion cell axons project down the optic nerve to the optic chiasm, and at the optic chiasm, the ones on the nasal half of the retina swap sides. The axons from the temporal half of the retina stay on the same side.
2) They project back to the lateral geniculate nucleus (LGN) in the thalamus.
3) Cells in the lateral geniculate nucleus send their axons through a region of white matter known as the optic radiation back to the occipital cortex, where the primary visual area is located.
What is phototransduction?
The conversion of light into electrical signals in the rods, cones and photosensitive ganglion cells of the retina of the eye
What are photoreceptors?
specialised cells in the retina which respond to light:
Rods- night vision
Cones- day vision
*most of the time the cones are being used
The rods are super-sensitive, so they saturate and become non-functional in high-light levels. They are used mostly in night vision.
The cones are less sensitive but work better in high-light levels. They are used for day vision.
Describe the structure of cone receptors.
> Inner Segment: nucleus and protein-making machinery etc.
> Outer Segment: bag containing tightly packed layers of phospholipid membrane which hold the chromophore (light-sensitive molecule) perpendicular to the light path, ensuring efficient trapping of light rays
> Axon: not really an axon, just a neurite, as it doesn’t fire action potentials, and doesn’t need to because it is a very small cell and can transmit information via electrotonic potentials
> Synaptic Terminal: releases glutamate
What is the resting potential of a cone photoreceptor, and how does it come about?
The resting potential of a cone photoreceptor is -45 mV.
They’re polarised as such because the inner segment has potassium channels that leak K+ out, and the outer segment have sodium channels that are continuously open, so they leak Na+ in.
Electrophysiology of cone photoreceptor at rest
· Photoreceptor nerve cells leak K+ all the time, producing a negative internal potential, resulting in resting membrane potential of -45mV (more depolarised than normal membrane potential -70mV).
· These cells are depolarised even at rest due to Na+ channels in the outer segment being open by default, allowing for the influx of Na+, and there is also glutamate being released from the synaptic terminal
· Glutamate, though usually excitatory, functions here as an inhibitory neurotransmitter
Electrophysiology of cone photoreceptor in response to increased light
· If the light striking the outer segment gets brighter, some of the Na+ channels close, causing the cells to become more negative inside (hyperpolarisation) as there is no more Na+ influx
·This prevents the release of inhibitory glutamate from the synaptic terminals, sending a signal along the visual pathway
Electrophysiology of cone photoreceptor in response to decreased light
· If the region striking the outer segment gets darker, more Na+ channels open, allowing for the influx of Na+ and depolarising the cell
· More glutamate is released from the synaptic terminal
What holds Na+ channels open in photoreceptors?
cGMP
Photopigment in cones
photopigment on membrane disc made of:
- opsin protein
- light-sensitive chromophore called retinal (11-cis retinaldehyde)
Transduction reaction when light strikes photopigment in cones
1) Unstable 11-cis bond in retinal ruptures and reforms in the more stable trans-configuration, resulting in all-trans retinaldehyde
2) Opsin is now linked to all-trans retinaldehyde and the photopigment is now activated and behaves in the same way that a G-protein coupled receptor would behave when activated
3) A single activated photopigment can activate many G-proteins, and each of those activated G-proteins go on to activate the enzyme responsible for destroying cGMP
4) There is a fall in intracellular cGMP, and cGMP diffuses away from the Na+ channels on the membrane, closing the Na+ channels
Transduction reaction when there is the termination of the light response
1) Activated photopigment is capped and retinal removed
2) G-proteins are not activated and therefore the enzymes which break down cGMP are not activated either
3) This allows a second enzyme to restore cGMP levels, which reopens the Na+ channels
4) Another molecule of 11-cis retinaldehyde is attached to the opsin, ready to respond to the next incoming photon
What causes loss of peripheral vision?
· If you have a disease attacking the peripheral retina (e.g. glaucoma, retinitis pigmentosa), peripheral vision is lost, however, the central visual field which allows you to see in detail is undestroyed and kept for a long time until quite later in the disease process.
What causes loss of central vision?
· If you have a disease attacking the central retina (e.g. age-related macular degeneration), central vision is lost, and you lose the ability to see in detail. Someone who has lost that tiny bit of their visual field, even though the rest of their vision is intact, will still be registered blind because although they may be able to navigate through the world, they won’t be able to recognise faces or read. Therefore, the central retina is hugely important.
- transient responses
- coarse detail vision, but responds well to fast movement
- half ‘on’ centre and half ‘off’ centre
Structure of peripheral retina
10x as many rods as cones, therefore cones are separated by a pool of rods, making big gaps in the sampling array
10x as many rods as cones, therefore cones are separated by a pool of rods, making big gaps in the sampling array
Where is the central retina located and what does it contain?
within the optic nerve head containing a dark structure called the fovea centralis
Foveal pit
region composed of closely packed small cones (not spaced and no rods) and there is no inner retinal tissue between them and light path, therefore there is no image blur and there is fine focus
What do ganglion cells in the central retina receive input from?
receive input from a single cone photoreceptor, meaning there is no convergence
this signal from the fine sampling array (tight cones) remains uncontaminated all the way back to the primary visual cortex
Peripheral Vision
Majority of the retina outside of the foveal pit serves only coarse vision:
· The visual image is optically blurred
· The cone photoreceptors are large and widely spaced (separated by a larger number of rods)
· The signals from many cones converge onto single ganglion cells via bipolar cells
· Poor vision
Central Vision
The fovea in the central retina is specialised for high resolution:
· Good focus- overlying retinal layers are absent
· Only cone photoreceptors- primarily red and green- are narrow and closely packed
· The signals from the photoreceptors are kept separate throughout the primary visual pathway and there is no convergence
· Fine detailed vision
The right-hand side of the image is focused on what side of both retinas? And where the side of the brain do they send information back to?
the left-hand side of both retina, sending information to the left side of the brain
The left-hand side of the image is focused on what side of both retinas?
right-hand side, sending information to the right side of the brain
Adaptation to light
after the photoreceptor responds to an increase in light, there is adaptation if the eyes stay in that same location for a length of time:
the brightness doesn’t change and the photoreceptor adapts and resets itself by going back to its resting potential. Now the photoreceptor is ready to respond to another change in brightness. This can happen over an enormous range of absolute illuminations.
Over the whole of this range, our photoreceptors are able to respond very sensitively to tiny changes in brightness very quickly without saturation because they constantly reset their membrane potentials through “adaptation”.
Lateral Inhibition in the Retina
The reduction of activity in one neurone by activity in neighbouring neurones
Central retinal ganglion cell receiving excitatory input from the single cone, but also inhibitory input from cones surrounding that one signal cone
The response of cells in the visual system depends upon the net result of excitatory and inhibitory signals it receives:
>If light decreases over the central cone, the cone becomes depolarised, subsequently depolarising the bipolar cell, and then depolarising the ganglion cell
>If light decreases over a surrounding cone, it will depolarise the inhibitory interneuron which will inhibit the bipolar cell and the ganglion cell
How do retinal ganglion cells respond to increases in illumination given that all photoreceptors are depolarised by decreases in illumination?
“on” central retinal ganglion cells coming from the central cone photoreceptor has an inverting synapse
-therefore, when the cone photoreceptor hyperpolarises to an increase in illumination, the inverting synapse depolarises bipolar cell, which excites the ganglion cell
Magnocellular retinal ganglion cells
> pool of cells which are very sensitive and designed to respond to fast-moving things, but they do so at the expense of being no good at seeing fine details
more convergence for coarse resolution
Parvocellular retinal ganglion cells
> pool of cells designed to pick up fine detail, but they are not very sensitive because they get very little input.
Which retinal ganglion cells are wavelength selective?
Parvocellular
- receive inputs from red and green photoreceptors
- encode wavelength by comparing responses from these inputs
Bistratified
- receive inputs from blue or “red+green=yellow” photoreceptors
- encode wavelength by comparing responses from these inputs
*These comparisons are the basis of colour vision
Primary Visual Cortex Input Distribution
Inferotemporal Visual Areas
- encode information about object identity
- receive input from parvocellular cells
Cortical Areas
-process colour
Parietal Visual Areas
- encode information about location and movement
- receive input from magnocellular neurones (don’t need fine detail)
