The Eye and Visual Pathways Flashcards
What is pressure in the eye provided by?
Pressure in the eye is provided by aqueous humour production into the anterior chamber by the ciliary body
This aqueous humour drains via the Canal of Schlemm into the Venous system!
What is behind the lens and what can happen as we age?
Behind the lens= vitreous humour, a transparent jelly - refracts light + gives a clear image.
W age, vitreous jelly proteins ‘clump together’, causing decline in sight + ‘floaters’
How does the eye focus an image?
1) To focus an image: the cornea forms an air-liquid interface, which initially refracts light
2) Behind the aqueous humour, the lens further refracts light to focus it onto the retina
compare how we look at close vs far away objects?
Far objects: ciliary muscles relax, suspensory ligaments tighten, pulling the lens into a long, thin shape. This refracts light rays to the back of the retina less
Close objects: ciliary muscles contract, suspensory ligaments slacken, the lens will bulge, refracting the light rays more
After focusing the image of the world onto the retina, we need to capture it on the retina.
What are the 2 layers of the retina?
What does one of the layers contain?
Neural retina + Retinal Pigment Epithelium
The neural retina contains rod+cones, which convert the light into an electrical signal
The retina is divided into the neural retina and retinal pigment epithelium. This can be further subdivided into more layers. Describe these
Back layer = RPE
Then photoreceptor layer - has rod and cones
Then inner Nuclear layer – this is a layer of interneurons that links the photoreceptors to the afferents
Retinal Ganglion Cell layer: frontmost layer, (the afferents bundle together to form the optic nerve)
Describe neural circuitry in the peripheral retina
What is the resolution like in the peripheral retina?
Cones are scattered in the peripheral retina
Several cone inputs (which are spread out) converge onto a single Bipolar Neurone. This relays the inputs from cones–> Ganglion cells.
This convergence creates a large RF –> poor resolution in peripheral retina
Resolution is also poor bc light must pass through cell layers + vitreous humour before reaching the photoreceptor layer – this blurs the image, no point wasting energy making clear image
Describe the fovea and 3 ways how its adapted to fine focus central vision
At centre of retina is the Fovea Centralis;
Within the fovea are exposed photoreceptors – give a clear image bc of no overlying layers/blood vessels. It only contains narrow + closely packed cones→ smaller RF→ higher resolution
Also, cones in the fovea are only red green sensitive, not blue = further reduces RF
No convergence of cone cell inputs onto the ganglion cells further increases image resolution
Describe the primary visual pathway
Retina sends visual signals along optic nerve→ optic chiasm.
In the optic chiasm, nasal fibres of each optic nerve cross over → go to LGN of Thalamus via the optic tracts
Cells from LGN then project to the PVC, in the Calcarine Sulcus of the Occipital lobe
Optic tract branches also project down to Brainstem nuclei, which control subconscious vision
Why are images inverted in the primary visual pathway?
In the pvp, left side of the image is focused on the right side of the retinae. Top and bottom also inverted.
Axons from the nasal retina (left) cross over at the optic chiasm
Axons from the temporal (right) side of the retina project ipsilaterally back to the LGN
So right side of both retinae project to RHS of the brain, and vice-versa.
Essentially, the same location of the visual world in Left and Right eye project to the same area of cortex, where a single cell will integrate inputs from both eyes
Use this diagram to explain the concept of a retinotopic map in the primary visual pathway
There are also projections to the LGN + cortex, which contain a ‘retinotopic map’ of the visual world:
The diagram shows more pink/purple in the cortex. This is bc its the centre, so focused into the central fovea.
The fovea has dense cones, each innervating mucho ganglion cells–> cortex. Hence cortex needs larger representation for this greater info.
Further out from the fovea, hay fewer ganglion cells. The brain receives sparse input, so needs less cortex dedicated to it
Distinguish between nasal and temporal hemifields
We have temporal and a nasal hemifield.
The temporal hemi fields focus on the nasal retina and vice versa.
The hemifields can be divided into superior and inferior quadrants.
A damaged temporal retina –> visual loss within the nasal hemi field of that same eye. This= scotoma
There are other forms of damage too- see below
Lesions within the visual pathway cause visual field defects called scotomas
The position of the lesion determines scotoma type and location
When identifying the lesion responsible for a scotoma, what 3 rules must we remember?
The retinal image is inverted!!!
Independent, asymmetrical or unilateral deficits occur due to lesions before the optic chiasm
Matching, bilateral deficits occur due to a lesion behind the optic chiasm
Each half of the brain maps the contralateral half of the visual field – the left side of the visual world is mapped by the right side of the brain, and the right side of the visual world is mapped by the left side of the brain
What does this diagram show and use it to find out where the lesion is
Monocular blindness= total blindness in one eye
Here, the left eye is blind, right eye is unaffected
Since the visual field defect is unilateral, it means that the lesion is before the optic chiasm (rules out 2, 3, 5)
Since the whole visual field of the left eye has been lost, the lesion must be in the optic nerve (1)
What can cause monocular blindness?
Optic nerve damage can be caused by:
Tumours, Cranial fractures + MS (MS only attacks white matter tracts, inc the O.nerve) → monocular blindness.
If optic nerve is incomplete, there will only be partial loss of sight in affected eye
What does this diagram show and use it to find out where the lesion is
Homonymous hemianopia: same hemifield lost in both eyes.
Since the deficit is bilateral and the same side in both eyes, the lesion is at or behind the optic chiasm
The right side has been lost, so the lesion must be in the left side of the brain
The lesion must be in the LGN, optic tract or whole of the visual cortex (1 or 5)
Bc there is a total loss of vision in the visual field, 5 is not likely bc the lesion in 5 still has a small section of the primary visual cortex intact, which would still give some limited vision
Hence the lesion is at 1, at the LGN, caused by eg tumour or stroke
Bilateral loss of vision on 1 side–> lesion must be behind optic chiasm and only on the left (rules out 1, 3, 5)
On the other diagram, we can see that 2 is in parietal white matter, which gets input from the upper half of the retina.
4 is in temporal white matter, which gets input from the lower half of the retina.
Remember superior visual field (damaged here) is focussed on the lower half of the retina, so the lesion must be at 4!!
Could be due to a tumour, stroke.
What is this + What causes it?
Tunnel vision - The deficit is asymmetrical between left + right eye, so the lesion must be before the optic chiasm(3) !
This is seen in Glaucoma: raised Intraocular Pressure compresses the Optic Nerve heads which damages the Peripheral Retinal axons!
What is this + What causes it?
where would the lesion be if the defect was symmetrical in both eyes + the cause?
Central Scotoma: centre of the visual field wiped out - Here its bilateral but asymmetrical- so the lesion must be before the Optic Chiasm (lesion 1)
A common cause= Age-related Macular Degeneration, a disease attacking the central retina independently in both eyes.
However, If the central scotoma was symmetrical in both eyes (homonymous), bilateral damage to the visual cortex (lesion 4) would be responsible – this happens in head trauma!
