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Neurology > Vision > Flashcards

Vision Flashcards

1
Q

What is the role of the eye?

A

The eye captures an image of the world

It needs the ability to focus the image

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2
Q

Outline the two distinctive regions used to navigate around the eye

A

From above can split eye into temporal and nasal half (named after temporal bone and the nose)

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3
Q

Explain where the temporal field focuses along the retina?

A

Temporal visual field focuses on the nasal part of the retina and vice versa because the optics of the eye inverse the image and flip it top to bottom. (top part of the world is focused onto the bottom of the retina etc.)

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4
Q

Describe the structure of the outer fibrous layer of the eye

A

Non-stretchy sclera acts as the anchoring point for the extraocular eye muscles that move the eye around.

The collagen fibres and cells at the front of the eye, align so that its a transparent layer to form the cornea.

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5
Q

Explain how the inner layers of the eye are kept rigid and stable

A

Non-stretchy sclera is flexible so allows the intraocular pressure keeps eye rigid, back surface smooth and stable, and distances between optics and retina correct

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6
Q

How is the intraocular pressure generated?

A

Intraocular pressure is generated by the aqueous humour (fluid that fills the eye). It’s produced by the ciliary body and is pushed outwards and reabsorbed by the angle of the eye. - v. slow flow

Balanced production and drainage of aqueous humour produces enough pressure to hold the eye rigid.

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7
Q

What is vitreous humour?

A

Vitreous humour is a jelly like structure behind the lens. It is hydrated by the aqueous humour

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8
Q

How does the vitreous humour change with age?

A

As you get older, the proteins within vitreous become clumped leaving watery patches. This causes the vitreous to pull away from the back of the eye, causing it to shrink => can cause visual problems

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9
Q

What are the optic structures of the eye?

A 
Cornea
Lens
Ciliary Body
Iris
Pupil
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10
Q

What is the role of the cornea and lens?

A

Cornea and lens bend light rays.

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11
Q

How odes cornea bend light entering the eye?

A

Cornea is the most powerful refractory surface in our eye, due to its curvature. As light touches water surface of cornea, it bends inwards

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12
Q

Where is the lens located in the eye?

A

Lens sits behind cornea

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13
Q

Describe the structure of the lens

A

Transparent and can change shape to alter focus

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14
Q

How is the lens suspended?

A

Lens is suspended by a ring of suspensory ligaments from the ciliary body

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15
Q

How does the lens change shape to alter focus?

A

Ciliary body contains a ring of muscle which changes the shape of the lens

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16
Q

Explain how muscle contraction changes lens size

A

When muscle contracts, muscle diameter reduces, lens = fatter

When the ring of muscle relaxes, muscle diameter is wider, lens is thinner

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17
Q

What is the iris of the eye?

A

Iris is a ring of muscle that creates the coloured part of the eye

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18
Q

What is the function of the iris?

A

Produces aperture in the middle which can close down to adapt to different light conditions

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19
Q

How does the pupil respond to light?

A

Pupil expands in dim light, narrows in bright light - due to retina

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20
Q

What is the significance of a small lens aperture?

A

Pupil maintains the smallest aperture possible for the illumination conditions as the smaller the aperture = better focus

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21
Q

When looking at a single point, where is the light reflected from?

A

Looking at a single point in visual space reflects light off in all directions

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22
Q

Describe what happens to the light reflecting off from a point?

A

Some rays strike the cornea and pass through
Some rays stopped by the iris

Those that pass through the pupil will be brought back to focus as a single point by the lens and cornea

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23
Q

Why are light rays converged?

A

The diverging light rays are bent to converge them to a single point to focus the image

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24
Q

How does a smaller pupil allow better focus?

A

Light passing through the edges of the lens won’t be focused accurately - smaller pupil = more accurate focus => better depth of field
This is because the lens has a great deal of optical aberrations

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25
Where is the image being seen, focused?
Image is focused to the back of the eye => retina Image blurs as passes through retinal tissue; scattered
26
Once focused to the retina, how do we see an image?
The eye transmits the image to the visual cortex
27
Describe the structure of the retina
Neural retina is the innermost layer | Retinal pigment epithelium outer layer
28
Why are the retinal pigment epithelium and the retina CNS structures?
Retinal pigment epithelium is a part of the retina and developed embryonically from the neural tube as was the retina => both CNS structures
29
What is the role of the retinal pigment epithelium?
provides biochemical support for photoreceptors, and holds the retina in place, prevents it from peeling away
30
Describe the structure of the optic nerve
Optic nerve is a CNS tract although it is referred to a nerve due to its shape CNS tracts and peripheral nerves are myelinated by different types of glial cells Optic nerve myelinated by oligodendrocytes as it is a tract
31
Which disorder is associated with the optic nerve degradation?
Multiple sclerosis initial symptoms involve problems with the optic nerve
32
What is the purpose of the neural retina?
Neural retina contains photoreceptors and afferents (retinal ganglion cells) Retinal ganglion cells axons run across the surface of the retina and go on to form the optic nerve
33
Outline the primary visual pathway
1. Ganglion cell axons project down the optic nerve to the optic chiasm. 2. Ganglion cells on the nasal side of the retina swap sides at the chiasm, temporal side cells remain the same side. 3. Ganglion cells project back to the LGN (lateral geniculate nucleus) - specific nucleus in thalamus that serves the visual system 4. LGN cells send their axons through the optic radiation (region of white matter), to the occipital cortex (primary visual area)
34
How is subconscious eye movement achieved?
Some of these axons form branches towards the brainstem, where they innervate different nuclei involved in subconscious activities e.g. subconscious eye movement
35
When are the different photoreceptors active?
Rod Photoreceptors: night vision | Cone photoreceptors: day vision
36
How do the two types of photoreceptors work?
Two types of cone cells are two different systems, but use the same neural circuits once light hits the retina → brain But before retina either rods or cones and their respected neural circuits are activated
37
What structures are found in the inner segment of cone receptors?
- nucleus - protein making machinary - axon (doesn't fire action potentials) - synaptic terminal
38
Explain why action potentials aren't required in cone cells?
Uses electrotonic potentials to transmit info from one end to another - doesn't need action potentials as its a small structure
39
Which neurotransmitter is released at cone cell synaptic terminals?
Cone receptors have a synaptic terminal that release glutamate (excitatory neurotransmitter)
40
How is glutamate release regulated in cone cells?
Glutamate release mediated by transduction apparatus in the outer segment
41
Describe the structure of the outer segment of cone cells
Outer segment is a bag containing tiny packed infolding of phospholipid membranes layers - these hold the chromophore (light sensitive area) in neat layers, perpendicular to the light path - ensures efficient trapping of the light rays
42
What effect doe slight have on cone cells during the day?
During the day light is hyperpolarizing in cone cells
43
Outline the resting membrane potential of a cone cell compared to a typical neuron
Typical nerve cell RMP = -70mV | Typical cone cell RMP = -45mV
44
Why do cone cells have a more positive RMP than typical nerve cells?
Cone cells are therefore depolarised even at rest - due to Na+ channels being open by default in outer segment
45
How doe slight hyperpolarise cone cells?
When brighter light strikes the cone cell outer segment, some of the Na+ channels will close - cell becomes more negative - prevents some glutamate release.
46
How do cone receptors respond to decreased light?
As outer segment region gets darker, more Na+ channels open | => cell depolarises causing more glutamate to be released
47
Why are cone cells so effective at responding to change sin light?
The biochemical cascade supports a very rapid response to changes in illumination
48
What allows the Na+ channels to remain open during the RMP in cone cells?
Na+ channel held open by intracellular messenger, cGMP
49
What is the chromophoric component of cone cells composed of?
Photo pigment composed of opsin protein and retinal (11-cis retinaldehyde) which is the chromophore
50
Describe the structure of 11-cis retinaldehyde
11-cis retinal composed of a C ring and C tail | All C-C links forming the C tail are in the trans formation apart from C11 which is in cis formation => 11-cis retinal
51
Explain how the 11-cis retinaldehyde initiates the response to light in cone cells
The cis configuration is less stable than the trans formation When light strikes the molecule, the unstable cis bond to rupture, and reform in the more stable trans configuration of all-trans retinaldehyde Opsin is now activated photopigment → acts in the same way an activated G coupled protein would
52
What is the effects of activated opsin?
Activated opsin photopigment activates many G proteins ⇒ activate enzymes which destroy cGMP Descreased [cGMP]i ⇒ cGMP moves away from Na+ channels allowing them to close
53
How is the light response terminated?
The cascade is terminated at several levels, including the phosphorylation of rhodopsin by rhodopsin kinase, the subsequent binding of phosphorylated rhodopsin by arrestin
54
What effect does termination of the light response have?
→ removes activated retinal → deactivates G proteins → stops enzymatic cGMP removal Allows build up of [cGMP]i to be restored ⇒ reopens Na+ channels
55
Explain how retinal processing occurs
You can only see in detail with the very centre of your visual field; small thumbnail of retina
56
Name disorders leading to loss of peripheral vision
Loss of peripheral vision: | E.g. Glaucoma, retinitis pigmentosa
57
Which disorders may cause a loss in central vision?
Age -related macular degeneration - destroys retina; lose ability to see detail
58
What is the role of the inter-neuron circuitry?
Interneuron circuitry extracts detail from the photoreceptor signals and transmits it to the ganglion cells
59
Why are we only able to see fine detail through the centre of the retina?
Cone photoreceptors in periphery are separated by pools of rod cells - cone cells concentrated at retina ← large gaps on sampling array in peripheral retina
60
Why is the peripheral retina not able to see as much detail?
Ganglion cells receive input from the photoreceptors via bipolar cells Bipolar cells pick up info from a pool of receptors (not just one) ⇒ convergence’ increases pixel size
61
What is the receptive field centre?
The concentrated cone cell area of the retina, directly linked to the ganglion is called the ganglion cells’ receptive field centre
62
What is the significance of the receptive field centre?
The bigger the receptive field centre = the less fine detail seen because input from each cone cell is being converged in summation on the ganglion cell
63
Where is the central retina located?
Central retina is within the optic nerve head, centred around the fovea centralis
64
What is the foveal pit?
In the centre is foveal pit - a region where the photoreceptors are uncovered, no retina between receptors and light path ⇒ no image blur Image blurs as passes through retinal tissue; scattered
65
Why is the sampling array better in the foveal pit compared to the peripheral retina?
Excellent sampling array, as no rod cells present and very thin cone cells packed closely together to maximise space
66
What type of cone cells are present in the foveal pit?
Only red and green cells in foveal pit (only red and green cells associated with fine detail, blue cone cells aren’t)
67
Why does the foveal pit allow us to see in greater detail than the peripheral retina?
Ganglion cells don’t receive converged input from these cone cells, only receive input from a single cell
68
Summarise the role of peripheral vision
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.
69
Outline the functions of central vision
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
70
What makes the centre of the retina so good at focusing on fine detail?
Centre of retina has more associated ganglion than peripheral
71
What is the role of photoreceptors?
Photoreceptors report changes in illumination from one moment to another
72
Explain how photoreceptors adapt, every time the eyes rest for a moment or two in the same location
Brightness of light strikes a particular photoreceptor→ a strong response produced (ie. depolarisation) If the eyes stay in the same location, brightness doesn’t change so photoreceptor adapts and resets to RMP → becomes new normal Due to adaptation, photoreceptors can respond very sensitively to small changes without saturating themselves
73
How do photoreceptors elicit responses in different locations using change sin brightness from one area?
Retinal circuitry pulls out changes in brightness from one place to another via lateral inhibition
74
Which ganglion are responsible for lateral inhibition?
Ganglion cell receives input from a single cone cell; which is its receptive field centre Other ganglion receives signals from inhibitory neurons which input info from the whole pool of cones surrounding the central cone ⇒ inhibitory
75
Explain how lateral inhbition works
E.g. Decreased light → depolarisation of cone→ depolarisation of bipolar cell → depolarises ganglion cell If light decreases on the blue cones → causes depolarisation on the cone, depolarising the inhibitory neurons which go on to inhibit the bipolar cell neuron and ganglion cell Whatever response is activated across the whole receptor field, is carried out - largely cancels out to produce no response
76
What causes the excitation of retinal ganglion cells?
Retinal ganglion cells centres may be excited by either decreases or increases in brightness Half of all retinal cells respond to increases in brightness
77
What are the 2 excitatory types of retinal ganglion cell?
- on centre ganglions | - off centre ganglions
78
Describe the excitation of off centre ganglion cells
The central photoreceptor depolarised by decreased illumination Bipolar and ganglion cells depolarised by excitatory synapses
79
How are on centre ganglions affected by excitation?
The central photoreceptor hyperpolarised (blue) by increased illumination Bipolar cell depolarized by inverting synapse, excites ganglion cell
80
What are the 2 classes of retinal ganglion cells?
Retinal ganglion cells can be divided into different classes - Magnocellular - Parvocellular
81
Outline the parvocellular class of retinal ganglion cells
- small cell with strong surround - sustained responses - fine detail vision, but only when image is stable - half “on” and half “off” centre
82
Describe the magnocellular retinal ganglion cells
- large cell with weak surround - transient responses - coarse detail vison, but responds well to fast movement - half “on” and half “off” centre
83
How are we able to differentiate colour?
Some retinal ganglion cells are wavelength selective - parvocellular - bistratified
84
Outline the selected wavelengths of parvocellular ganglion cells
selective inputs from “red” or “green” photoreceptors by comparing these responses they can encode wavelength RED vs GREEN
85
What wavelengths are selected for in bistratified retinal ganglion cells?
selective inputs from “blue” or “red+green” photoreceptors by comparing these responses they can encode wavelength BLUE vs YELLOW
86
What is the visual role of higher cortical centres of the brain?
Higher visual cortical areas have different roles and send signals to different parts of the brain
87
What effect may lesions in the cortical area of the brain have?
Lesions in the cortical area processing colour cause colour blindness
88
How are different brain regions innervated to see different detail?
Inferotemporal areas get input mainly from parvocellular ganglion to identify detail Parietal visual areas have a great deal of magnocellular ganglion to see broad shapes

Neurology (20 decks)

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