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Neuro > Vision > Flashcards

Vision Flashcards

1
Q

Retina

A

Nerve cells and glia forming a thin, layered structure that lines the back of the eye, including: photoreceptors, interneurons and retinal ganglion cells

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2
Q

Photoreceptors

A

Light-sensitive sensory cells (rods for low light levels, cones for daylight vision)

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3
Q

Interneurons

A

A circuit formed that extracts salient information from the pattern of photoreceptor responses

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4
Q

Retinal ganglion cells

A

Retinal afferents

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5
Q

Lateral Geniculate Nucleus (LGN)

A

The thalamic nucleus that relays visual information from the retina to the primary visual cortex, in the pathway that perceives perception.

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6
Q

Primary visual cortex

A

Receives input from the retina via the LGN and distributes it to many “higher” visual areas for further and more specific processing.

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7
Q

Binocular cell

A

Receives input from both eyes, and can hence contribute to depth perception

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8
Q

Fovea

A

A tiny region in the centre of the retina, where the point in visual space that you are directly looking at is focussed, that is structurally adapted for fine detailed vision. Often used as synonymous with “macula”, a more extensive region distinguished by yellow pigmentation.

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9
Q

Photopigment

A

The molecule that captures a photon of light and triggers a change in a photoreceptor’s membrane potential. Consists of a protein, “opsin”, and a light-sensitive chromophore, “11-cis retinal”.

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10
Q

Receptive field

A

A region of the visual image that affects the responses of a given cell.

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11
Q

Describe the structure of the eye

A
  • Horizontal section of the head taken with an MRI. - - The two eyes with the optic nerves leading back to the optic chiasm.
  • There is a temporal half and a nasal half.
  • Towards the nose is the nasal visual field.
  • The temporal visual field is focusing on the nasal part of the retina and visa versa. This is because the optics of the eye will innervate the image.
  • The upper part is focusing on the bottom part of the retina.
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12
Q

Which part of the retina focuses on the temporal visual field?

A

Nasal Retina

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13
Q

Which part of the retina focuses on the nasual visual field?

A

Temporal Retina

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14
Q

What is the sclera?

A

The white part of the eye that creates an anchoring point for the extra-ocular eye muscles. It is flexible and is needed to be held ridged.

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15
Q

Why does the eye need a solid body?

A

A solid body is needed to create a non-stretch layer which creates a ridge outside.

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16
Q

What holds the sclera ridge?

A

Intra-ocular pressure

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17
Q

How is the cornea made transparent?

A

The collagen fibres align in a way that makes the cornea transparent

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18
Q

How is pressure created in the eye?

A

By the production of aqueous humour. It flows out of the ciliary body and reabsorbed at the angle of the eye. It is a slow flow. The production and drainage needs to be balanced to produce enough pressure to hold the eye ridge.

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19
Q

Where is aqueous humour made?

A

It is produced by the ciliary body at the front of the eye supporting the lens.

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20
Q

What is the vitreous humour?

A

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

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21
Q

How are floaters created in vision?

A

In older people, the proteins in the vitreous clumps leaving watery patches and pulls away the back of the eye shrinking down.

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22
Q

What is the cornea?

A

The curved structure at the front of the eye that bends light rays. As the light passes from the air to the water via this curved interface, the light rays are bent inwards. This is part of the optics of the eye. Responsible for focussing light rays

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23
Q

What is the lens?

A

Behind the cornea. It changes shape which can change the focus of the eye. It is suspended by a ring of suspensory ligaments from the ciliary body. Provides additional variable “fine” focus

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24
Q

What is the ciliary body?

A

Contains a ring of muscle that can change the shape of the lens.

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25
Q

What happens when the ciliary body muscle contracts?

A
  • When the muscle contracts, the diameter is smaller which makes the lens fatter for close vision.
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26
Q

What happens when the ciliary body muscle relaxes?

A
  • When the muscle relaxes, the diameter flatters which flatterns the lens for distance vision.
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27
Q

Function of the Iris

A

The ring of muscle that creates the coloured part of the eye. It function is to produce the aperture in the middle called the pupil. Controls how much light enters the eye via the pupil

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28
Q

Function of the pupil

A

The pupil expands in dark light and widens in lots of light. The pupil maintains the smallest apeture for the illumination conditions

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29
Q

Function of the retina

A

Adjusts the eye for different brigthness

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30
Q

Describe the path of light rays to the eye

A
  • Light reflects off in all directions and some of the photons strike the cornea and past through.
  • Some are stopped by the iris and some pass through the pupil and brought back to focus at a single point by the cornea and the lens.
  • The diverging light rays are bent to bring them back to a single point as part of the focussing of the image.
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31
Q

Smaller pupil

A

The smaller the pupil, the more accurate the focus and the better depth of field.

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32
Q

Describe the structure of the retina

A
  • At the back of the eye is the retina where the image focuses.
  • Consists of the neural retina
  • Also behind the neural retina is the retinal pigment epithelium (RPE).
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33
Q

What is the retinal pigment epithelium?

A

It provides chemical support for the photoreceptors and holds the retina in place to prevent it from pulling away. It is developed embryonically from the neural tube same as the retina.

34
Q

Which type of cells marinate the optic nerve?

A

Oligodendrocytes as it is part of the CNS

35
Q

What is one of the first symptoms of MS?

A

Porblems with the optic nerve

36
Q

What forms the optic nerve?

A

It has photoreceptors and afferents (retinal ganglion cells) these have axons that run across the surface of the retina and form the optic nerve.

37
Q

What is the primary visual pathway?

A
  • The ganglion cell axons project down the optic nerve to the optic chiasm.
  • At the optic chiasm, the ones on the nasal half of the retina swap, whilist the ones on the temporal half of the retina stay on the same side.
  • They project back to the lateral geniculate nucleus (LGN). This sends the axons through the optic radiation back to the occipital cortex where the primary visual area is.
  • Some axons make branches and head to the brainstem where they innovate a number of different nuclei that are involved in subconscious things like control of eye movements.
38
Q

What is the optic radiation?

A

A region of white matter

39
Q

Summarise the primary visual cortex pathway

A
  1. Ganglion cell axons project down to the optic nerve.
  2. Then they project to the optic chiasm
  3. At the optic chiasm, the axons on the nasal side of the retina switch and the ones on the temporal half of the retina stay
  4. They project back to the lateral geniculate nucleus (LGN)
  5. Then this sends axons to the optic radiation
  6. These are then transferred back to the primary visual cortex which is involved in conscious vision.
40
Q

What is the involuntary visual control pathway?

A
  1. Same pathway as above apart from
  2. Some of the axons make branches that head to the brainstem
  3. The brainstem is involved in subconscious movement like control of eye movements
41
Q

When can the rods and cones work together?

A

There is a period during twilight where the light is between the levels for the rods and the cones where they are both active and supporting one another.

42
Q

What neural circuit do each the rods and cones use?

A

They both use the same neural circuitry into the brain. At the level of the retina, only either the rods or the cones are active at one time.

43
Q

How does the artificial light affect the rods and cones?

A

When the rods are used on their own it is during the night, but most of the time due to artifical light, we are using cone photoreceptors all the time.

44
Q

Structure of the cone photoreceptors

A

Contains the inner segments and the outer segment.
- The inner segment contains the nucleus, and protein making machinery
- Also has an axon
- Contains the synaptic terminal
-

45
Q

Function of axon in cone photoreceptors

A

Doesn’t fire action potentials as there is no voltage gated channels but has a synaptic terminal that releases glutamate.

46
Q

Function of the synaptic terminal in cone photoreceptors

A

Cone receptor doesn’t fire action potentials because it is a very small cell that can use electrotonic potentials to transmit information from one end to the another. Releases glutamate depending on the level of depolarisation. It is a graded potential that is being produced by the transduction apparatus.

47
Q

Function of the outer segment of the cone photoreceptors

A

Contains the transduction apparatus. The outer segment is a bag containing tightly packed layer of phospholipid membrane. This is the same as the outer membrane.

48
Q

Chromophore

A

Contained in the phospholipid membrane layers.

Light sensitive, in neat layers, perpendicular to the light path. This ensures efficient trapping of light rays.

49
Q

Describe the release of glutamate from the cone receptors

A
  • All nerve cells leak potassium producing a negative internal potential. In cone receptors, the resting potential is -40 to -45mV. They are depolarised even at rest. This is due to sodium channels in the outer segments open by default. - If the light hitting the outer segment is brightened, some of the sodium channels close which hyperpolarises the cell. This closes some of the release of glutamate. This is the signal sent along the visual pathway. Light is hyperpolarising rather than depolarising.
  • If light hitting the outer segment is darker, more sodium channels open which depolarises the cell so it releases more glutamate.
50
Q

What is transduction?

A

Initiation of the light response. The outer membrane contains membrane discs.

51
Q

Describe the initiation of the light response

A
  • On the outer membrane, there is a sodium channel which is held open by an intracellular messenger called cGMP. This means the channel will be open and leaking sodium.
  • There are photo pigments on the membrane made out of opsin protein and a molecular of 11-cis retinaldehyde or retinal.
  • This retinal is the chromophore - light sensitive area. It is made up of a carbon ring and tail. All of the carbon to carbon links are in the trans configuration apart from the 11 position that is in the cis configuration. This is less stable.
  • When light hits the molecule, it causes the unstable bond to rupture and when it reforms, it forms a more stable formation trans configuration, all-trans retinaldehyde.
  • The opsin is now an activated photopigment. It behaves in the same way that a GPCR would act.
52
Q

Describe the amplifying biochemical cascade that happens after opsin is activated

A

After opsin is activated, it activates a G protein. A single activated photopigment can activate many G proteins. Each G protein activates an enzyme and these destroy cGMP. There is a fall in cGMP and it diffuses away from the sodium channels allowing them to close.

53
Q

How is transduction terminated?

A
  • No linger response is wanted otherwise there will be a trail of after images. Each photon needs to produce a transient reponse.
  • The G-proteins inactivate and the photopigment needs to be deactivated to stop the G proteins.
  • There is a cascade of biochemical events that cap the trans-retinal and remove it.
  • This will stop activating the G-proteins, stops the enzyme and this will allow a second enzyme to reactivate cGMP and reopen the channels.
  • Another molecule of 11-cis retinal will bind to the photopigment to respond to the next incoming photon.
54
Q

What does the sensitive cones mean?

A

The cones are extremely sensitive and are able to produce a large response to a small change of illumination. This is due to the amplifiying cascade - each photon will activate a lot of G-proteins down the line which will activate an enzyme which will produce a rapid change in cGMP levels. This fast peak is produced by the cascade.

55
Q

What happens if a disease attachs the peripheral retina?

A

There will be loss of peripheral vision. This can happen in a disease like glaucoma or retinitis pigmentaosa and the visual field is lost but there is middle vision still.

56
Q

What happens if there is loss of the central retina?

A

Loss of central vision caused by a disease like age-related macular degeneration which will destroy the central retina that will mean the vision will lose the ability to see detail. This means that a person will lose the ability to see detail. This person is registered blind as they cannot read or make out details.

57
Q

Structure of the peripheral retina

A
  • Ganglion cells at the base and photoreceptors at the top.

- In the middle, there is a circuitry system that is extracting detail from the photoreceptor signal.

58
Q

Ratio of cones to rods in the peripheral retina

A

In the peripheral retina, there is 10x as many rods as cones. The cones are separated by pools of rods. Even in these light conditions, the rods are non-functional. They are too sensitive to cope with the brightness.

59
Q

Function of the ganglion cells

A

Receive input from the photoreceptors via bipolar cells and get input from a whole pool of photoreceptors. The pixel size is bigger than a single photoreceptor.

60
Q

What is the ganglion cell receptor field centre?

A

The part of the visualisation that is directly linked to the ganglion cell. The bigger the centre, the less fine detail is being filtered out.

61
Q

Why is the focus not good when it passes through the peripheral vision?

A

Light comes in and has to pass through the outer segment of the retina before encountering hte cones. In this process, it passes through a dense capillary bed as well as all the nuclei. The light is scattered before it hits the outer segments and the focus isn’t good.

62
Q

Function of the central vision

A

Can see fine detail

63
Q

Structure of the central vision

A

Blood vessels heading towards the fovea centralis - central point of the retina, the converging point.
Foveal pit - region where the photoreceptors are uncovered - there is no retina. This is where there is no image blur.

64
Q

What is the foveal pit?

A

Area of uncovered photoreceptors, no retina. Excellent sampling array as there is no rods here. They are specialised, thin cones and only red and green cones. These are associated with fine details -blue cones are not.
The ganglion cells only receive input from a single cone so there is no convergence, uncontaiminated all the way back to the primary visual cortex.

65
Q

Describe the side of the brains view

A

The right hand side of the brain is looking at the legt hand side of the visual image from both eyes. This is because there is swapping over of the nasal part of the retina so the left hand side of the image is projected to the right hand side of both eyes.

66
Q

What is the retinotopic map in the cortex?

A

Each point in visual space is focussed on matching points in the two retina. This is projected back to the same point in the cortex.

67
Q

How does the photoreceptor change in different illumination conditions?

A
  • If the brightness changes in the same location, the photoreceptor adapts and resets returning to resting potential. It is ready to respond to another photon of light.
  • The cell can continue to respond to sensitive changes in light, every time it stops changing for a moment it adapts itself.
68
Q

How do the retinal ganglion cells report changes in illumination?

A
  • The retina changes in response to brightness overtime. This is designed to pull out changes in circuitry from one place to the neighbouring place. It does this with lateral inhibition.
  • The ganglion cell receives input from a single cone in the central vision.
  • The ganglion also receives input from inhibitory interneurons that pick up information from a whole pool of cones surrounding the centre cone. These have an inhibitory effect.
  • If light decreases over the central cone or another cone, it will have the same effect and inhibit.
  • Whatever happens over the entire field will produce a mixture of excitation and inhibition - but it will usually cancel out if the brightness is the same in all areas.
69
Q

How is the response of retinal ganglion cells to brightness controlled?

A
  • By the “on” and “off” centres. The retina sends information about increases and decrease in illumination equally even though the receptors respond in only one direction.
70
Q

What is the response of a central photoreceptor with an “off” centre?

A

A central photoreceptor depolarised (red) by decreased illumination.
This means bipolar and ganglion cells depolarised by excitatory synapses and there is no stimulus sent along the visual pathway.

71
Q

What is the response of a central photoreceptor with an “on” centre?

A

A central photoreceptor hyperpolarised (blue) by increased illumination. The bipolar cell depolarised by inverting synapse, excites ganglion cells.

72
Q

What are the two different classes of retinal ganglion cells?

A

Parvocellular and Magnocellular ganglion cells

73
Q

Parvocellular cells and tuning

A
  • Small field with strong surround
  • Fine resolution
  • Accurately follows changes in light
  • Needs stable light
74
Q

Magnocellular cells and tuning

A
  • Large field with weak surround
  • Coarse resolution
  • Transient responses to change
  • Responds well to fast movement
  • Due to more convergence
75
Q

Wavelength selective ganglion cells

A
  • Parvocellular

- Bistratified

76
Q

Wavelength selective parvocellular cells

A
  • Selective inputs from “red” or “green” photoreceptors
  • Comparing these responses they can encode wavelength.
  • Colour sensitive
77
Q

Wavelength selective bistratified cells

A
  • Selective inputs from “blue” or “red+green” (yellow) photoreceptors
  • Compare these responses by encoding the wavelength
78
Q

Cells in the visual cortex

A

Different cells that also have elongated fields not just relative brightness. Also respond to the presence of contrast or contrast in specific directions.
Also orientation sensitive - depending on direction, velocity, elongate contour etc.
- Difficult to respond to vision so their response has meaning that there is something specific going on.

79
Q

How does the primary visual cortex distribute information?

A
  • Can go to different parts of the cortex but also to different parts of the brain:
  • Parietal visual areas
  • Cortical area
  • Inferotemporal visual areas
80
Q

Function of the parietal visual area

A

Encodes information about location and movement

81
Q

Function of the cortical area

A

Processes colour

82
Q

Function of the inferotemporal visual area

A

Encode information about object identity

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