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Neurophysiology & Human Behaviour > Vision (Retina) > Flashcards

Vision (Retina) Flashcards

1
Q

What is the range of wavelengths for visible light?

A

390nm - 700nm

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2
Q

What are the ways in which light can be quantified?

A
  • Emission: Amount of light emitted by an object (candela, cd).
  • Illuminance: Amount of light falling onto particular surface (lux).
  • Luminance: Amount of light reflected by an object (cd/m2).
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3
Q

What are the characteristics of light detected by the eyes?

A
  • Wavelength
  • Intensity
  • Position
  • Changes in time
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4
Q

What is the absolute range of intensities over which the eyes are able to operate without saturation?

A

1010

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5
Q

Why are the eyes unlikely to ever have to deal with very wide range of luminances in an environment?

A

Objects in the same visual field are likely to be illuminated by the same light source.

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6
Q

What is the albedo of an object?

A

Albedo = Luminance/Illuminance

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7
Q

What is visual acuity?

A

The resolution of an image formed by the eyes (closest 2 points can be together an still be distinguishable as 2 points)

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8
Q

What are the primary factors that affect acuity?

A
  • Optics
  • Retinal mosaic
  • Neural processing
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9
Q

What are the optical factors that affect acuity?

A
  1. Chromatic aberrations
  2. Spherical aberrations
  3. Lens defects (refractive errors)
  4. Glare
  5. Diffraction
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10
Q

How are the eyes adapted to minimise chromatic aberrations?

A
  • Few blue cones in centre fovea
  • Yellow filter over fovea that absorbs blue light
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11
Q

How are the eyes adapted to minimise spherical aberrations?

A
  • The cornea is ellipsoid in shape, which is more ideal.
  • Refractive index of lens decreases further out in the lens.
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12
Q

What properties of the eyes cause diffraction?

A
  1. Diameter of lens
  2. Diameter of pupil
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13
Q

How is diffraction in the eyes quantified?

A

d = 1.22λ/D

d = diameter of diffraction pointspread

λ = Wavelength of light

D = Diameter of lens

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14
Q

How are the eyes adapted to minimise glare?

A
  • Müller cells guide light through retinal vascular layer to minimise scattering of light
  • Pigment epithelium beneath retina contains black pigment melanin that absorbs light and minimises reflected light
  • Blood vessels are pushed aside over fovea
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15
Q

What are the different refractive errors?

A
  • Emmetropia: Normal.
  • Hypermetropia: Long-sighted, corrected with convex lens.
  • Myopia: Short-sighted, corrected with concave lens.
  • Presbyopia: Long-sightedness as a result of loss of lens elasticity with age.
  • Astigmatism: Oval-shaped lens results in light in one plane being focused onto different point compared to another plane. This is corrected with cylindrical lens.
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16
Q

How does pupil size affect visual acuity?

A
  • As pupil size increases, contribution of diffraction to pointspread decreases but contribution of other aberrations and glare increases.
  • As pupil size decreases, contribution of diffraction increases, but other aberrations and glare decreases.
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17
Q

How does the pointspread function limit visual acuity?

A

Pointspread function of 2 points must be distinct and not merge into one in order for the points to be distinguishable.

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18
Q

How does the retinal mosaic limit visual acuity?

A

In order for 2 points to be distinguishable from each other, there needs to be at least 3 photoreceptors; 1 at each peak and 1 in the trough between the 2 functions. In other words, the minimum receptor spacing needs to be 1/2 the width of the pointspread function (Nyquist limit).

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19
Q

How does neural processing limit visual acuity?

A

Light from 2 sources need to remain distinct signals as they ascend to the brain in order for the brain to be able to distinguish them. However, in rods, there is a lot of convergence and information pooling, thus reducing acuity.

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20
Q

What is the absolute visual acuity of the eyes in a ‘normal’ person?

A

35-40 arc seconds

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21
Q

What are the focusing elements of the eyes?

A
  • Cornea: Contributes 2/3 (~48D) of overall focusing power of eyes.
  • Lens: Contributes 1/3 (~18-26D) of overall focusing power of eyes.
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22
Q

What is the structure of the cornea?

A

Thick layer of regularly arranged transparent collagen fibres sandwhiched between an external epithelium and an internal endothelium.

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23
Q

What is the structure of the lens?

A

The lens is made up of long ribbon-like cells (filled with transparent protein crystallin) tightly packed together.

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24
Q

What is significant about the development process of the lens?

A

As the lens develops, cells are added from the periphery. The oldest cells are in the middle and have the greatest refractive index, helping to counter spherical aberrations.

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25
Q

What are the functions of the lens?

A
  • Accommodation and allowing eyes to focus on objects at different distances.
  • Absorbs high energy UV light and prevents damage to the retina.
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26
Q

What is the process of accommodation?

A
  • When the ciliary muscles are relaxed, the suspensory ligaments are tensed and pull the lens out, making it less convex and decreasing its power. This is for focusing on distant objects.
  • When the ciliary muscles are contracted, the suspensory ligaments are taut and allow the lens to collpase inwards, making it more convex and increasing its power. This is for focusing on near objects.
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27
Q

What is the near reflex?

A
  • Ciliary muscles contract and lenses become more convex, increasing focusing power.
  • Pupils constrict to allow for greater tolerance for focusing errors for objects not quite in focus (so they look acceptably sharp regardless), increasing depth of field.
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28
Q

What is the pathway of the pupillary reflex?

A
  1. Information regarding light intensity is carried via the optic nerve [II] and optic tract to the pretectum in the midbrain.
  2. Intermediate neurones project from this point to the Edinger-Westphal nucleus bilaterally where they synapse with parasympathetic fibres.
  3. Parasympathetic fibres are carried by the oculomotor nerve [III] to the sphincter pupillae muscles and cause contraction, leading to pupillary constriction.
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29
Q

What is the significance of photoreceptor arrangement in the retina?

A
  • Retinal photoreceptors are located on the outermost layer of the retina, with the retinal nerve fibres and blood vessels between them and the incident light.
  • This is problematic as it causes light to scatter before it reaches the photoreceptors.
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30
Q

What is the structure of a photoreceptor?

A
  1. Outer segment
  2. Inner segment
  3. Cell body
  4. Axon + synaptic terminal
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31
Q

What are the similarities and differences between the outer segments of rods and cones?

A

Similarities:

  • Both outer segments are connected to the inner segment by a “connecting cilium”.
  • Both outer segments contain folds of membrane into discs.

Differences:

  • Rods have cylindrical outer segment while cones have conical outer segment.
  • Folds of membrane in rods are separated from the PM while they are continuous with the PM in cones.
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32
Q

What is the function of outer segment discs?

A

Maximises the surface area within the outer segment for photopigments and enzymes to be embedded.

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33
Q

What is the life-cyce of an outer segment disc?

A
  • They are formed near the base of the outer segment from inward invagination of the PM.
  • They break off near the top of the outer segment and remnants are phagocytosed by the pigment epithelium.
34
Q

What is the structure of retinal photopigments?

A
  • All photopigments contain a G-protein opsin embedded within the membranes, bound to a chromophore in the binding cleft.
  • The chromophore is 11-cis retinal.
  • In cones, 11-cis retinal is also the chromophore, but opsins are different.
35
Q

What determines the absorption spectrum of a photopigment?

A

Interactions between the opsins and 11-cis retinal determines its absorption spectrum.

36
Q

What is the peak absorption wavelength of rhodopsin in rods?

A

~500nm

37
Q

What is the molecular mechanism of light transduction (in rods)?

A
  1. In resting state (no light), retinal is in 11-cis isoform. Guanylyl cyclase is active, producing cGMP from GTP.
  2. cGMP binds to non-selective cyclic nucleotide-gated ion channels and activates them. This causes an inward depolarising Na+/Ca2+ current (dark current).
  3. In the dark, the retinal photoreceptors are depolarised at a membrane potential of ~-30mV.
  4. On absorption of single photon, 11-cis retinal isomerises to form all-trans retinal. This promotes conformational change in bound opsin to form activate metarhodopsin II (R*).
  5. R* promotes exchange of GDP for GTP on G-protein transducin, which is activated and dissociates into Gα and Gβγ subunits.
  6. Gα activates PDE, which hydrolyses cGMP to GMP, decreasing intracellular [cGMP].
  7. Resultant decrease in [cGMP] causes closure of CNs, inward current and thus hyperpolarisation of the photoreceptor to ~-75mV.
38
Q

What is the mechanism behind termination of the phototransduction cascade?

A
  1. Metarhodopsin II is phosphorylated by protein kinase on intracellular putative phosophorylation site.
  2. Phosophorylation allows binding of arrestin, blocking interaction with transducin.
  3. Intrinsic GTPase activity in Gα subunit of transducin results in self-deactivation after a period of time and termination of the cascade (in absence of active rhodopsin).
39
Q

What is the process of photopigment regeneration?

A
  1. Metarhodopsin II is highly unstable and all-trans retinal dissociated within minutes (receptor is bleached).
  2. All-trans retinal is transported into retinal pigment epithelium (RPE) where it is regenerated and converted back to 11-cis retinal.
  3. Regenerated pigment is transported back into photoreceptor cells where they re-associate with the opsin.
40
Q

What is the chemical process of photopigment regeneration?

A
  1. All-trans retinal reduced to all-trans retinol (in photoreceptor).
  2. Esterification to all-trans retinyl ester.
  3. Conversion to 11-cis retinol via multi-step reaction (catalysed by isomerohydrolase enzyme).
  4. Oxidation to 11-cis retinal.
41
Q

How long does the pigment regeneration process take in rods?

A

Up to 30 minutes

42
Q

Why is the cone pigment regeneration process quicker than the rod process?

A
  1. Private regeneration pathway via Müller cells.
  2. Quicker pigment-opsin reassociation.
43
Q

What are the differences between rod transduction and cone transduction?

A
  • Cones are 50x less sensitive compared to rods.
  • Cones respond much faster compared to rods.
44
Q

What are the types of colour blindness?

A
  1. Protanopia: Absence of red cone
  2. Deuteranopia: Absence of green cone
  3. Tritanopia: Absence of blue cone
  4. Anomalous trichromacy:
    - Protanomaly: Defective red cone results in green-shifted absorption spectrum
    - Deuteranomaly: Defective green cone results in red-shifted absorption spectrum
45
Q

What types of mutations can result in colour blindness?

A
  • Unbalanced translocations: Resulting in an opsin gene not being inherited.
  • Balanced intergenic translocations: Chimera of red/green gene inherited (with shifted absorption spectrum). Type of colour blindness depends on dominant gene in chimera (e.g. green dominance causes deuteranomaly.
46
Q

Which chromosomes are the cone pigment genes located?

A

X-chromosome:

  • Red
  • Green

Chromosome 7:

  • Blue
47
Q

What are the types of cells present within the retina?

A
  • Photoreceptors
  • Bipolar cells
  • Horizontal cells
  • Amacrine cells
  • Ganglion cells
  • Müller cells
48
Q

What are the layers of the retina (outward to inward)?

A
  1. Photoreceptor outer segment
  2. Outer nuclear layer: Photoreceptor nuclei
  3. Outer plexiform layer: Synpases between photoreceptors, bipolar cells and horizontal cells
  4. Inner nuclear layer: Nuclei of bipolar cells, horizontal cell and amacrine cells
  5. Inner plexiform layer: Sypapses between bipolar cells, ganglion cells and amacrine cells
  6. Ganglion cell layer: Cell bodies of ganglion cells
49
Q

What type of conduction is used in cells of the retina?

A

All cells in the retina, apart from ganglion cells, use passive conduction.

50
Q

What is the function of convergence/divergence?

A
  • Convergence: Discards irrelevant information (for signal economy) and allows information polling which increases sensitivity
  • Divergence: Allows analysis of different aspects of the same image simultaneously (parallel streams)
51
Q

What is the structure of photoreceptor synapses?

A
  • Photorecepors always synapse with both horizontal and bipolar cells.
  • In the pre-synaptic terminal are structures called synaptic ribbons that hold NT in place. Their presence makes photoreceptors “ribbon synpases” that are specialised for transmitting graded information.
  • Synpases can be flat or invaginating.
52
Q

What are the differences between rod and cone receptor synapses?

A
  • Rod receptor synapses (rod spherules) are smaller than cone synapses (cone pedicles).
  • Each rod only synapses with one bipolar cell but one bipolar cell may synapse with multiple rods (convergence) while each cone synapses with multiple bipolar cells (divergence).
53
Q

What is the receptive field of a retinal cell?

A

An area of the retina (and associated area of visual space) in which light can evoke a response.

54
Q

What are the properties of receptive fields?

A
  1. Which field is stimulated defines the position of stimulus
  2. How big the receptive field is defines the level of convergence
  3. It contains both excitatory and inhibitory regions
55
Q

What are the types of bipolar cell receptive fields?

A
  • On-centre receptive fields: Maximum response of bipolar cell is achieved when the excitatory centre is fully lit but part of inhibitory surround in occluded. Minimum response when centre fully occluded but sorround is partially lit.
  • Off-centre receptive fields: Maximum response of bipolar cell is achieved when the inhibitory centre is fully occluded but theh excitatory surrounds in partially lit. Minimum response when centre fully lit but surrounds partially occluded.
56
Q

What is the purpose of receptive fields?

A
  • Forms the basis of lateral inhibition, which has 3 purposes:
    1. Emphasises borders and enhances contrast
    2. Discards redundant information for signalling economy
    3. Prevents satruation of cell possessing receptive field even when absolute light intensity is very high
57
Q

What is the mechanism of off-centre receptive fields?

A

When light falls onto inhibitory centre:

  1. Photoreceptors in centre hyperpolarise.
  2. Because synapse between photoreceptor and ganglion cell is sign preserving (excitatory Glu synapse), hyperpolarisation of photoreceptor leads to less NT release and less stimulation of bipolar cells and hyperpolarisation.

When light falls onto excitatory surround:

  1. Photoreceptors in surround hyperpolarise.
  2. Because synapse between photoreceptor and horizontal cell is sign preserving, hyperpolarisation of photoreceptor leads to less NT release and less stimulation of horizontal cells and hyperpolarisation.
  3. Hyperpolarisation of horizontal cell causes less inhibition on photoreceptor, resulting in more NT release and thus bipolar cell depolarisation.
58
Q

What is the mechanism of on-centre receptive fields?

A

When light falls onto excitatory centre:

  1. Photoreceptors in centre hyperpolarise.
  2. Because synapse between photoreceptor and ganglion cell is sign reversing (inhibitory Glu synapse), hyperpolarisation of photoreceptor leads to less NT release and less inhibition of bipolar cells and depolarisation.

When light falls onto inhibitory surround:

  1. Photoreceptors in surround hyperpolarise.
  2. Because synapse between photoreceptor and horizontal cell is sign preserving, hyperpolarisation of photoreceptor leads to less NT release and less stimulation of horizontal cells and hyperpolarisation.
  3. Hyperpolarisation of horizontal cell causes less inhibition on photoreceptor, resulting in more inhibitory NT release and thus bipolar cell hyperpolarisation.
59
Q

What types of contacts and NT are used in photoreceptor-bipolar synapses of on-centre/off-centre bipolar cells?

A
  1. On-centre (sign-inverting):
    - Metabotrophic Glutamate (mGluR6) receptors. Glu binding results in activation of cation channel-inhibiting G-proteins, resulting in hyperpolarisation.
    - Invaginating contacts.
  2. Off-centre (sign-preserving):
    - Ionotrophic AMPA glutamate receptors. Glu binding results in opening of non-selective cation channel, inward current, and depolarisation.
    - Flat contacts.
60
Q

What are the types of bipolar cells?

A
  1. Midget bipolar cells:
    - Small receptive fields
    - Only synapses with one cone
    - Involved in high acuity imaging
    - Colour information is preserved
  2. Diffuse bipolar cells:
    - Large receptive fields
    - Synapses with 5-10 cones
    - Greater sensitivity but lower resolution compared to midget bipolars
    - Colour information is lost
61
Q

How are parallel streams established?

A
  • One cone may synapse onto multiple types of bipolar cells.
  • This allows different aspects of the image (which different bipolar cells are specialised to process) to be processed simultaneously.
62
Q

What type of synapse in the bipolar-ganglion cell synapse?

A

Excitatory (sign-preserving) ionotropic Glu synapses

63
Q

What are the functions of magnocellular ganglion cells?

A

Detection of movement

64
Q

What are the functions of parvocellular ganglion cells?

A

Detection of fine form and colour

65
Q

What are the proportions of ganglion cells?

A

Magnocellular: 10%

Parvocellular: 80%

66
Q

Which cells make up the 10% of ganglion cells not magno/parvocellular?

A

Intrinsiclly photosensitive retinal ganglion cells (ipRGCs) that detect absolute light levels, which is needed in:

  1. Pupillary reflex
  2. Circadian rhythms
67
Q

What is the rod pathway in mesopic/scotopic vision?

A

Mesopic:

Rod → Cone

Scotopic:

Rod → Rod bipolar (on-centre) → AII amacrine → Cone (on-centre) bipolar → On-centre ganglion

AII amacrine → Cone (off-centre) bipolar (inhibitory glycinergic)

68
Q

What is the nature of convergence in rod pathway?

A

1500 rods → 100 rod bipolar cells → 5 AII amacrine cells → 4 cone bipolars → 1 ganglion cell

69
Q

What mediates changes in rod pathway from scotopic to mesopic vision?

A
  • Dopaminergic A18 amacrine cells.
  • They secrete dopamine when light intensities are high, which serves to decouple AII amacrine cells from cone bipolar cells.
70
Q

What is the consequence of rod switching between 2 pathways?

A

In scotopic range, rod signals actually undergo processing in bipolar cells. This doesn’t occur in mesopic range when rods simply complement cone response.

71
Q

How can visual acuity be measured?

A

Scientific:

  • 2-point discrimination
  • Grating patterns

Non-scientific:

  • Snellen chart
  • Landolt chart
  • FrACT test
72
Q

What is the optimum diameter of pupils?

A

~3 mm

73
Q

What causes the Argyll-Robertson pupil?

A

Damage to pretectum by neurosyphilis

74
Q

What are the characteristics of the Argyll-Robertson pupil?

A

The pupil constricts in response to accommodation (i.e. when viewing near objects) but not in response to consensual light reflex (i.e. when viewing bright light).

75
Q

What type of NT is used in photoreceptor-bipolar cell in lateral inhibition (on/off-centre receptive fields)?

A

GABA

76
Q

What are the contents of the optic nerve?

A

Axons of retinal ganglion cells

77
Q

What is the direct pathway and what NT is used is all synpases of the pathway?

A
  • Photoreceptor → Bipolar cell → Ganglion cell
  • Glutamate
78
Q

What is the synaptic triad?

A

Synapse between photoreceptors, bipolar cells and horizontal cells involved in lateral inhibition.

79
Q

How is aqueous humour drained?

A
  1. Trabecular meshwork
  2. Canal of Schlemm
80
Q

In which region of the retina is rod density the highest?

A

Parafoveal region (~20o either side of fovea)

Neurophysiology & Human Behaviour (20 decks)

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