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Psychophysics H > Abnormal visual development - genetics > Flashcards

Abnormal visual development - genetics Flashcards

1
Q

what are the two mechanisms of visual development and when in life do these occur

A
  • nature: innate, mainly pre natal life

- nurture: visual environment, post natal life

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2
Q

what are the properties in the nature aspect of visual development

A
  • gene & molecular expressions
  • establish the basic ‘hardwire’ - primitive function connectivity between different pars of the visual system
  • rudimentary vision at birth
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3
Q

what are the properties in the nurture aspect of visual development

A
  • experience-dependent visual acuity
  • stabilises the exiting ‘hardwire’ and refines the ‘software’ (i.e. need to be exposed to to visual activity), optimises and adapts functional connectivity of the system
  • mature perception and fine discriminations

we operate the software, so its user dependent

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4
Q

name a disorder that can be caused by pre natal/genetic causes

A

albinism

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5
Q

what implications does a disorder that can be caused by pre natal/genetics have

A
  • disrupts the basic ‘hardwire’
  • permanent and severe visual defects
  • treatment: palliative only (can’t cure them, only help them)
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6
Q

name a disorder than can be caused by post-natal/experimental causes

A

amblyopia

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7
Q

what implications does a disorder that can be caused by post-natal/experimental causes have

A
  • disrupts the promotional (maintaining) and refinement processes
  • arrested or altered/sub-optimal development of visual stimulus
  • potential for (partial) recovery
  • treatment: visual stimulation-based ‘cures’ e.g. patching in amblyopia
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8
Q

how prevalent is human albinism

A

~1 : 16,000

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9
Q

what are the genetic and phenotypic variations in, with albinism

A

variations in melanin synthesis and expression

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10
Q

what does oculo-cutanous albinism OCA affect on a human

A

affects hair, skin and eyes

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11
Q

what is the cause of oculo-cutanous albinism

A

the absence of tyrosinase due to a gene mutation, which is the enzyme that produces melanin,
resulting in no melanocytes in the skin
no melanin in the RPE, choroid and iris so can see the blood flow and tissue through the iris

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12
Q

what does ocular albinism OA affect on a human

A

the pigment inside the eyes only (so not the iris) and is x-linked so affects males only

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13
Q

where is x-linked male only ocular albinism inherited from

A

from the maternal line

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14
Q

how is melanin defective in ocular albinism

A

has a problem with packaging melanin into melanosomes, but only with the melanin producing cells derived from the neural tube e.g. the RPE and ciliary retina
so melanocytes of the skin and iris don’t have this problem as they’re not tube derived

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15
Q

how does tyrosine produce melanin (which is defective in oculo-cutanous albinism)

A

it catalyses the conversion of tyrosine to DOPA and DOPA to dopaquinone

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16
Q

what is a direct consequence of albinism in the eye

A

absent/reduced ocular pigment

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17
Q

what implications does absent/reduced ocular pigment in the eye have, name two problems

A
  • iris (trans-iridial illumination): causes marked photophobia, so even in low illumination the px has to wear sunglasses
  • fundus (no black box effect): light bounced around in the eye, causes visual acuity loss, as there is no pigment to absorb the the light so when light goes into the eye, it is not focussed on the correct optical apparatus e.g. the retina. also there is no fovea or macula
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18
Q

how does embryonic RPE melanin expression begin, and what does it do

A

normally begins at the eye-cup stages of early embryonic development (even before the RGC’s develop in the neural retina)
it regulates the development of the adjoining neural retinal cells and hence development on the RPE

embryonic RPE melanin expression is absent or reduced in albinos

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19
Q

what are two consequences of an absent or under developed embryonic RPE melanin expression

A

neural retinal abnormalities:

  • fovea is underdeveloped
  • retinal pathways are mainly crossed
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20
Q

what is the outcomes of the fovea underdeveloped as a consequences of an absent or under developed embryonic RPE melanin expression (in both OC and OCA), name two

A
  • absent/poorly defined pit + hypoplasia (absence of cone cells in fovea, and reduced amount next to fovea)
  • further effects on visual acuity ~ 6/40 both eyes, so cannot drive as they can’t see the number plate at 25 metres

as the pit in our fovea is meant to allow light to go straight onto our foveal outer segments of cones and helps to reduce light scatter and improve our acuity

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21
Q

what is the outcomes of the retinal pathways mainly crossed as a consequences of an absent or under developed embryonic RPE melanin expression (in both OC and OCA)

A
  • temporal axons are misrouted across the midline at the optic chiasm (they cross over with the nasal axons)
  • visual pathways are then essentially monocular and dominated by the opposite eye so…
  • binocular depth perception is absent or reduced and px will have a strabismus or nystagmus i.e. fixation instabilities as va is so poor and no fovea
22
Q

what is severely disrupted as a consequence of misrouted temporal axons to the opposite side of the brain and what is the consequence of this

A

LGN layers and visual maps are disrupted
as a result, the stimulation of nasal or temporal hemi-fields of the left eye causes activity mainly in the opposite right cortex of albinos, and only a bit of activity in the left cortex due to a small amount of axon connection to temporal fibres of the left eye (like its suppose to)

23
Q

what are the VEP results of an albino due to hemispheric asymmetry

A

due to chiasmatic misrouting, there will be pronounced contralateral dominance of VEP activity, as temporal as well as nasal axons cross over to the other side of the cortex, e.g. in left cortex, after R eye stimulation and right cortex after L eye stimulation
pattern reversal is also reduced in amplitude and has a longer latency responses via the ipsilateral eye (as the temporal axons do not stay on that side like they should have done)

24
Q

how do you treat albinism

A

with low vision aids such as dark glasses, hand held & illuminated magnifiers to combat reduced visual acuity (as albinos can’t read without low vision aids)
this will only be able to improve the quality of life

25
Q

as well as genetic mutations causing congenital disorders such as albinism, what the effects can genetic mutations cause and name an example

A

more progressive effects, so the symptoms do not appear until later in childhood or even in adult life e.g. hereditary rod-cone degenerative or ‘dystrophic’ diseases such as retinitis pigmentosa

26
Q

how many mutations can cause isolated retinitis pigmentosa and hence what is the prevalence of developing it

A

> 40 ~1:4000

27
Q

which 3 types of gene mutations cause retinitis pigmentosa

A

autosomal dominant, autosomal recessive and x-linked types

28
Q

what do the gene mutations in retinitis pigmentosa result in

A

excess retinal pigment

29
Q

what is the numerous non-isolated syndrome associated with retinitis pigmentosa

A

usher’s syndrome (with deafness), is a congenital disorder

30
Q

what is the common retinal pathology in early stage retinitis pigmentosa

A

rod death peripherally:

causes night blindness (prolonged/defective dark adaptation in dim light) and peripheral VF loss

31
Q

what is the common retinal pathology in progressive stages of retinitis pigmentosa

A

rod and cone death centrally i.e. loss of photoreceptor layer: ‘tunnel vision’ so can only see central 10 degrees as it is cone dominated

32
Q

what is the common retinal pathology in the end stages of retinitis pigmentosa

A

profound photophobia and inability to navigate independently i.e. total blindness

33
Q

what are the 5 places that have defective genes/proteins in, with retinitis pigmentosa

A
  • RPE
  • rods
  • cytoskeletal integrity
  • disc trafficking
  • energy and oxygen consumption
34
Q

which protein is defective in the RPE due to retinitis pigmentosa

A

retinol metabolism is defective e.g. RPE65 and disc phagocytosis

35
Q

which protein is defective in rods, due to retinitis pigmentosa, name 3 defects

A

rhodopsin production is defective, visual transduction and ion channels

36
Q

what are the two consequences of common pathways to photoreceptor cell death in retinitis pigmentosa

A
  • oxidative stress: due to reactive oxygen species which leads to metabolic exhaustion
  • metabolic exhaustion: neurons in the retina get tired out
37
Q

which layer of the retina are the cell bodies of rods and cones usually found

A

inner nuclear layer

38
Q

what is left remaining of the photoreceptors in early stages of retinitis pigmentosa

A

only cone outer segments (COS) and their nuclei (ONL) remaining. as a result the outer nuclear layer in very thin as all rods have died out and only cones are remaining

39
Q

why is there a bone spicule appearance on the fundus of a retinitis pigmentosa px

A

because the photo receptor layer (ONL) degenerates, so RPE cells come into direct contact (where the rods die) with blood vessels in the inner retinal layers and migrate along them, forming the bone spicule appearance

this appearance starts off at the peripheral retina first and progresses into the whole retina in later stages

40
Q

what does a objective test for retinitis pigmentosa e.g. ERG show in the late stages, and how must this test be carried out

A

must be carried out in the dark, adapt the px to do this
shows no ERG responses, it is flat, due to non functioning rods or cones

so it the px has a bone spicule on their retina and lack of responses with ERG, the outcome is likely to be retinitis pigmentosa

41
Q

what should a normal ERG response look like when dark adapting a px

A

the ‘a’ wave which shows when theres photoreceptor hyperpolarisation to light onset (i.e. response to light in the dark) should have a -ve deflection
and
the ‘b’ wave which shows when theres, rod ON-bipolar cell depolarisation and cone ON and OFF bipolar cell responses should have a +ve deflection (which occurs after the ‘a’ wave)

42
Q

what symptoms does a px with early stage retinitis pigmentosa have, name three

A

Rods only affected:

  • peripheral VF defects
  • nightblindness
  • bumps into things
43
Q

what symptoms does a px with late stage retinitis pigmentosa have, name four

A

Rods and cones affected:

  • tunnel vision - VF restricted to central 2-3 degrees
  • photophobia
  • can’t read
  • house bound
44
Q

how does the concept of using trifield prisms as a low vision aid used in ‘treating’ somone with retinitis pigmentosa

A

glasses with one plano lens which the px can see straight ahead with with one eye, and one prism where the px can see peripherally with the other eye (so the line of site is pointed peripherally) to combat the tunnel vision

45
Q

what is a disadvantage of trifield prisms as a low vision aid used in ‘treating’ somone with retinitis pigmentosa

A

it leads/causes binocular confusion, as it acts like a squint, so only a few RP sufferers can tolerate them

46
Q

name two (novel) therapies used to treat retinitis pigmentosa

A
  • intra-vitreal injections

- sub-retinal implants

47
Q

what is used in intra-vitreal injections to treat retinitis pigmentosa and what are the side effects

A

growth factors (BDNF, bFGF, CNTF) injected into the vitreous

  • via encapsulated cells to promote photoreceptor survival
  • side effects: neovascularisation or cataract
48
Q

how are stem cells and/or gene replacements used to treat retinitis pigmentosa

A
  • cellular proliferation to replace the dead photoreceptors

- viral vectors carrying the specific missing gene (injected into the vitreous)

49
Q

what is the defective gene in retinitis pigmentosa, where clinical trials have now begun on

A

RPE65

50
Q

what is the concept of using retinal prosthetics in treating retinitis pigmentosa

A

micro electrodes are implanted behind the retina or in-front and the electrodes take over the jobs of destroyed photoreceptors. it helps when theres regions of intact photoreceptors next to the implant so they both can co-operate with each other.
this is useful for early and mid stage RP, but not useful for late stage RP

Psychophysics H (11 decks)

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