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Neural Biology 1 > RETINA 4 : Macula > Flashcards

RETINA 4 : Macula Flashcards

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1
Q

What is the the External Limiting Membrane ?

A

is not a membrane at all, but a series of tight junctions between photoreceptors and Müller cells

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2
Q

What do the tight junctions appear as on the electron micrograph ?

A

as dots

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3
Q

Why do we have tight junctions in the external limiting membrane?

A
  • dont allow passage of substances

- ensures stuff in each layer remain in their own layer and doesnt go into the other layers of retina

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4
Q

What could be another function of the external limiting membrane ?

A

as a structural function to keep the photoreceptors in place

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5
Q

What is layer 4?

A

outer nuclear layer

-has nuclei of Rods and cones

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6
Q

What does the thickness of the 4th layer depend on ?

A
  • depends where you are in retina

- made up of a few rows of nuclei

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7
Q

What are the types of nuclei ?

A

smaller darker

paler oval

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8
Q

What are the smaller darker nuclei like ?

A

5 x 5um
lie towards inner retina
there are several rows of smaller darker rod nuclei than cones

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9
Q

What are the smaller darker nuclei ?

A

the rod nuclei
-more of this than cones
darker nuclei lie virtual to external limiting membrane

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10
Q

What are the pale CONE oval nuclei like ?

A

-only have single row
-they lie in a single line -closest to ELM
more pale - more oval - each nucleus belongs to one cone etc.

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11
Q

Where does the centre of the macular region lie ?

A

4mm temporal

  1. 8mm inferior to the optic disc
    - this is where we do most of our day time vision
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12
Q

What does the macular region consist of ?

A

4 regions

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13
Q

What are the 4 regions of the macula?

A

A- foveola- central region
B- fovea- outside of foveola
C- parafovea- outside fovea
D- perifovea- outside parafovea

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14
Q

What is the centre of the macula region?

A

where foveola is .

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15
Q

What is the centre of the macula region?

A

where foveola is (characterised by a dip)

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16
Q

What is shown in the histological view/light micrograph o the centre of macula region?

A

only shows 5 layers of the retina the rest have been pushed aside

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17
Q

How can you see the foveal dip easily ?

A

using an OCT

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18
Q

What is the dip caused by?

A

by the inner 5 layers of the retina being ‘pushed aside

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19
Q

What is the dip a source of?

A

is the source of the ‘foveal reflex seen with an ophthalmoscope.

  • when you traverse the retina with an ophthalmoscope you get a foveal reflex which is a bright spot
  • The slope walls act as a mirror and Get a bright spot
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20
Q

What is a foveal reflex?

A

bright spot

21
Q

How is the distribution or rods and cones in retina?

A

not even in the retina

  • in some areas you get more cones
  • in other areas you get more rods
22
Q

What does the x axis show in graph on slide?

A

where you are in retina- horizontally

23
Q

what does y axis show in graph on slide?

A

density of photoreceptors

24
Q

What is the density of cones in the peripheral/temporal retina?

A

low

25
Q

What does the y axis show?

A

density of photoreceptors

  • black line with grey shading is density of cones]
  • light line density of rods
26
Q

What is the density of cones as we approach the macula region / esp the foveola?

A

high

27
Q

What is the density of cones as we approach the macula region / esp the foveola?

A

high

  • highest cone density is in the centre of foveola- where dip is in retina
  • and find no rods at all
28
Q

What does the fovea contain ?

A

The foveola contains a very high density of cones and no rods.

  • The density of rods increases, and that of the cones decreases, going towards the periphery.
  • rods reaches maximum in periphery
29
Q

What does the tangential section through foveolar receptor outer segments show?

A

through foveolar receptor outer segments revealing a homogenous ‘mosaic’ composed of only cones
=photoreceptors are much smaller and of one type
-only cones high density in the centre

30
Q

What does the section of the ellipsoid layer of rods and cones show?

A

Parafoveal retinal ‘mosaic’ cut at the level of the inner segments showing both rods and cones

  • large number of rods and less number of cones in periphery
  • cones big
31
Q

Sumamrise the distribution of the rods and cones in the retina

A

central foveola- high density of cones

periphery-high density of rods

32
Q

What is the density of cones in the macula region?

A

very high in the fovea (161,900 cones mm-2),

-however not all cones are here

33
Q

How many cones are in the foveola?

A

foveola is a small retinal area - so if you work out area and look at density of cones- in foveola we have 60,00 cones out of 5million cones we have in total

  • only 1% of our cones are in the foveola
  • just that in foveola the density of cones is very high compared to the small retinal are
34
Q

What are the peripheral cones like ?

A

‘short and fat’

35
Q

What are the cones in the foveola like?

A

long and thin

36
Q

What is the difference in cones from the periphery to the centre of macula region?

A

decrease in thickness is so you can get more of them in their to get a higher density
-in order to reach a high packing density in foveola for them to be packed of lots of them.

37
Q

Where do the peripheral and outer segments of the cones stop ?

A
  • peripheral cones usually stop short of the RPE and the outer segments of cones within the fovea go all the way to the RPE.
    (NB the outer segment discs are still attached to the plasma membrane, and not free floating)
38
Q

What do the cones in the foveola look like?

A
  • look much more like rods- long and thin - which is a rod characteristic
  • in order to make cones have a. high packing density- they need to be reduced in thickness
  • However they still have (NB the outer segment discs are still attached to the plasma membrane, and not free floating)
39
Q

What are the cell bodies of the foveolar cones connected to ?

A

are connected to their synaptic region via Henle fibres (-axons of the cone photoreceptors and they make contact with bipolar and horizontal cells in the outer plexiform layer )

  • run towards periphery of retina
  • the synapses occur in the periphery- and inner 5 layers of retina have been pushed aside
40
Q

What is the sturcture foveola?

A

avascular- no inner retinal blood vessels in a region in the fovea.- due to inner 5 layers being pushed aside( as they contain the blood vessels)

41
Q

What is the foveola?

A
  • foveola region have the highest acuity- highest resolving power- due to cones being densely packed- can sample the image more frequently and in crisp detail
  • cones are tightly packed ensuring a higher ‘sampling frequency’.
  • these are the factors which result in high acuity in fovea
42
Q

What do the layers being pushed aside mean?

A

This, along with the inner 5 layers of the retina being ‘pushed aside’ hence no blood vessels, ensures that light scatter is minimised.

43
Q

What do the the Henle fibres and Müller cells contain in the macula region ?

A

the macula pigment, which is a mixture of the carotenoids zeaxanthin and lutein

44
Q

Why does the macula region have a slight yellow tinge?

A

This yellow pigmentation removes short wavelengths, (blue) which serves to reduce the amount of scatter (as short wavelengths are more prone to Rayleigh scatter) and lessen the amount of chromatic aberration. Potential acuity is thus further increased
-short wavelengths such as blue light is not good if you want good VA- as it is scattered more and prone to more chromatic aberration- lower VA- so put yellow pigment to stop this.

45
Q

What do Carotenoids also act as?

A

also act as antioxidants- they can absorb the damaging free radicals- can get carotenoids from eating the right food

46
Q

What does the macula region do?

A

serve to increase VA

47
Q

What is the retina protected by?

A

protected by the macula pigment as well as short wavelengths are over which help increase image quality

48
Q

Why is blue light not good?

A

blue light is not good if you want good VA- as it is scattered more and prone to more chromatic aberration- lower VA- so put yellow pigment to stop this.

49
Q

What does the Frozen section through the fovea of a rhesus monkey viewed in white light and blue light show ?

A

The yellow macular pigment appears dark area in blue light because short wavelengths are absorbed.
(fovea and foveola are contained with this macula pigment)

Neural Biology 1 (58 decks)

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