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Neural Biology 1 > retina- photoreceptor structure > Flashcards

retina- photoreceptor structure Flashcards

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1
Q

What does the photoreceptors account for?

A

the 2,3,4,5 layers of the retina

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2
Q

What do the photoreceptors have in common in the retina?

A

have the same basic structure

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3
Q

What is the structure of the photoreceptors?

A
  • Cell body (with nucleus )- nuclei of the photorecepptor make up the out nuclear layer of retina - layer 4
  • sclera - inner segment (part of photoreceptor)- it is divided by 2 sections - myoid(nearest to nucleus) and ellipsoid
  • the ellipsoid is connected to the final part of photoreceptor which is the Outer segment
  • Outer segment and ellipsoid are connected by a thin connective cilium
  • outer segment and inner segment make up the rod and cone layer - layer 2 of retina
  • then have external limiting layer - 3rd layer
  • then outer nuclear membrane - 4th layer
  • Going inwards - axon which connects cell body to synaptic region= photoreceptors make synapses with horizontal cells and bipolar cells in Outer plexiform layer- layer 5
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4
Q

What are the photoreceptor layers?

A

(2) Rod and cone layer
(3) External limiting membrane
(4) Outer nuclear layer
(5) Outer plexiform Layer

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5
Q

What are the 2 distinct type of photorecpetor?

A

There are two distinct types of photoreceptor; rods,
subserving low light level high sensitivity (scotopic)
vision, and cones, delivering high acuity but low
sensitivity (photopic) vision.

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6
Q

what are the two parts of the inner segment?

A

1.ellipsoids
- contain many
mitochondria
-power house of the photoreceptor

  1. myoid
    - is filled
    with endoplasmic
    reticulum and
    neurotubules
    - proteins are made
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7
Q

How do Cone inner segments appear?

A

are much ‘fatter’ than those of rods

  • because the ellipsoid contains much more mitochondria as cones need a lot more energy
  • photopic vision requires much more energy
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8
Q

What is the inner segment is connected by?

A

to the outer segment by a thin eccentrically placed cilium

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9
Q

What is the inner segment ?

A

is where the proteins are made with endoplasmic reticulum in myoid and where energy of the photoreceptor is produced which is in the ellipsoid

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10
Q

What does a tangential section reveal ?

A

that the connecting cilium which connects inner segment to outer segment is made up of 9 microtubule doublets, indicating its similarity to things such as bacterial flagella

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11
Q

What are Calycal processes ?

A

Calycal processes extend from the inner segment to envelope the lower half of the outer segment

  • common in cones than rods
  • have 13 per cone
  • originates from ellipsoid
  • gives stability to the bottom half of the photoreceptor
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12
Q

What is the outer segment composed of?

A
  • composed of ‘discs’ whose membranes contain the visual pigment (so it is here that light is absorbed and transduction occurs)
  • The structure of the of the outer segments of rods and cones differ
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13
Q

What is in rod outer segment ?

A

This is composed of stacked membranous
discs which are free from the plasma membrane
-plasma membrane goes all around the outer segment
- contained within these plasma membranes are discs
-each rid outer segments have thousands of membrane bound discs within them
- and it’s within these discs are the visual pigment
-each disc is free floating/independent of its neighbour

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14
Q

So, what actually forms the rod outer segment ?

A

independent disc/ free floating disc to make up the rod outer segment

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15
Q

Where are new discs formed?

A
  • formed at the base of the outer segment by evagination of the connecting cilium membrane
  • connecting cilium buckles out and grows in size till it pinch off from the membrane to become free floating.
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16
Q

What are the new discs in rods?

A

In rods the new discs ‘pinch off’ from the plasma membrane

17
Q

How are the new discs formed in rods?

A

At the base of the rod outer segments some discs are are attached to the plasma membrane before they ‘pinch off’ and become ‘free floating’ in the rod outer segment

18
Q

What happens in new discs in cones?

A

Unlike in rods, cone discs do not ‘pinch off’ and stayed attached to the
plasma membrane - not free floating
-formation is same but just doesnt pinch off and not free floating

19
Q

What is the difference between rods and cones discs?

A

Rod outer segment discs are ‘free floating’/ not attached to plasma membrane, while those of the cones remain attached to the plasma membrane

20
Q

What does a tangential section of a rod outer segment show ?

A

it shows that the discs are lobulated.

whereas cone discs are smooth

21
Q

What is another difference of the rod and cone outer segments?

A
  • Rod outer segments go all the way to the RPE to be enveloped by it processes (‘like the fingers of a glove’/ long extensions)- which come from RPE
  • Cone outer segments stop short of the RPE (which then sends out long villi)- RPE have to send processes down to the cones- cone dont go all the way to RPE
22
Q

What is another structural difference of rod and cones?

A

Cone outer segments are sometimes conical, while those of rods tend to be more ‘rod-like’!

23
Q

What are the differences between Rods and cones?

A

-Rods - Scotopic vision (sensitivity)- low light level
Cones-Photopic vision (acuity)- high light level

Rods-Thinner inner segments Cones-Fatter inner segment as they have more mitochondria

Rods-Free floating outer segment discs
Cones-Discs attached

Rods-Outer segment reaches RPE
Cones-RPE sends fingers to outer segment

Rod-Outer segment ‘rod-like’ Cone-Outer segment often ‘conical’

Rods-Discs lobulated Cone-Discs smooth

Rods-Discs shed/renewed after dawn
Cones-Discs shed after dusk

Rods-(Small dark vitreal nuclei Cones-Nuclei lighter and larger and scleral

Rods-Synaptic spherule Cones- Synaptic pedicle)

Rods- One visual pigment Cones- 3 spectral pigment types

Rods - High gain enzyme cascade
Cones-Lower gain cascade

24
Q

What do little black dots show ?

A

where proteins are being made

25
Q

What is the technique of autoradiography?

A

-has contributed greatly to our understanding of outer segment disc renewal

26
Q

How do you do autoradiography?

A

-Radioactively labelled amino acids are injected and later visualised in histological section as black silver grains.

27
Q

What happens in rods when you inject radioactively amino acids?

A
  • when injected
  • after few hours, you find them in the myoid region
  • as the myoid region is where the E.R is
  • so amino acid are being incorporated into new proteins in the myoid regions
  • show as black grains
  • after a few hours later - the amino acids which are now part of proteins - become incorporated in Golgi apparatus- and then go to base of connecting cilium
28
Q

How is antoradiography done ?

A
  • proteins made inE.R
  • proteins go to Golgi apparatus - become modified and packaged
  • then transported to membrane at the base of connecting cilium
  • adding more new proteins to base of connecting cilium , the old ,membrane hence old proteins get pushed upward, you can see the connecting cilium membrane start to buckle and then it keeps adding and pushing upwards until you get newly formed discs at the base of rod.
  • Injected amino acid is incorporated into the visual pigment protein (opsin), which is inserted into the membrane of the connecting cilium. This buckles and forms a new disc containing the opsin
29
Q

What happens after the whole process?

A
  • New discs are clearly seen as a dense band of radioactivity at the base of the outer segment
  • This band of radioactivity moves up the outer segment with time, before being shed from its tip
  • this dark band of radioactivity
30
Q

summary of the fate of injected amino acids in rods ?

A
  • new proteins made in the myoid
  • they get incorporated into the membrane of connecting cilium
  • they form new discs at the base of the outer segment
  • with time as new discs get added the old discs get pushed up the outer segment until they reach the tip of the outer segment at which point they’re engulfed and destroyed by the RPE although some debris remains and accumulates as lipofuscin
31
Q

what happens to injected amino acids in cones?

A
  • when cones are injected with radioactive amino acids, events in the inner segment are the same as in rods.
  • the outer segment never has a dense band of radioactivity instead it becomes diffusely labelled
  • this is most likely because rod discs are individual entities, while cone discs are all attached via the plasma membrane
32
Q

when are cones shed?

A

shed at night ( when they are not being used )

33
Q

when are rods shed?

A

shed during the day

34
Q

what is retinitis pigmentosa caused by ?

A
  • RPE are unable to phagocytise the photoreceptor outer segment so all this debris fills up in the retina
  • this leads to blindness
35
Q

what happens in retinal detachment ?

A

retina falls away from RPE

Neural Biology 1 (58 decks)

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