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MVST1B NHB > Vision L3 > Flashcards

Vision L3 Flashcards

1
Q

is the retina a thin neural sheet?

A

yep

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2
Q

The retina is a thin neural sheet which, in addition to the photoreceptors themselves, contains four major classes of interneurons:

what are they?

A

bipolar cells (BC),

horizontal cells (HC),

amacrine cells (AC) and

ganglion cells (GC)

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3
Q

There are also glial elements known as …

(retina sheet)

A

There are also glial elements known as Müller cells (MC).

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4
Q

terina has _ well defined layers

A

5 layers

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5
Q

These cells are distributed in five well-defined layers, which can be subdivided into three ____ layers (containing cell bodies) and two _____ layers (containing axons and cell processes).

A

These cells are distributed in five well-defined layers, which can be subdivided into three nuclear layers (containing cell bodies) and two plexiform layers (containing axons and cell processes).

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6
Q
A
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7
Q

descibe the outer uclear layer

A

photoreceptor cell bodies

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8
Q

describe the Outer plexiform layer (OPL

A

synapses between photoreceptors, bipolars and horizontal cells.

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9
Q

describe the Inner nuclear layer (INL)

A

bipolar, horizontal and amacrine cell bodies

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10
Q

describe the Inner plexiform layer (IPL)

A

synapses between bipolar, amacrine and ganglion cells.

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11
Q

describe the Ganglion cell layer (GCL)

A

cell bodies of ganglion cells.

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12
Q

describe the direct pathway of information flow in the retina

which neurotransmitter do they use?

A

The direct pathway of information flow is from photoreceptors to bipolar cells (IB/FB) to ganglion cells (G).

All of these cells use glutamate as neurotransmitter.

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13
Q

describe lateral inhibition

A

Lateral inhibition is mediated by horizontal cells (H) which are GABA-ergic.

Amacrine cells (A) mediate a diverse collection of interactions in the inner retina, and use many different transmitters.

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14
Q

Photoreceptors synapse only with _____ cells and -_____ cells;

A

Photoreceptors synapse only with bipolar cells and horizontal cells;

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15
Q

ganglion cells receive input from _____ and ______ cells.

A

ganglion cells receive input from bipolar and amacrine cells.

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16
Q

The output of the retina is carried by the axons of ganglion cells, which together form the _____ nerve.

A

The output of the retina is carried by the axons of ganglion cells, which together form the optic nerve.

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17
Q

Synaptic processing is characterised by both

____ and _____- of signals

A

Synaptic processing is characterised by both

convergence and divergence of signals

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18
Q

Ganglion cell axons are the ONLY output (together forming the optic nerve).

is there any input into the retina?

A

Ganglion cell axons are the ONLY output (together forming the optic nerve).

No efferent input to retina

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19
Q

Apart from ______ cells, all use ______ potentials

A

Apart from ganglion cells, all use graded potentials

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20
Q

which retinal cells use APs

A

ganglion cells

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21
Q

amacrine cells usually only fire spikes in response to _____ stimuli.

A

amacrine cells usually only fire spikes in response to strong stimuli.

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22
Q

why are graded potentials used in the retina?

A

Graded potentials are a more efficient means of

transmitting information over short distances.

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23
Q

Overall, the retina exhibits ________, possessing 6 million cones, 120 million rods, but only 1.5 million ganglion cells

A

Overall, the retina exhibits convergence, possessing 6 million cones, 120 million rods, but only 1.5 million ganglion cells

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24
Q

is divergence even across the retina?

A

the retina does not sample and process visual information uniformly.

The highest resolution is achieved only in foveal regions where there are ~3x more ganglion cells than cones (net divergence); but in peripheral retina there is only one ganglion cell for every ~16 cones (net convergence and loss of spatial information).

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25
Q

describe photoreceptro synapses?

A

Photoreceptor synapses have a characteristic presynaptic ribbon: a modified presynaptic density characteristic of synapses that transmit graded signals. The postsynaptic targets always include processes from both bipolar and horizontal cells, leading to the term synaptic triad.

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26
Q

describe the cone pedicle?

A

Cone terminals end in a large synaptic swelling - the cone pedicle. There may be up to 30 synapses at each cone pedicle, reflecting divergence of the cone signal to numerous bipolar cells. Cones form both invaginating and flat synaptic contacts (more on this later).

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27
Q

describe the rod spherule?

A

Rod synaptic terminals (spherules) are smaller forming only one synapse per rod (no divergence). However many rods feed into a single bipolar cell, indicating convergence and a loss of spatial resolution.

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28
Q

define a receptive field

A

A visual cell’s receptive field can be defined as the area on the retina (or its projection in the visual field) from which its activity can be influenced by light.

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29
Q

what are centre surround receptive fieldss?

A

Retinal neurons typically exhibit centre- surround receptive fields, with a circular receptive field centre and a concentric surrounding annulus.

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30
Q

on and off centre pathways locate what regions of images?

A

bright and dark regions / boundaries

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31
Q

describe an on centre receptive field

A

An on-centre receptive field is excited by light in the central region and inhibited by illumination of the surrounding annulus.

32
Q

describe an off centre reeptive field

A

Conversely, an off-centre receptive field is inhibited by illumination of the centre and excited by the surround.

33
Q

Centre-surround antagonism provides an example of ……

A

a general principle found in most sensory systems. The subdivision into on- and off-centre pathways allows the localisation of bright and dark regions of the visual image.

34
Q

Off-centre bipolar cells _______, like the photoreceptor, to a “centre” stimulus, but ________ to a “surround”.

A

Off-centre bipolar cells hyperpolarize, like the photoreceptor, to a “centre” stimulus, but depolarize to a “surround”.

35
Q

decribe how off centre photoreceptotrs interact with bipolar cells?

A

In the dark the cone continually releases transmitter (glutamate) which depolarizes the bipolar cell by opening cation channels (ionotropic glutamate receptors).

When the cone is illuminated, it hyperpolarizes, transmitter release is reduced and therefore the bipolar cell also hyperpolarizes.

36
Q

describre how the surroudning inhibition is created in off-centre receptive fields

A

The antagonistic surround is generated by lateral inhibition mediated by horizontal cells, which have broad dendritic fields, collecting inputs from a large number of cones, and which form inhibitory feedback synapses back onto the photoreceptors. Horizontal cells also hyperpolarize in response to light and therefore when only the surround is stimulated the central photoreceptor (now not illuminated) will depolarise (as it receives less inhibition from horizontal cell feedback).

37
Q

describe how on centre bipolar cells repsond?

A

On-centre bipolar cells respond in the opposite way, depolarizing to a centre stimulus, via a metabotropic glutamate receptor (mGluR6).

Like rhodopsin, this activates a G protein resulting in the closure of cation channels in response to photoreceptor transmitter release in darkness (a sign-inverting synapse).

Details of the transduction cascade are unclear, but it may operate via direct inhibition of the channel by the G protein α subunit.

When light hyperpolarises the photoreceptor, thereby decreasing transmitter release, this inhibition is relieved and the channels open, so the on- centre bipolar cell depolarizes.

38
Q

T or F

Every cone contacts both on- and off-centre bipolar cells.

A

T

39
Q

good picture

A
40
Q

Sign-inverting synapses to on- centre bipolar cells are formed at the ______ contacts,

A

Sign-inverting synapses to on- centre bipolar cells are formed at the invaginating contacts,

41
Q

Sign-preserving synapses to off-centre bipolar cells correspond to _____ contacts, using ionotropic AMPA receptors.

A

Sign-preserving synapses to off-centre bipolar cells correspond to flat contacts, using ionotropic AMPA receptors.

42
Q

T or F

every coen contacts both Diffuse & Midget Bipolar cells

A

Diffuse & Midget Bipolar cells

21

43
Q

describe midget bipolar cells?

A

In primates:

most bipolar cells are midget bipolar cells,

which receive input from just one cone

and have small receptive fields.

They thus have the potential to signal the finest spatial detail in the image.

Their signals are also colour-specific.

44
Q

describe diffuse bipolar cells

A

Diffuse bipolar cells have:

larger receptive fields,

and collect input from 5-10 cones.

Convergence means that their responses are more sensitive,

but spatial detail and colour signals are lost.

45
Q

Both diffuse and midget bipolar cells come in on- and off-centre classes.

T or F

A

T

46
Q

describe what is meant by parallel streams in the retina?

A
47
Q

Bipolar-to-ganglion cell synapses are _______

A

Bipolar-to-ganglion cell synapses are excitatory

48
Q

Bipolar-to-ganglion cell synapses are excitatory, consequently the ganglion cells have a _____ receptive field structure to bipolar cells.

A

Bipolar-to-ganglion cell synapses are excitatory, consequently the ganglion cells have a similar receptive field structure to bipolar cells.

49
Q

how is the receptive field of ganglion cells modified to that from bipolar ells?

A

lateral interactions from amacrine cells

50
Q

Bipolar signals transmitted to ganglion cells by ______ (ionotropic) glutamatergic synapse in the ______ plexiform layer

A

Bipolar signals transmitted to ganglion cells by excitatory (ionotropic) glutamatergic synapse in the inner plexiform layer

51
Q

An on-centre cell will respond when …..

A

An on-centre cell will respond when a light turned on in the centre of its field, or when a light is turned off in its surround.

52
Q

A diffuse light covers both excitatory and inhibitory regions, which is outputted as…

A

A diffuse light covers both excitatory and inhibitory regions, which cancel each other out, leaving at most only a weak response.

53
Q

picturw

A
54
Q

ganglionic cells fire….

A

APs

55
Q

activity of ganglion cells is determined by ?

A

Activity determined by balance of stimulation in centre vs surround

56
Q

Activity determined by balance of stimulation in centre vs surround.

for ganglionic cells - what is the effect of this

A
  • Avoids saturation
  • Emphasises borders
  • Ignores redundant information
57
Q

for ganglionic cells:

Lateral inhibition emphasises …

A

Lateral inhibition emphasises contrast borders

58
Q

Lateral inhibition stresses borders.

T or F

A
59
Q

2 classes of ganglion cells?

A

magnocellular (M) and parvocellular (P) classes (again, both these classes come in “on” and “off” centre cells).

60
Q

The majority (80%) of primate ganglion cells belong to the _________ (_) class (also known, rather confusingly as midget ganglion cells)

A

The majority (80%) of primate ganglion cells belong to the parvocellular (P) class (also known, rather confusingly as midget ganglion cells)

61
Q

do parvocellular cells give a more sustained response?

A

yep

62
Q

describe parvocellular ganglion cells

A
  • give a more sustained response
  • smaller dendritic fields
  • in the fovea the centre of the receptive field collects signals from only one cone via one midget bipolar cell
  • Their antagonistic surround is made up of complementary (other colour) cone input.
  • Alternatively, the receptive field centre might be driven by a blue cone and the surround by both green and red cones to yield yellow.
63
Q

are parvocellular ganglion cells colour encode?

A

yep

64
Q

Parvocellular (80%) vs

Magnocellular (10%)

A
65
Q

describe magnocellular ganglion cells

A
  • large dendritic fields
  • collecting input (central) from many photoreceptors - including both R and G cones - via diffuse bipolar cells.
  • They respond more rapidly
  • generate transient responses to step changes in contrast.
  • They are more sensitive than P cells and have larger diameter axons (faster conduction velocity).
  • They project to distinct layers in the LGN and cortex, which appear to be specialised for detection and processing of motion stimuli.
66
Q

fill in table

A
67
Q

give 5 Parvocellular colour opponent classes

A
68
Q

describe amacrine cells

A
  • over 50 classes
  • not much known about function
  • Some have extensive dendritic fields and are likely to supplement the horizontal cells in contributing to inhibitory surrounds by feedback at both bipolar and ganglion cell level.
  • Others are likely to be involved in modulatory functions, such as the role of the dopaminergic amacrine cell in adjusting the eye for photopic and scotopic vision
69
Q

_ ganglion cells receive significantly more amacrine input than the _ cells; this may contribute to their transient

A

M ganglion cells receive significantly more amacrine input than the P cells; this may contribute to their transient

70
Q

Colour vision is also lost in the ______ intensity range, and the overall spectral sensitivity is shifted from ~560 nm (the average peak sensitivity of the red and green cones) to the rod peak sensitivity of ~_____ nm (______ shift)

A

Colour vision is also lost in the scotopic intensity range, and the overall spectral sensitivity is shifted from ~560 nm (the average peak sensitivity of the red and green cones) to the rod peak sensitivity of ~500 nm (Purkinje shift)

71
Q

describe signals paths in the fully dark adapted retine?

A

In the fully dark-adapted retina rods pass signals to specialised rod-bipolar cells (all of “on” type) which connect exclusively with rod AII amacrine cells. These connect via gap- junctions to “on centre” cone bipolar cells which transmit the signal to “on” ganglion cells in the usual way

72
Q

how massive is the convergence in the rod pathway

A

There is massive convergence in this pathway: (~1500 rods > 100 rod-bipolars > 5 AII amacrines > 4 cone bipolars> 1 ganglion cell); together with the rod’s ability to detect single photons, this accounts for our extraordinary sensitivity to low luminances.

73
Q

image for rod pathway convergence

A
74
Q

describe retianal processing at mesopic ranges?

A

At slightly higher intensities (twilight) an alternative simpler route involves electrical synapses from rod spherules directly to cone pedicles. The rod signals thereby hijack the cone pathway at mesopic intensities when both rods and cones can function.

75
Q

fat

A

mamba

MVST1B NHB (25 decks)

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