Vision L3

Question 1

is the retina a thin neural sheet?

Answer 1

yep

Question 2

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

what are they?

Answer 2

bipolar cells (BC),

horizontal cells (HC),

amacrine cells (AC) and

ganglion cells (GC)

Question 3

There are also glial elements known as …

(retina sheet)

Answer 3

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

Question 4

terina has _ well defined layers

Answer 4

5 layers

Question 5

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).

Answer 5

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).

Question 6

Answer 6

Question 7

descibe the outer uclear layer

Answer 7

photoreceptor cell bodies

Question 8

describe the Outer plexiform layer (OPL

Answer 8

synapses between photoreceptors, bipolars and horizontal cells.

Question 9

describe the Inner nuclear layer (INL)

Answer 9

bipolar, horizontal and amacrine cell bodies

Question 10

describe the Inner plexiform layer (IPL)

Answer 10

synapses between bipolar, amacrine and ganglion cells.

Question 11

describe the Ganglion cell layer (GCL)

Answer 11

cell bodies of ganglion cells.

Question 12

describe the direct pathway of information flow in the retina

which neurotransmitter do they use?

Answer 12

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.

Question 13

describe lateral inhibition

Answer 13

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.

Question 14

Photoreceptors synapse only with _____ cells and -_____ cells;

Answer 14

Photoreceptors synapse only with bipolar cells and horizontal cells;

Question 15

ganglion cells receive input from _____ and ______ cells.

Answer 15

ganglion cells receive input from bipolar and amacrine cells.

Question 16

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

Answer 16

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

Question 17

Synaptic processing is characterised by both

____ and _____- of signals

Answer 17

Synaptic processing is characterised by both

convergence and divergence of signals

Question 18

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

is there any input into the retina?

Answer 18

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

No efferent input to retina

Question 19

Apart from ______ cells, all use ______ potentials

Answer 19

Apart from ganglion cells, all use graded potentials

Question 20

which retinal cells use APs

Answer 20

ganglion cells

Question 21

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

Answer 21

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

Question 22

why are graded potentials used in the retina?

Answer 22

Graded potentials are a more efficient means of

transmitting information over short distances.

Question 23

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

Answer 23

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

Question 24

is divergence even across the retina?

Answer 24

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).

Question 25

describe photoreceptro synapses?

Answer 25

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.

Question 26

describe the cone pedicle?

Answer 26

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).

Question 27

describe the rod spherule?

Answer 27

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.

Question 28

define a receptive field

Answer 28

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.

Question 29

what are centre surround receptive fieldss?

Answer 29

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

Question 30

on and off centre pathways locate what regions of images?

Answer 30

bright and dark regions / boundaries

Question 31

describe an on centre receptive field

Answer 31

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

Question 32

describe an off centre reeptive field

Answer 32

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

Question 33

Centre-surround antagonism provides an example of ......

Answer 33

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.

Question 34

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

Answer 34

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

Question 35

decribe how off centre photoreceptotrs interact with bipolar cells?

Answer 35

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**.

Question 36

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

Answer 36

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).

Question 37

describe how on centre bipolar cells repsond?

Answer 37

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.

Question 38

T or F ## Footnote Every cone contacts both on- and off-centre bipolar cells.

Answer 38

T

Question 39

good picture

Answer 39

Question 40

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

Answer 40

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

Question 41

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

Answer 41

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

Question 42

T or F every coen contacts both Diffuse & Midget Bipolar cells

Answer 42

Diffuse & Midget Bipolar cells 21

Question 43

describe midget bipolar cells?

Answer 43

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.**

Question 44

describe diffuse bipolar cells

Answer 44

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.**

Question 45

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

Answer 45

T

Question 46

describe what is meant by parallel streams in the retina?

Answer 46

Question 47

Bipolar-to-ganglion cell synapses are \_\_\_\_\_\_\_

Answer 47

Bipolar-to-ganglion cell synapses are excitatory

Question 48

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

Answer 48

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

Question 49

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

Answer 49

lateral interactions from amacrine cells

Question 50

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

Answer 50

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

Question 51

An on-centre cell will respond when .....

Answer 51

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.

Question 52

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

Answer 52

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

Question 53

picturw

Answer 53

Question 54

ganglionic cells fire....

Answer 54

APs

Question 55

activity of ganglion cells is determined by ?

Answer 55

Activity determined by balance of stimulation in centre vs surround

Question 56

Activity determined by balance of stimulation in centre vs surround. for ganglionic cells - what is the effect of this

Answer 56

* Avoids saturation * Emphasises borders * Ignores redundant information

Question 57

for ganglionic cells: ## Footnote Lateral inhibition emphasises ...

Answer 57

Lateral inhibition emphasises contrast borders

Question 58

Lateral inhibition stresses borders. T or F

Answer 58

Question 59

2 classes of ganglion cells?

Answer 59

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

Question 60

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

Answer 60

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

Question 61

do parvocellular cells give a more sustained response?

Answer 61

yep

Question 62

describe parvocellular ganglion cells

Answer 62

* 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.

Question 63

are parvocellular ganglion cells colour encode?

Answer 63

yep

Question 64

Parvocellular (80%) vs Magnocellular (10%)

Answer 64

Question 65

describe magnocellular ganglion cells

Answer 65

* 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.

Question 66

fill in table

Answer 66

Question 67

give 5 Parvocellular colour opponent classes

Answer 67

Question 68

describe amacrine cells

Answer 68

* 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

Question 69

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

Answer 69

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

Question 70

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)

Answer 70

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)

Question 71

describe signals paths in the fully dark adapted retine?

Answer 71

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

Question 72

how massive is the convergence in the rod pathway

Answer 72

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.

Question 73

image for rod pathway convergence

Answer 73

Question 74

describe retianal processing at mesopic ranges?

Answer 74

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.

Question 75

fat

Answer 75

mamba