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BIOL 404 FINAL > Visual System > Flashcards

Visual System Flashcards

1
Q

what are the main challenges of the visual system?

A
  1. detecting and coding the light signals (brightness, frequency)
  2. use the light information for visual and non-visual behaviours
  3. dynamically modulating this process in a context-specific manner
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2
Q

light is detected by ____ in a thin layer of the eye called the _____

A

photoreceptors, retina

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3
Q

photoreceptors project to _____, which project to the ______

A

bipolar cells, retinal ganglion cells (RGC)

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4
Q

what make up the optic nerve?

A

retinal ganglion cells

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5
Q

information dealing with the _________, crosses over at the ______ before terminating in the _______ of the ________.

A

contralateral visual field, optic chiasm, lateral geniculate nucleus, thalamus

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6
Q

where is the light initially detected in the visual system?

A

photoreceptors in the retina

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7
Q

at what point do RGC axons cross over in the visual pathway?

A

at the optic chiasm

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8
Q

where do RGCs project after leaving the optic nerve?

A

to the lateral geniculate nucleus and other non-visual areas

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9
Q

where does the info from the LGN ultimately project to?

A

the primary visual cortex in the occipital lobe

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10
Q

Label this diagram:

A

A: optic nerve
B: optic chiasm
C: LGN
D: optic radiation
E: striate cortex
F: optic tract
G: hypothalamus
H: pretectum
I: superior colliculus

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11
Q

circadian timekeeping: what is the typical activity pattern of nocturnal mice?

A

they are more active at night

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12
Q

circadian timekeeping: how do mice respond to changes in the light/dark cycle?

A

they are sharply entrained to dark and light cycles; if you change the phase of the light/dark cycle, they can phase shift to be concurrent.

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13
Q

what is the significance of IPRGCs in circadian timekeeping?

A

detecting light even without rods or cones

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14
Q

if mice are kept in complete darkness, they still keep a ______ but their cycle shifts a bit every day.

A

12 hour endogenous rhythm

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15
Q

what is lost if lesion in the retina?

A

endogenous rhythm

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16
Q

_____ are maintained in the absence of _____

A

circadian cycles, rod and cone photoreceptors

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17
Q

what are IPRGCs?

A

intrinsically photosensitive retinal ganglion cells

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18
Q

what does maintained circadian cycle in the absence of rod and cone photoreceptors indicate?

A

that there must be some way of detecting light without rods or cones, and that there must be some other photopigment elsewhere in the retina.

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19
Q

what did the study involving fluorescent dye injection into the hypothalamic suprachiasmatic nucleus aim to invesitgate?

A

examine the response of ganglion cells projecting to the SCN to light stimulation

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20
Q

fluorescent dye injection into the hypothalamic suprachiasmatic nucleus: where did the dye travel to?

A

through the optic nerves back into the retina to label ganglion cells that project to the SCN

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21
Q

fluorescent dye injection into the hypothalamic suprachiasmatic nucleus: what was the observed response in ganglion cells to light?

A

depolarization

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22
Q

fluorescent dye injection into the hypothalamic suprachiasmatic nucleus: what happened to the ganglion cell response when cobalt (Cav) channel blocker) was applied?

A

depolarlization persisted even w the application, indicating an independent light response

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23
Q

fluorescent dye injection into the hypothalamic suprachiasmatic nucleus: how did it provide evidence for the cells generating their own light response?

A

isolating retinal GCs from the retina and finding persistent responses confirmed the generation of an independent light response

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24
Q

what was observed when measuring the response of ganglion cells to light?

A

depolarization in response to light, indicating light sensitivity

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25
Q

depolarization persisted even when _____ was applied

A

cobalt (Cav channel blocker)

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26
Q

As synaptic transmission depends on these voltage-gated calcium channels, this indicates…

A

that the cell generates its own light response through possession of a photopigment.

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27
Q

These types of RGCs are called _________: also known as ________.

A

intrinsically photosensitive retinal ganglion cells (IPRGCs), melanopsin-containing retinal ganglion cells

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28
Q

IPRGCs produce…

A

a large, sustained response at shorter wavelengths (blue light); this is why it is not advisable to look at a lot of blue light before bed.

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29
Q

IPRGCs encode for…

A

the absolute levels of intensity (if slowly adapting).

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30
Q

IRPGCs contain…

A

an opsin called melanopsin that drives this response; tagging this with GFP reveals 5 types of IPRGCs that stratify in different places in the inner plexiform layer.

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31
Q

melanopsin is encoded by…

A

the OPN4 gene

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32
Q

what are some type of IPRGCs involved in?

A

driving the pupillary light reflex (PLR): how pupils get smaller in brightness and larger in darkness.

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33
Q

what does OPN4 KO show?

A

OPN4 knockout mice using diphtheria toxin show severe deficits in the pupillary light reflex (PLR).

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34
Q

the _____ is the main system responsible for ______ in response to light

A

parasym, pupil constriction

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35
Q

Some IPRGC axons exit the optic tract and synapse at the ________.

A

pretectal olivary nucleus.
*others project to the LGN.

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36
Q

pretectal neurons _____ and project to the______

A

cross over, Edinger-Westphal nucleus

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37
Q

Preganglionic parasympathetic fibers travel with the _______ and synapse at the _______.

A

oculomotor (CN III; 3n) nerve, ciliary ganglion

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38
Q

The postganglionic parasympathetic neurons (short ciliary nerves) travel to and innervate…

A

the contraction of the iris sphincter muscle via ACh at the neuromuscular junction, resulting in pupil constriction

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39
Q

Ciliary ganglion cells provide _______ to the muscles of the iris. Contraction of these muscles result in ______.

A

acetylcholine input, pupil constriction

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40
Q

Both _______ and _______ systems are required for pupil dilation.

A

parasympathetic, sympathetic nervous

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41
Q

rods

A
  • Are very sensitive, used at dusk and night.
  • Saturate in moderate to bright light.
  • Have poor acuity (sharpness)
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42
Q

cones

A
  • Not very sensitive; only used at daytime.
  • Never saturate
  • Have good acuity
  • Provide colour vision in humans.
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43
Q

in dark conditions, cyclic nucleotide-gated (CNG) channels in the outer segment of the photoreceptor are…

A

constitutively open due to being bound by cGMP.

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44
Q

what is the ion movement in dark conditions?

A

Sodium and calcium ions move into the cell through open CNG channels.

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45
Q

what is the resting potential during dark conditions?

A

Depolarized, closer to 0 mV.

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46
Q

Define dark current in photoreceptors.

A

The continuous, constitutive firing activity of photoreceptors under dark conditions, maintained by open cyclic nucleotide-gated (CNG) channels and ion movements.

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47
Q

what balances the dark current?

A

K+ ions flowing in from the inner segment

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48
Q

in light conditions photoreceptors contain…

A

opsins (rhodopsin), which contain retinal pigments.

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49
Q

what happens when light hits the retina?

A

it absorbs the light energy and uses this to change from a cis to trans configuration.

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50
Q

what does the conformational change activate?

A

activates a G protein which activates downstream pathways.

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51
Q

what do these G proteins catalyze?

A

the exchange of GDP for GTP on multiple transducin molecules.

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52
Q

what does the activated G-alpha subunit do?

A

stimulates cGMP phosphodiesterase, which breaks down cGMP.

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53
Q

what happens with cGMP-gated channels?

A

are therefore deactivated, while the K+ channel stays active, and the photoreceptors are hyperpolarized and stop firing.

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54
Q

how is this pathway amplified?

A

one rhodopsin can activate many G proteins and In turn, a single G-alpha subunit can activate many cGMP phosphodiesterases, each of which can catabolize many cGMPs.

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55
Q

T/F: one photon can activate only one rhodopsin

A

true

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56
Q

It is difficult to turn off the amplification pathway once it is activated; this requires _____.

A

specific enzymes

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57
Q

Rhodopsin kinase:

A

phosphorylates activated rhodopsin to make it susceptible to binding to arrestin.

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58
Q

arrestin:

A

binds phosphorylated rhodopsin, effectively “arresting” the photoresponse.

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59
Q

what are three main points of control/regulation upon sustained illumination (decreased Ca2+ due to channels closing):

A
  1. RK - rhodopsin kinase
  2. GC guanylate cyclase
  3. cGMP-gated channels
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60
Q

Rhodopsin Kinase (RK) in regulation of photoresponse

A

increased rhodopsin kinase activity (which terminates residual rhodopsin activity) leads to depolarization.

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61
Q

Guanylate cyclase (GC) in regulation of photoresponse

A

converts GTP to cGMP; increased GC activity (higher cGMP levels) results in stronger depolarization.

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62
Q

cGMP-gated channels in regulation of photoresponse

A

are more sensitive, which results in stronger depolarization.

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63
Q

______ is the main modulator of the photoresponse.

A

calcium

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64
Q

When a photoreceptor is stimulated by a light flash, the response grows in a ________.

A

graded way (not all or nothing).

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65
Q

what does graded response allow for?

A

the weakest flash to be perceived, which is advantageous.

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66
Q

when the response eventually reaches saturation what happens

A

does not increase in amplitude, although it becomes increasingly sustained.

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67
Q

Through a series of enzymes, the all-trans retinol is converted back to a cis configuration in the pigment epithelium, what is the key enzyme for this?

A

RPE 65

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68
Q

where do photoreceptors lie?

A

in the outermost layer of the retina

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69
Q

when are photoreceptors depolarized?

A

in dark conditions

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70
Q

what happens with photoreceptors when they depolarize?

A

they release glutamate on bipolar cells, which then signal to ganglion cells.

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71
Q

the signal from photoreceptors is _____ until it reaches the _____; which need to….

A

graded, ganglion cells, transduce the graded signal into a binary signal (action potential).

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72
Q

The image, after passing through the lens of the eye and hitting the retina, is _______

A

inverted vertically and horizontally.

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73
Q

Binocular visual field: seen by both eyes.

A

Monocular visual field: seen by only a single.

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74
Q

The visual system is completely crossed…

A

the left visual hemifield is “seen” by the right visual cortex and vice versa.

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75
Q

The nasal part of the visual field for each eye stays ______, while the temporal part crosses over at the _______.

A

ipsilateral, optic chiasm

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76
Q

what is anopsia

A

a vision deficit

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77
Q

Bitemporal hetero nomous hemi anopsia:

A

lesion at optic chiasm; crossing over won’t occur and the temporal areas of the visual field won’t be perceived.

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78
Q

Left homonymous hemi anopsia:

A

lesion after optic chiasm on one side; crossing over occurs, but the information for one side of the visual field won’t be perceived.

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79
Q

Where are inputs from the two eyes relayed before reaching the visual cortex?

A

Dorsolateral lateral geniculate nucleus of the thalamus (dLGN).

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80
Q

how many layers of the LGN are there?

A

6

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81
Q

contralateral input projects to which layers?

A

1, 4, 6

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82
Q

ipsilateral input projects to which layers?

A

2, 3, 5

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83
Q

The inputs from the two eyes remain ______ in a _____

A

segregated, cell-specific manner (different cells types in different layers of the LGN).

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84
Q

parvocellular cells

A

located in LGN layers 3-6; are small, with small receptive fields, and are important for fine spatial acuity.
- Receive input from midget ganglion cells.

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85
Q

magnocellular cells

A

located in LGN layer 1-2; have large receptive fields, and are important for motion.

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86
Q

koniocellular cells

A

ocated in between the layers of the LGN; involved in colour vision.

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87
Q

Visual information is represented _______ in the visual cortex in the ______.

A

topographically, occipital lobe

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88
Q

the _____ divides the upper and lower visual cortex

A

calcarine sulcus
- The upper visual cortex receives the lower half of the visual field.
- The lower visual cortex receives the upper half of the visual field.

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89
Q

The fovea is …

A

disproportionately more represented spatially in the visual cortex than the periphery.

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90
Q

Information from one side of the visual field is processed by the

A

contralateral visual cortex.

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91
Q

how many cones in the retina?

A

5 million

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92
Q

where is there a concentration of cones?

A

fovea

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93
Q

fovea

A

a region about 1.5 mm in diameter.

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94
Q

our most acute vision (denoted by higher cone centrations) is limited to where?

A

the foveola, which is a region of diameter ~0.4 mm in diameter.

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95
Q

there are very few _____ in the center of the eye

A

rods

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96
Q

The foveal pit is devoid of rod photoreceptors and of secondary and tertiary neurons (bipolar and ganglion cells), allowing light to directly stimulate cones and give us ______

A

maximal visual acuity.

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97
Q

Although cones are primarily concentrated in the fovea, the surround does not appear blurry because we are always scanning an image using rapid, ballistic eye movements called ______.

A

saccades.

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98
Q

Saccades have a maximal velocity of..?

A

900 degrees per second.

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99
Q

Colour vision is mediated by three things:

A
  • Cone type
  • Retinal circuits (ON and OFF pathways)
  • Horizontal cell mediated surround inhibition.
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100
Q

We have three sensors in our eyes that detect three primary colours:

A

Red, green, and blue sensors.

101
Q

RED + GREEN = _____
RED + BLUE = ______
GREEN + BLUE = ______

A

CYAN, MAGENTA, CYAN

102
Q

Each cone and rod have a peak ____

A

spectral sensitivity.

103
Q

Short (S):

A

respond maximally to short wavelengths (blue).
- peak: around 430 nm.

104
Q

Medium (M):

A

respond maximally to medium wavelengths (green).
- Peak: around 525 nm.

105
Q

Long (L)

A

respond maximally to long wavelengths (red).
- Peak around 550 nm.

106
Q

Even if you only have one photoreceptor, it doesn’t mean that you will only see in that colour;

A

each photoreceptor can absorb other wavelengths outside of their maximal peak wavelength.

107
Q

Achromatic channels:

A

Compare activation of all three types of cones to measure brightness.
- L + M + S

108
Q

Red-green channels:

A

compare activation of L and M cones in an antagonistic way to measure red-green activation.
- L - M

109
Q

Yellow-blue channels:

A

compare combined red and green (which makes yellow) input with blue cones to measure yellow-blue activation.
- S - (L + M)

110
Q

ishihara colour test

A

Uses pseudochromatic plates to test colour vision.

111
Q

The outer plexiform layer contains synapses among and between…

A

retinal photoreceptors, and inner plexiform layer horizontal cells and bipolar cells.

112
Q

what type of synapses are in the OPL?

A

ribbon synapses

113
Q

What is the nature of synapses in the outer plexiform layer?

A

Many postsynaptic cells connect.
Vesicles loaded onto electron-dense ribbons.

114
Q

function of ribbon synapses

A
  • Supports tonic (sustained) vesicle release during depolarization.
  • Acts like a conveyor belt delivering vesicles to postsynaptic cells when depolarization occurs.
115
Q

vesicles release _____ across the synaptic clef

A

glutamate

116
Q

what is the IPL formed by?

A

by interlaced dendrites of RGCs and cells of the inner nuclear layer.

117
Q

what cell bodies are in the IPL?

A

Bipolar cells.
Horizontal cells.
Amacrine cells.

118
Q

What are the two main types of bipolar cells in the visual system?

A

off and on bipolar cells

119
Q

off bipolar cells:

A
  • Activated by darkening of the stimulus (depolarization of photoreceptor).
  • Separate dark input into a specific channel.
120
Q

on bipolar cells:

A
  • Activated by brightening of the stimulus (hyperpolarization of photoreceptor).
  • Glutamate is inhibitory for ON bipolar cells, mediated by the MGLUR6 receptor
121
Q

for ON bipolar cells _____ is inhibitory. this is mediated by _____

A

glutamate, MGLUR6 receptor

122
Q

What does mGluR6 do in visual processing?

A
  • Closes TRPM1 channels.
  • Activates G protein Go.
  • Fast kinetics due to dendritic tip localization
123
Q

What is the role of G protein Go in TRPM1 channel modulation?

A
  • Activated by mGluR6.
  • Alpha and gamma subunits close TRPM1 channels
124
Q

How does Guanine Nucleotide Exchange Factor (GEF) contribute to TRPM1 modulation?

A
  • Facilitates GDP release from G protein Go.
  • Enables GTP binding.
125
Q

What is the association between TRPM1 mutation and night blindness in Appaloosa horses?

A
  • TRPM1 mutation causes night blindness.
  • Homozygotes show no B wave response to light in ERG
126
Q

How is the TRPM1 gene related to the leopard complex in Appaloosa horses?

A
  • TRPM1 gene close to gene specifying leopard complex.
  • Both affect coat patterns in horses.
127
Q

What did the Electroretinogram (ERG) reveal in horses with the TRPM1 mutation?

A
  • A wave: Photoreceptor response.
  • B wave: ON-bipolar cell response.
  • Homozygotes showed no B wave response to light
128
Q

ON and OFF circuits in the visual system ________ throughout but exhibit some crosstalk

A

remain segregated

129
Q

Where do ON and OFF bipolar cells terminate in the inner plexiform layer (IPL)?

A

in different places in the inner plexiform layer (IPL).

130
Q

Where do OFF bipolar cells ramify, and to which ganglion cells are they connected?

A

closer to the outer retina and are connected to OFF ganglion cells.

131
Q

Where do ON bipolar cells ramify, and to which ganglion cells are they connected?

A

closer to the inner retina and are connected to ON ganglion cells.

132
Q

Why are graded potentials used in the retina?

A

don’t have to travel a large distance within the retina.

133
Q

Why do ganglion cells generate action potentials?

A

because they need to travel from the retina to the LGN (longer distance).

134
Q

How do ganglion cells transduce graded potentials into action potentials?

A
  • Graded potentials (analog) from bipolar cells (glutamate release) are transduced into action potentials (digital) in ganglion cells.
  • Disadvantage: Not continuous/descriptive.
  • Advantage: Can travel longer distances.
135
Q

When are OFF ganglion cells active, and what happens in response to light?

A
  • OFF ganglion cells are active when there is no light.
  • When photoreceptors hyperpolarize in response to light, no glutamate activates OFF bipolar cells, resulting in no activation of OFF ganglion cells.
136
Q

When are ON ganglion cells active, and what happens in response to light?

A
  • ON ganglion cells are active when there is light.
  • When photoreceptors hyperpolarize in response to light, the absence of glutamate means less mGluR6-mediated inhibition, allowing ON-bipolar cells to release glutamate onto ON-ganglion cells.
137
Q

Where do horizontal cells lie in the retina?

A

Horizontal cells lie in the Outer Plexiform Layer (OPL).

138
Q

What is the role of horizontal cells in the retina?

A

make inhibitory feedback synapses with cone photoreceptors.

139
Q

What type of receptors do horizontal cells contain?

A

AMPA receptors

140
Q

How are horizontal cells depolarized in the dark

A

continuously

141
Q

What is the reversal potential of cGMP channels?

A

0 mV

142
Q

How do K+ channels influence the reversal potential of cGMP channels?

A

K+ channels bring the cGMP channels’ potential down to -30 mV.

143
Q

What is the reversal potential of chloride channels in GABA receptors?

A

The reversal potential of chloride in GABA receptor channels is -60 mV.

144
Q

How does GABA feedback inhibition affect the potential of photoreceptors?

A

brings the potential of photoreceptors down to around -40 mV.

145
Q

what components contribute to the dark potential?

A

relies on cGMP channels, K+ channels, and GABA receptor chloride channels.

146
Q

What happens when a GABA antagonist is applied to a photoreceptor?

A

results in more depolarization.

147
Q

What is surround inhibition in the context of retinal ganglion cells?

A

refers to stimulation to the center of the receptive field causing excitation, while stimulation to the surround of the receptive field causes inhibition.

148
Q

How do horizontal cells contribute to surround inhibition in retinal ganglion cells?

A

mediates surround inhibition to retinal ganglion cells.

149
Q

What does illumination cause in the center photoreceptor of the receptive field?

A

causes center photoreceptor hyperpolarization.

150
Q

How do horizontal cells respond to center photoreceptor hyperpolarization?

A

causes horizontal cell hyperpolarization.

151
Q

What is the consequence of center photoreceptor hyperpolarization on the surround photoreceptor?

A

causes surround photoreceptor depolarization.

152
Q

What is the resulting effect on center and surround ganglion cells due to the interaction of photoreceptors and horizontal cells?

A

more excitation of center ganglion cells and inhibition of surround ganglion cells.

153
Q

How do horizontal cells respond to spot size in the context of surround inhibition?

A

can mediate a surround to large spots that activate them, but activate minimally for small spots.

154
Q

What contributes to red-green opponency in ganglion cells?

A

through the combined inputs of horizontal and bipolar cells.

155
Q

How is red-green opponency exemplified in a red-ON/green-OFF opponent cell?

A

receives:
- Excitatory L-cone input to the receptive-field center.
- Inhibitory M-cone input to the receptive-field surround.

156
Q

From which layers of the inner retina do ganglion cell dendrites collect?

A

both layers of the inner retina.

157
Q

Which cones do yellow bipolar cells collect from, and to which layer do they connect?

A

Collect from red and green cones.
Connect to the OFF layer.

158
Q

Which cones do blue bipolar cells collect from, and to which layer do they connect?

A

Collect from blue cones.
Connect to the ON layer.

159
Q

How do yellow and blue bipolar cells converge in the blue-yellow pathway?

A

converge at a blue-ON/yellow-OFF ganglion cell.

160
Q

What was the experimental setup to study receptive fields in cats?

A

A light stimulus was presented to a cat.
- Action potentials were recorded from an electrode.

161
Q

What was observed regarding retinal ganglion cells during the experiment?

A
  • There was a spot that corresponded to a maximal response in a retinal ganglion cell.
  • Moving away from this spot resulted in less response.
  • Retinal ganglion cells can have concentric receptive fields.
162
Q

Describe the responses of on-center retinal ganglion cells.

A
  • Moving to the edge of the center results in a drop in firing rate.
  • Moving into the surround results in a lower firing rate than the basal firing rate.
163
Q

What is responsible for the inhibition observed in on-center retinal ganglion cells?

A
  • Horizontal cells can’t be responsible as they only respond to large spots of light.
  • Instead, amacrine cells are involved in this inhibition.
164
Q

What are the abbreviations for the layers in the retina?

A

ONL: Outer Nuclear Layer
OPL: Outer Plexiform Layer
INL: Inner Nuclear Layer
IPL: Inner Plexiform Layer
GCL: Ganglion Cell Layer

165
Q

What do HC, BC, AC, and RGC stand for?

A

HC: Horizontal Cell
BC: Bipolar Cell
AC: Amacrine Cell
RGC: Retinal Ganglion Cell

166
Q

Describe the synaptic connections in the retina.

A
  • Photoreceptors make glutamatergic synapses with ON/OFF bipolar cells.
  • Bipolar cells make connections to RGCs.
167
Q

What is the role of amacrine cells in the retina?

A
  • Inhibitory: Release GABA and glycine.
  • Mediate center-surround inhibition.
168
Q

How was surround inhibition measured in the experiment mentioned?

A

Voltage-Clamp Mode: Voltage clamped at -60 mV.
Chloride Reversal Potential: No driving force for chloride.

169
Q

What happens when exposed to light in voltage-clamp mode?

A

GABAergic channels on amacrine cells open.
No chloride conductance due to the voltage clamp.

170
Q

How did the excitatory postsynaptic currents (EPSCs) change with the size of the light spot?

A

Maximal response at 150pA.
- Larger sizes result in a decrease in excitatory current for both ON and OFF ganglion cells.

171
Q

What are the two possible causes of the observed inhibition?

A
  1. Horizontal cells blocking photoreceptors.
  2. Bipolar cells being inhibited.
172
Q

What was the effect of applying a GABA antagonist picrotoxin in the experiment?

A

Inhibition reduced until around 400pA.
- Activation of GABA receptors required for surround/lateral inhibition

173
Q

How was the experimental setup modified to investigate spike activity?

A
  • Voltage held at 0 mV (glutamate reversal potential).
  • Introduction of a driving force for GABA receptors.
174
Q

What were the changes observed in inhibitory postsynaptic currents (IPSCs) with increasing diameter of the light spot?

A
  • IPSC at light onset increases and then levels off with diameter increase.
  • Indicates simultaneous excitation and inhibition of retinal ganglion cells.
175
Q

What role do ON bipolar cells and amacrine cells play in the observed response?

A
  • ON bipolar cells excite inhibitory amacrine cells.
  • Depolarization of amacrine cells leads to GABA release onto ON ganglion cells.
176
Q

What does the experiment reveal about the nature of retinal ganglion cell response?

A

Simultaneous excitation and inhibition of retinal ganglion cells demonstrated.

177
Q

What is the role of horizontal cells in lateral inhibition?

A
  • Illuminate and center photoreceptor hyperpolarizes.
  • This causes horizontal cell hyperpolarization.
  • Results in surround photoreceptor depolarization and ON-bipolar cell inhibition.
178
Q

How do amacrine cells contribute to lateral inhibition?

A
  • Receive depolarizing input from ON bipolar cells.
  • Make inhibitory connections with bipolar and retinal ganglion cells.
179
Q

What experimental approach was used to determine the inhibitory connection for lateral GABAergic inhibition?

A
  • Measured IPSCs of ganglion cells at 0 mV (no glutamate current).
  • Applied a sodium channel blocker (tetrodotoxin; TTX).
180
Q

What was the effect observed with the application of Tetrodotoxin (TTX) in the study?

A
  • Loss of IPSCs.
  • Indicates dependence on sodium channels opened in response to depolarization.
181
Q

In the study, what comparison was made between amacrine cells and horizontal cells?

A
  • Amacrine cells depolarize and fire action potentials (spikes) in response to light.
  • Horizontal cells get hyperpolarized.
182
Q

What was the conclusion regarding the mediation of inhibition in lateral inhibition?

A

Amacrine cells mediate inhibition over horizontal cells.
- Highlighted by the loss of IPSCs with TTX

183
Q

What does the loss of IPSCs with sodium channel blockade suggest?

A

Loss of IPSCs with TTX.
- Indicates involvement of sodium channels opened in response to depolarization.

184
Q

What conditions lead to the maximal response in retinal ganglion cells?

A
  • Illuminate the maximal amount of the center.
  • Illuminate the minimal amount of the surround.
185
Q

How does lateral inhibition contribute to the Mach bands illusion?

A

Along the boundary between adjacent shades of grey in Mach bands, lateral inhibition makes:
- Darker areas falsely appear even darker.
- Lighter areas falsely appear even lighter.

186
Q

How is spatial frequency measured in the context of vision, and what unit is used?

A

Spatial Frequency Measurement:
- Measured in terms of the number of cycles within one degree of visual angle.
- One cycle consists of one dark and one light bar.
Visual Acuity Standard (20/20 Vision):
- Corresponds to 30 cycles per degree.

187
Q

How is contrast defined in the context of vision, and what contributes to it?

A

Definition:
- Contrast in vision is due to the amplitude of a sine wave.
Visual System Sensitivity:
- The visual system is highly contrast-sensitive.
Contrast Sensitivity (CS):
- CS is the ability to perceive sharp and clear outlines of very small objects.
Effect of Low Contrast:
- At low contrast, it becomes harder to distinguish between white and black areas.

188
Q

How is the visual system tuned to specific spatial frequencies?

A

The visual system is tuned to a particular spatial frequency (SF).

189
Q

For a bar with a spatial frequency, what conditions lead to optimal activation of the center-surround mechanism?

A

High enough to activate the center but not too high for the surround, optimal activation is achieved

190
Q

What are the characteristics of the center and surround in the visual system’s center-surround mechanism?

A

Surrounds: Sensitive to low SF.
Center: Sensitive to high SF.

191
Q

How does the retina process information, considering the large number of photoreceptors and fewer optic nerve fibers?

A

In the retina, parallel pathways extract various image features such as motion, texture, and orientation.
With 10 million photoreceptors and fewer optic nerve fibers, the retina compresses and extracts information

192
Q

How many types of bipolar cells are there in the primate retina?

A

12 different types

193
Q

Where do bipolar cells collect information from, and to which layer do they send it?

A

collect information from photoreceptor dendrites in the outer plexiform layer and send it to ganglion cells in the inner plexiform layer

194
Q

How is the inner plexiform layer (IPL) stratified, and how do different types of bipolar cells contribute?

A
  • There are 5 strata in the IPL, and different types of bipolar cells project to different strata.
  • Bipolar cells terminating in the middle of the IPL are more rapidly adapting/transient.
  • Bipolar cells terminating near the edges of the IPL are more slowly adapting/sustained.
195
Q

How many types of amacrine cells are there, and what is a notable characteristic of their morphology?

A

There are 35+ types (sometimes 30-50) of amacrine cells.
- Amacrine cells exhibit a lot of morphological diversity.

196
Q

What is a common characteristic of the stratification of amacrine cells, and why is it significant?

A

Amacrine cells are usually multistratified.
- Multistratification allows for the combination of channel information (e.g., from ON and OFF layers), enabling early integration of information in the retina

197
Q

How many morphologically distinct types of retinal ganglion cells (RGCs) are there?

A

35

198
Q

Red-green & blue-yellow surround RGCs:

A

Red-green & blue-yellow surround RGCs:

199
Q

Midget RGCs:

A

Have small receptive fields, responsible for high spatial acuity.

200
Q

Magnocellular Cells:

A

Have large arbors, responsible for sensing motion.

201
Q

What is the significance of mosaics in the retina, and why are they essential?

A
  • Each retinal cell type forms a complete mosaic.
  • Mosaics are repeated all over the retina to effectively sample the visual world.
  • Ensures even feature selectivity across the retina.
  • Any part of the retina has almost all of the cell types.
202
Q

What is the concept of “Rod Piggybacking”?

A
  • Rod photoreceptors evolved separately from cones.
  • Activation of one rod by light activates a single ON-bipolar cell.
  • A2 amacrine cells “piggyback” through electrical and glycinergic synapses.
203
Q

How do rod photoreceptors connect to the cone pathway through piggybacking?

A

Activation of one rod by light activates a single ON-bipolar cell.

204
Q

What role do A2 amacrine cells play in rod piggybacking?

A
  • A2 amacrine cells “piggyback” through electrical and glycinergic synapses.
  • electrical Synapses: Sign-preserving to ON-bipolar cells in the cone pathway.
  • Glycinergic Synapses: Sign-inverting to OFF-bipolar cells in the cone pathway.
205
Q

How is the visual information processed in the LGN, and how are the pathways segregated?

A
  • 80-90% of input into the LGN comes from various sources.
  • Magno-, Parvo-, and Koniocellular pathways remain segregated in the LGN.
206
Q

Where do magnocellular cells send their input in the LGN?

A

Send input to layers 1 and 2 (magnocellular layers) of the LGN.

207
Q

Where do midget ganglion cells send their input in the LGN?

A
  • Send input to layers 3, 4, 5, and 6 (parvocellular layers) of the LGN.
  • Layers remain separated in layer 4 of the visual cortex.
208
Q

What do neurons in the primary visual cortex selectively respond to, and how was this studied?

A
  • Neurons in the primary visual cortex respond selectively to oriented edges.
  • Studied through responses to visual stimuli in cats
209
Q

Describe the experimental setup in the study on orientation selective neurons.

A
  • Small spots of light to the cornea did not evoke responses.
  • Neurons responded when a bar of a specific orientation was presented
210
Q

What did researchers find when they pushed an electrode through different layers of the visual cortex?

A
  • Different layers exhibited similar receptive fields with some overlap.
  • Similar orientation tuning across layers
211
Q

What type of organization was observed in the tuning preferences of neurons in the primary visual cortex?

A

Tuning preferences (e.g., orientation) exhibited a columnar organization in V1.

212
Q

How is orientation preference mapped in the primary visual cortex?

A
  • Functional imaging reveals orderly mapping of orientation preference.
  • Different preferred orientations are organized in pinwheel clusters.
213
Q

What happens to the orientation information in the primary visual cortex

A

Orientation information is processed and integrated in higher visual processing centers.

214
Q

What additional feature, besides orientation selectivity, are cells in V1 sensitive to?

A

Cells in V1 encode not only for orientation selectivity but also for the direction of motion.

215
Q

What is the function of ocular dominance columns in V1?

A

Cells in V1 of a certain hemisphere process visual input from one receptive field but receive input from both eyes.

216
Q

How are the inputs from the eyes organized in ocular dominance columns?

A

Ocular dominance columns segregate input from the two eyes and are organized in an alternating pattern.

217
Q

What relationship was observed between ocular dominance columns and orientation columns?

A

While no obvious relationship was found between orientation and ocular dominance columns, the center of pinwheel orientation columns often aligned with the center of ocular dominance columns.

218
Q

What happens if there is damage to one eye?

A

If one eye is damaged, input from the other eye can take over the ocular dominance columns of the damaged eye

219
Q

What is the primary function of the striate cortex?

A

The striate cortex is involved in processing visual information and is the first cortical visual area that receives input from the lateral geniculate nucleus in the thalamus.

220
Q

Where does the mixing of pathways from the two eyes first occur in the visual system?

A

Mixing of pathways from the two eyes occurs in higher areas of the striate cortex.

221
Q

Where does the integration of information from both eyes first start in the visual system?

A

In the striate cortex, there is an overlap of ocular dominance columns, marking the region where information from both eyes begins to be integrated together.

222
Q

What is a requirement for binocular fusion?

A

Binocular fusion, the merging of input from both eyes, requires retinal correspondence or parity.

223
Q

What is the horopter?

A

The horopter is an imaginary 3D surface in front of the viewer. It includes the object being fixated on and all other points in 3D space that project to corresponding positions in the two retinas.

224
Q

What are the two types of retinal disparities?

A
  1. Crossed Disparity:
    Greater difference for close objects.
  2. Uncrossed Disparity:
    Smaller difference for distant objects.
225
Q

What is stereopsis?

A

Depth cues processed from information from both eyes by comparing retinal images. Analyzing the disparity between these images allows the determination of depth and distance of objects in the visual field

226
Q

How is stereopsis processed in the visual cortex?

A

coincidence detector neurons receive input from both eyes and encode for the disparity. They only fire for a specific disparity, enabling the perception of depth and spatial relationships

227
Q

Under what conditions do coincidence detector neurons respond?

A

The neuron responds only when a line is simultaneously presented to both eyes and has the correct:
Orientation
Direction of motion
Binocular disparity

228
Q

Name some cues for depth perception other than binocular disparity.

A

Interposition (occlusion)
Relative size
Linear perspective
Cast shadows

229
Q

Define size constancy.

A

The perception of an object as having a fixed size, regardless of changes in the visual angle due to variations in distance.

230
Q

What are the ventral and dorsal pathways in visual processing?

A

Ventral Pathway:

Location: From occipital to temporal lobe.
Function: Involved in object identification and recognition.

Dorsal Pathway:

Location: From occipital to parietal lobe.
Function: Involved in processing the object’s spatial location relative to the viewer.

231
Q

Describe the dissociation observed in patient DF’s visual processing.

A

Damage: Inferior temporal lobe (ventral stream).
Observation: Inaccurate perception of envelope slot orientation but proficient performance when instructed to mail a letter through it.

232
Q

What are the three levels proposed by David Marr to understand brain function?

A
  • Computational Level: Why? Identifies the problem the brain is trying to solve.
  • Algorithmic Level: What rules? Specifies the algorithm the brain uses for problem-solving.
  • Implementational Level: How? Explains how the algorithm is physically implemented in the system
233
Q

What features do studies show that neurons in the primate somatosensory cortex code for?

A

Neurons code for stimulus features such as orientation (angle) and direction (movement) in Area 2 (S2)

234
Q

How is direction selectivity studied in the barrel cortex of rodents?

A

Neurons in the barrel cortex respond differently when whiskers are moved in various directions, showing preference for specific directions.

235
Q

What is essential for animal communication and human speech in the auditory system?

A

Neurons in the primary auditory cortex respond preferentially to the direction of FM (frequency-modulated) sweeps, crucial for communication and speech.

236
Q

How many types of ON-OFF DSRGCs are there, and what do they encode for?

A

4, Each type encodes for cardinal (NSEW) directions.

237
Q

What is the optokinetic reflex, and which type of retinal ganglion cells drive it?

A

Eye movement to track a moving object.
- ON DSGCs drive the optokinetic reflex during pursuit movements.

238
Q

What happened when picrotoxin (GABA receptor antagonist) was applied to direction-selective retinal ganglion cells (DSRGCs)?

A

Picrotoxin abolished direction selectivity, specifically in the null direction.

239
Q

In voltage clamp experiments, what did researchers discover about GABA’s role in direction selectivity?

A

Clamp Potential: At 0mV (glutamate reversal potential), isolating inhibitory currents showed minimal response.
Clamp Potential: At -60 mV (chloride reversal potential), the firing response remained similar.
Conclusion: GABA doesn’t mediate bipolar cell connections to RGCs; instead, Startburst Amacrine Cells (SACs) mediate the inhibition in the null direction.

240
Q

What is the role of Startburst Amacrine Cells (SACs) in direction selectivity?

A

SACs are only activated in the null direction, inhibiting Direction Selective Retinal Ganglion Cells (DSRGCs).

241
Q

What neurotransmitters are associated with Starburst Amacrine Cells (SACs)?

A

GABAergic: SACs release GABA.
Cholinergic: SACs release acetylcholine (Ach)

242
Q

What is the consequence of ablating Starburst Amacrine Cells (SACs)?

A

Loss of Direction Selectivity: Ablation of SACs leads to the loss of direction selectivity, particularly in the mediation of inhibition in the null direction.

243
Q

Where in the visual system is direction selectivity first observed?

A

Distal Dendrites: Direction selectivity is first observed at the distal dendrites of SACs, where GABA is released.
Note: Acetylcholine (Ach) is also released in this region.

244
Q

How do dendrites of Direction Selective (DS) cells stratify in the retina’s inner synaptic layer?

A

Two Levels: DS cells extend dendrites into two levels of the retina’s inner synaptic layer.
ON and OFF Sublayers: One set of dendrites stratifies in the ON sublayer, and the other set stratifies in the OFF sublayer.

245
Q

From which cells do DS cells receive excitatory input, and what do these cells respond to?

A

ON and OFF Bipolar Cells: DS cells receive excitatory input from ON and OFF bipolar cells.
Response: These bipolar cells respond to objects either brighter (ON type) or darker (OFF type) than the background.

246
Q

How do DS cells interact with Starburst Amacrine Cells (SACs)?

A

Extensive Contact: DS cells contact extensively with SACs.
SAC Subtypes: SACs come in two subtypes, one in the ON sublayer and one in the OFF sublayer.

247
Q

How do SACs respond to motion, and what is their role in direction selectivity?

A

Dendritic Ca2+ Signals: SACs generate larger dendritic Ca2+ signals when motion is from their somata towards their dendritic tips.
Role: SACs play a role in direction selectivity by integrating excitatory and inhibitory synaptic inputs for detecting objects brighter or darker than the background.

248
Q

What physiological aspects confirm direction selectivity in DS cells?

A

Spiking Response: Movement in the preferred direction evokes a depolarization and a burst of action potentials (spikes).
Excitatory and Inhibitory Conductance: Direction selectivity reflects in both excitatory and inhibitory synaptic inputs, which prefer opposite directions.
Convergence of Pathways: Four separate direction-selective synaptic pathways converge onto a single DS cell, involving excitatory and inhibitory synapses from ON and OFF pathways.

BIOL 404 FINAL (8 decks)

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