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CNS > Unit 8 - Vision Physiology > Flashcards

Unit 8 - Vision Physiology Flashcards

1
Q

Colour of melanin

A

Brown

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2
Q

Vascularisation of iris

A

Highly vascularised

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3
Q

What cells are found near the iris SM

A

Pin cells

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4
Q

Overview of structures of eye

A
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5
Q

Where do we try and aim for light to hit

A

Fovea - centre of macula

Highest acuity - photoreceptors are packed in this region - (blindness called macular degeneration)

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6
Q

Function of outer layers of eye

A

Protective

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7
Q

What structures change the shape of the lens

A

Ciliary bodies

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8
Q

What structures absorb light

A

Pigmentation (melanin) in the choroid layer absorb light

Albinos have poor eyesight due to low levels of melanin (pink irises)

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9
Q

Accomodation by lens

A

Capacity to focus light on the retina

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10
Q

RODS

Vision type

Acuity

Adaptation

Number of rods in the eyes

A
  • Responsible for night vision
  • Very sensitive to light - low threshold so high level of photopigment
  • Low acuity - much amplification of signal, convergence and summation of signal at bipolar cells
  • Adapt slowly (summation in 100ms)
  • 100 million rods but less dense at fovea
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11
Q

CONES

Type of vision

Sensitivity (hence threshold)

Acuity

Different types

Region cones are found in

Resolution

A
  • Less sensitive ⇒ high threshold
  • High acuity - less signal amplification, little convergence
  • Concentrated in fovea (macula region - 3 million in each eye)
  • 3 types - red, green, blue
  • High temporal resolution (adapt early)
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12
Q

Change in MP as a result of light hitting rods & cones

A

Hyperpolarisation

Graded response

RMP is higher than in other cells

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13
Q

Structure of rod vs cone

A
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14
Q

Where do rods & cones lie in the retina

A

Choroid plexus lies beneath

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15
Q

What is present in higher conc in rods

A

Photopigment

⇒ more sensitive to light

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16
Q

Pigment present in the eyes

A

Rhodopsin (opsin + retinal)

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17
Q

Retinal isoforms

How is 1 changed to the other

A

11-cis retinal - non-activated rhodopsin

All-trans retinal - activated

Light alters shape of retinal molecule to trans

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18
Q

Which retinal isoform is activated

A

All-trans retinal

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19
Q

Chain of events once 11-cis retinal → all-trans retinal

A

Opsin and retinal split // metarhodopsin II (activated rhodopsin)

Excites electrical change in photoreceptors

Rods and different cones contain different isoforms of opsin

Varied absorption characteristics

Scotopsin (rods), photopsins (cones)

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20
Q

Scotopsin

A

Rods

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21
Q

Photopsins

A

Cones

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22
Q

What is retinal a derivative of

A

Vitamin A (carrots)

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23
Q

Prevalence of colour blindness among males vs females

A

8% males

0.5% females

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24
Q

Categories of photopsins

A

S (blue) 420 nm

M (green) 540 nm

L (red) 570 nm

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25
Q

What opsins are X linked

A

M & L

Constitute 90-95% of opsins

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26
Q

M cone and colour blindness

A

75% of colour blindness cases

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27
Q

Normal (trichromat)

A
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28
Q

Protanopia

A
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29
Q

Deuteranopia

A
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30
Q

S photopsin

A

blue

420 nm

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31
Q

M photopsin

A

green

540 nm

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32
Q

L photopsin

A

red

570 nm

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33
Q

MOA of dark current (happens in the dark)

A

Influx of Na+

Depolarisation

Membrane potential = -40 mV

cGMP splits Na+ channels open

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34
Q

Effect of activation of photopigment

A

Reduces cGMP levels (NB light causes hyperpolarisation)

GTP → cGMP → 5’GMP (INACTIVE)

GTP → cGMP // guanylyl cyclase

cGMP → 5’GMP // cGMP phosphodiesterase

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35
Q

Enzymes involved in metabolism of GTP

A

GTP → cGMP // guanylyl cyclase

cGMP → 5’GMP // cGMP phosphodiesterase

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36
Q

Function of cGMP

How is this affected by splitting of opsin and retinal

A

Opsin and retinal are split - metarhodopsin II created

This leads to LOWERING of levels of cGMP because metarhodopsin II activates cGMP phosphodiesterase

(NB - cGMP → 5’GMP // cGMP phosphodiesterase)

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37
Q

How does metarhodopsin II activate cGMP phosphodiesterase

what effect does this have

When is cGMP PD active

A

Via a G protein - transducin

Levels of cGMP are lowered

Phosphodiesterase is active in light

CLOSURE of cGMP gated ion channels

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38
Q

Flow of ions in the dark

MP created

A
  • Na+ inward current in OUTER segment
  • K+ outward current in inner segment
  • Creates MP of -40 mV
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39
Q

Movement of ions in the light

A

Na+ inward current is stopped

K+ outward current is maintained

Creates hyperpolarisation

40
Q

Arrangement of neurons in retina

A
41
Q

Axons of what structures form the optic nerve

A

Axons of ganglion cells

42
Q

Where is there a blind spot in our eye

A

No rods and cones where the optic nerve leaves the eye

43
Q

Where are cones found

A

Macula - specifically fovea

44
Q

What NT do rods and cones release

How does the release vary in dark and light

A

Glutamate

NT released in dark

Reduced glutamate release in light

Hyperpolarisation moves potential away from threshold, reducing the likelihood of NT being released

45
Q

What receptors are found on bipolar cells

A

Glutamate receptors

IONOTROPIC - AMPA and kainate

METABOTROPIC - mGluR6

46
Q

What does lateral inhibition allow

A

Transmission of high integrity info towards the brain

47
Q

Bipolar cells express ____

What effect does this have on the signal transmitted

A

EITHER ionotropic or metabotropic receptors

Equal and opposite respons to light signal

48
Q

Centre for metabotropic receptors on bipolar cells

A

On centre

49
Q

Centre for ionotropic receptors on bipolar cells

A

Off centre

(Signal passed to ganglion cells)

50
Q

Ionotropic glutamate receptors found on bipolar cells - MOA

A

AMPA and Kainate

Glu binding leads to activation of ligand gated Na+ ion channel and subsequent depolarisation of the bipolar cell

Off centre

51
Q

Metabotropic glutamate receptors found on bipolar cells - MOA

A

mGluR6

Glu binding leads to closure of cGMP gated Na+ channels and subsequent hyperpolarisation of bipolar cell

On centre

52
Q

Effect of lateral inhibition

A

Enhanced quality of sensory info through lateral inhibition

53
Q

What do the on vs off centres allow for

A

Contrast info which enables visual acuity

e.g.

enables ability to detect stimuli against a background

1/2 of cells check for stimuli brighter than background

Other 1/2 check for stimuli darker than background

= and opposite responses in on & off centre cells

54
Q

Pathway of light through cells of retina

NT of bipolar cells

what are the receptors on ganglion cells and what do they cause

A

Bipolar cells - Glu as NT

Ionotropic receptors on ganglion cells - AMPA, kainate and NMDA

Depolarisation produced in ganglion cells

APs generated

55
Q

Cells involved in lateral inhibition

A

Horizontal cells

Amacrine cells

+ve signal to 1 cell and -ve signal to another

APs generated

56
Q

P cells - parvocellular vision

A

Numerous

Small receptive fields (smaller jigsaw pieces - a piece of this retina is sent to brain and put back together in the visual cortex - carried ganglion cell by ganglion cell)

Colour sensitive

High visual acuity

High spatial resolution

57
Q

M cells - magnocellular division

A

Large receptive fields (large jigsaw pieces)

Fast conducting

Motion perception, rapidly changing detail (highly myelinated)

High temporal resolution

58
Q

K cells

A

Transmit colour blue (used to be part of P cell group)

59
Q

What does the arrangement of the visual pathway allow for

A

Depth perception

60
Q

Subcortical targets of visual pathway

A

Pretectum

Superior colliculus

Suprachiasmatic nucleus of hypothalamus

Lateral geniculate nucleus of thalamus

(cerebral cortex - perception of vision)

61
Q

Pretectum

A

Blinking response

62
Q

Superior colliculus

A

Head and eye movements (can see movement out of side of eye)

63
Q

Suprachiasmatic nucleus of hypothalamus

A

Circadian rhythms

64
Q

Lateral geniculate nucleus of thalamus

A

90% of ganglion fibres

65
Q

10% of ganglion fibres go to

A

Pretectum

Superior colliculus

Suprachiasmatic nucleus of hypothalamus

66
Q

Visual pathway

A
67
Q

Synonym for tunnel vision

A

Bilateral hemianopia

68
Q

Visual pathway structures

A
69
Q

Lateral geniculate nucleus

proportion of retinal axons

organisation

1/2 of representation is ______

How is it divided

A

90% of retinal axons

Retinotopic organisation/map (jigsaw)

50% of representation = fovea

6 layers of cell bodies separated by intralaminar layers

70
Q

Layer 1 & 2 (ventral)

A

Magnocellular layer

Receive input from M cells

Motion - big receptive field

71
Q

Layers 3-6 (dorsal)

A

Parvocellular layer

Receive input from P cells

Fine detail

72
Q

1, 4, 6

A

Contralateral nasal hemiretina

73
Q

2, 3, 5

A

Ipsilateral temporal hemiretina

74
Q

Breakdown of lateral geniculate nucleus

A
75
Q

Sensory signal from LGN →

A

4Cβ

76
Q

Magnocellular cells →

A

4Cα

(allows us to actually see)

77
Q

Difference between layers 3, 4, 5

A

3 & 5 - thicker in motor output

4 - thick in sensory cortex

78
Q

Organisation of cerebral cortex

A
79
Q

BA of primary visual cortex

A

17

visual area 1

6 layers of cells

80
Q

Where do inputs from LGN enter

A

In layer 4

81
Q

sublayers of layer 4 (where input from LGN enters)

A

4A

4B

4α

4β

82
Q

4Cβ

Axons of P cells terminate on simple cells - detail info

A
83
Q

4Cα

A

Axons of M cells terminate on simple cells

Motion info

84
Q

Layers 2 & 3

A

Axons from intralaminar region of LGN terminate in layers 2 & 3 where they innervate blobs - colour info

85
Q

Arrangement of layers

A
86
Q

Structure of blobs

A
87
Q

Magnocellular pathway

A

V1 → V2 → V3 → MT

Dorsal pathway

Posterior parietal cortex

V3 = orientation, position

MT = object movement

88
Q

Parvocellular pathway

A

V1 → V2 → V4

Ventral pathway

Inferior temporal cortex

V4 - colour, form, shape, texture

89
Q

M

A

Dorsal/parietal pathway

Motion

Change

90
Q

P

A

Ventral/temporal pathway

Form

Detail

91
Q

Where is the fovea found

A

Follow dotted line to retina

92
Q
A
93
Q
A
94
Q
A
95
Q
A
96
Q
A

CNS (36 decks)

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