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Systems Neuroscience > Vision > Flashcards

Vision Flashcards

1
Q

What is the sclera?

A

It is the tough outer covering of the eyeball.

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2
Q

What is the conjunctiva?

A

It is the thin transparent membrane that covers the back surface of the eyelids and the eyeball.

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3
Q

Name the two muscles that allow the iris to control how much light enters the eye.

A

Pupillary sphincter muscle.
Dilator muscle.

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4
Q

What is the role of the ciliary muscle?

A

It alters the shape of the lens to focus light onto the retina.

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4
Q

Does the lens get thicker or thinner to focus on nearby objects?

A

Thicker.

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4
Q

Does the lens get thicker or thinner to focus on distant objects?

A

Thinner.

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5
Q

What does the retina contain?

A

Blood vessels and photoreceptors.

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5
Q

Name the two main types of photoreceptors.

A

Rods.
Cones.

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6
Q

Which elements of vision do we perceive using rods?

A

Night and movement sensitivity.
Peripheral vision.

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7
Q

Which elements of vision do we perceive using cones?

A

Sharp details.
Central vison.
Colour.

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8
Q

Where in the retina is there a high density of cones?

A

The macula.

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9
Q

Why is there a high density of cones in the macula?

A

To facilitate the high resolution and detail that we perceive within an image.

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10
Q

Name the nine layers of cells and synapses that exist in the retina.

A

Pigment epithelium.
Photoreceptor layer.
Outer lining membrane.
Outer nuclear layer.
Outer plexiform layer.
Inner nuclear layer.
Inner plexiform layer.
Ganglion cell layer.
Nerve fibre layer.

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11
Q

Which cells are located in the pigment epithelium?

A

Pigmented cuboidal cells.

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12
Q

What are the two main functions of pigmented cuboidal cells?

A

They contain melanin with absorbs light not captured by the retina to protect the photoreceptors from damaging levels of light.
They provide glucose and essential ions to the photoreceptors.

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13
Q

Are there more cones or rods in the retina?

A

Rods outnumber cones approximately 20:1 across most of the retina.

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14
Q

Which region of the retina contains more cones than rods?

A

The fovea because it only contains cones.

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15
Q

Which cells are located in the photoreceptor layer?

A

Rods and cones.

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16
Q

What occurs in the outer plexiform layer?

A

Synaptic interaction between photoreceptors and horizontal and bipolar cells.

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17
Q

Which cells are located in the inner nuclear layer?

A

Amacrine cells.
Horizontal cells.
Bipolar cells.

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18
Q

What are the two main functions of amacrine cells?

A

Act as interneurons.
Modulate ganglion cell activity.

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19
Q

What are the two main functions of horizontal cells?

A

Act as interneurons.
Process photoreceptor signalling.

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20
Q

What is the main function of bipolar cells?

A

Process photoreceptor signalling.

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21
Q

What occurs in the inner plexiform layer?

A

Synaptic interactions between bipolar cells, amacrine processes and ganglion cell dendrites.

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22
Q

Which cells are located in the ganglion cell layer?

A

Cell bodies of multipolar ganglion cells.

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23
Q

Which area of the retina has the greatest density of ganglion cells?

A

The centre of the fovea.

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24
Q

What are the two unique features of ganglion cells?

A

They are the only source of output from the retina.
They are the only retinal cells capable of firing action potentials.

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25
Q

Name the synaptic relay that visual information travels to upon leaving the optic nerve.

A

The lateral geniculate nucleus in the dorsal thalamus.

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26
Q

Where is visual information processed, interpreted and remembered?

A

In the cerebral cortex.

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27
Q

What is the fovea?

A

A depression in the centre of the macula.

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28
Q

What is special about the fovea?

A

It is the region of greatest visual acuity in the retina.

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29
Q

How is such a high density of cones packed into the fovea?

A

The diameter of the cone outer segments is decreased.

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30
Q

How do specific structures work to ensure that the fovea has high visual acuity?

A

Blood vessels are diverted away from the fovea.
Cell body layers and processes are displaced around the fovea.
These limit the scattering of light before it hits the photoreceptors.

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31
Q

What is the foveola?

A

The centre of the fovea.

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32
Q

Name the three functional regions within a photoreceptor.

A

Outer segment.
Inner segment.
Synaptic terminal.

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33
Q

What is the role of the outer segment of photoreceptors?

A

It is involved in phototransduction.

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34
Q

What is the structure of the outer segment of photoreceptors?

A

Consists of a stack of membranous discs formed by infolding of the plasma membrane.
Contains light absorbing photopigments.

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35
Q

How does the structure of the outer segment differ between rod and cone cells?

A

In rods, the membranous discs pinch off from the plasma membrane so they are free-floating.
In cones, the membranous discs remain attached to the plasma membrane.

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36
Q

How are photoreceptor outer segments renewed?

A

Discs are shed from the distal end of the outer segment.
Pigment epithelial cells remove these discarded discs by phagocytosis.
New discs are added to the proximal end of the outer segment.

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37
Q

Which organelles does the inner segment of photoreceptors contain?

A

Nucleus.
ER.
Golgi apparatus.
Mitochondria.
Ribosomes.

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38
Q

What connects the photoreceptor inner segment to the outer segment?

A

Connecting cilium (CC).

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39
Q

What are the two main functions of the photoreceptor inner segment?

A

Protein synthesis.
Energy production.

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40
Q

What is the function of the photoreceptor synaptic terminal?

A

To make synaptic contact with other cells.

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41
Q

Why do rods capture more light than cones?

A

They contain more photosensitive pigment.

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42
Q

What issues can loss of rods lead to?

A

Night blindness.
Loss of peripheral vision.

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43
Q

What issues can loss of cones lead to?

A

Blindness.

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44
Q

Why are cones more sensitive to direct axial light rays?

A

Due to their conical shape.

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45
Q

How do photoreceptors respond to light?

A

Graded changes in membrane potential (NOT action potentials).

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46
Q

How does the response time differ between rods and cones?

A

Rods have a slow response time.
Cones have a fast response time.

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47
Q

Why does light have to go through the retina to the back to reach the photoreceptors?

A

The retina is inverted, so light has to pass through all the neuronal cell layers and blood vessels before reaching the photoreceptors.

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48
Q

Why are photoreceptors embedded within the pigment epithelium?

A

The pigment epithelium provides blood flow, glucose and essential ions.
It also helps degrade and replace the discs in the photoreceptor outer segment, and regenerates photopigment in the discs.

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49
Q

What are the two main functions of Muller cells?

A

Support the function and survival of retinal neurons.
Transfer light through the inner retina by projecting the visual information forward from the photoreceptors to the bundle of optic nerves.

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50
Q

How are Muller cells directly in the path of light as it enters the retina?

A

They impinge onto photoreceptors.

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51
Q

Why are rods more sensitive to light than cones?

A

Multiple rods converge onto and activate one bipolar cell, whereas only one cone activates one bipolar cell.

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52
Q

Rods and cones transmit signals to bipolar and horizontal cells via which type of synapse?

A

Chemical synapse.

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53
Q

What is phototransduction?

A

The conversion of light energy into a graded change in membrane potential.

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54
Q

What is the RMP of photoreceptors?

A

-40mV.

55
Q

What happens to the membrane potential of a photoreceptor when light impinges on the photopigment disc layer?

A

It is hyperpolarised.

56
Q

What does cGMP stand for?

A

Cyclic guanosine monophosphate.

57
Q

How are photoreceptors depolarised in complete darkness?

A

cGMP is continually produced in photoreceptors by the enzyme guanylate cyclase.
cGMP binds to the cytoplasmic side of sodium channels in the outer segment membrane.
The channels remain open, allowing continual sodium influx into the cell.
This depolarises the cell.

58
Q

Which process does depolarisation of photoreceptors facilitate?

A

This facilitates calcium entry at the base of the inner segment.
This causes the consistent release of glutamate from the photoreceptor.

59
Q

What is the role of sodium-potassium pumps in the inner segment of depolarised photoreceptors?

A

Potassium flows out across the inner segment through non-gated potassium channels.
Sodium-potassium pumps maintain the concentration balance between sodium and potassium.

60
Q

What is the dark current?

A

The movement of positive charge (sodium influx) across the membrane.

61
Q

What is the membrane potential of hyperpolarised photoreceptors?

A

-65mV.

62
Q

How does light cause photoreceptors to hyperpolarise?

A

Light reduces cGMP levels.
Sodium channels close.
The membrane potential becomes more negative.

63
Q

Name the photopigment found in rods.

A

Rhodopsin.

64
Q

What type of receptor is rhodopsin?

A

G-protein coupled receptor.

65
Q

What is the receptor protein of rhodopsin?

A

Opsin.

66
Q

What is the prebound chemical agonist on rhodopsin?

A

Retinal.

67
Q

How does light reduce levels of cGMP via rhodopsin?

A

Rhodopsin absorbs the electromagnetic radiation from light energy.
This causes a conformational change in retinal.
This activates the opsin.
This changes the wavelength absorption by rhodopsin, bleaching it from purple to yellow.
This stimulates transducin.
This activates the phosphodiesterase (PDE) enzyme that breaks down cGMP.

68
Q

What does photoreceptor hyperpolarisation lead to?

A

Reduction in calcium influx.
Reduction in glutamate release.

69
Q

What is transducin?

A

G-protein in the membrane of the stacked discs in the rod outer segment.

70
Q

Describe the conformational change that occurs in retinal due to light absorption.

A

Light breaks down the double bond in retinal, converting it from cis- to trans-retinal.

71
Q

What type of process is phototransduction?

A

A biochemical cascade that facilitates signal amplification.

72
Q

How does phototransduction facilitate signal amplification?

A

Each molecule of rhodopsin activates many G-proteins.
This activates more phosphodiesterase enzyme which breaks down more cGMP.

73
Q

Why do we need signal amplification in vision?

A

So that we can detect even just a single photon of light energy.

74
Q

Why is vision in bright light dependent on cones?

A

Prolonged light exposure causes rods to become saturated.
Levels of cGMP are reduced, leading to little or no response.

75
Q

Why might one cone be activated at a different wavelength compared to another?

A

Each cone contains one of three opsins: blue, green or red.
Each opsin is activated at a different wavelength.

76
Q

Which wavelength activates blue cones?

A

445nm (short).

77
Q

Which wavelength activates green cones?

A

508nm (medium).

78
Q

Which wavelength activates red cones?

A

565nm (long).

79
Q

Which cones are least abundant in the retina?

A

Blue cones.

80
Q

What does the ratio of green : red cones normally vary between?

A

Between 1:1 to 4:1.

81
Q

What is scotopic vision?

A

The ability to see in low light conditions (night vision).

82
Q

What is mesopic vision?

A

The ability to see in moderate light conditions (twilight vision).

83
Q

What is photopic vision?

A

The ability to see in well-lit conditions (daytime vision).

84
Q

Which elements of vision does bright light improve?

A

Colour vision.
Visual acuity.

85
Q

Human vision is dependent on a linear process that is facilitated by what?

A

The ribbon synapse of rods.

86
Q

What is the typical structure of a ribbon synapse?

A

One synaptic ribbon faces one postsynaptic density.

87
Q

Why are ribbon synapses necessary?

A

They produce rapid synaptic transmission that facilitates the rapid adaptation required.

88
Q

Why is rapid adaptation required in the visual system?

A

Light continually changes, so the synapses between cells in the retina must rapidly convert graded light energy signals into action potentials.

89
Q

Describe the three different pools of synaptic vesicles that are released at the ribbon synapse.

A
  1. Pool is rapidly releasable on the synaptic bar near the membrane.
  2. Pool is releasable on the synaptic bar but it is slightly further from the membrane.
  3. Pool is a large filling pool in the cytoplasm around the synaptic bar.
90
Q

Why is low order calcium dependency essential in the linear process of human vision?

A

Low level release of calcium is sufficient for vesicle release of NT at the ribbon synapse. The higher the calcium, the more NT released. This allows rods to relay small changes in light energy at the first synapse from photoreceptors to bipolar and horizontal cells.

91
Q

How do rods adapt to light one they have become saturated?

A

In bright continuous light, calcium channels close.
Intracellular calcium levels drop.
This reduces phosphodiesterase activity.
More cGMP is produced.
This binds to rod channels and they open.
The channels have a higher affinity for cGMP in low calcium levels, so the more cGMP that is bound, the more that becomes bound.

92
Q

Light energy is transduced into an electrical signal that travels via the retinal ganglion cells which project up to where?

A

The dorsal lateral geniculate nucleus of the thalamus.

93
Q

Where do lateral geniculate nucleus (LGN) neurons project their axons to?

A

To the primary visual cortex and then to the secondary visual cortex in the occipital lobe where they terminate.

94
Q

How is visual information integrated with information from other pathways?

A

LGN axons fan out as optic radiations of the internal capsule and travel through the temporal, parietal and occipital lobes.

95
Q

Describe the two other key pathways that influence our sense of vision.

A

Some retinal ganglion cells project to the superior colliculus.
Some retinal ganglion cells project to the suprachiasmatic nucleus.

96
Q

What is the role of the superior colliculus?

A

It controls reflexes and the orientation of the head and eye movement.

97
Q

What is the role of the suprachiasmatic nucleus?

A

It controls circadian rhythm.

98
Q

What do the axons of the retinal ganglion cells form?

A

The optic nerves.

99
Q

What occurs at the optic chiasm?

A

The optic nerves cross over (decussate) to form the optic tracts.

100
Q

What is the purpose of crossing over?

A

The eye inverts the images that we see.
Crossing over of information from both the left and right visual fields ensures that none of the information is lost when it is processed in the visual cortex.

101
Q

How many more neurons does the LGN contain compared to the retina?

A

10x.

102
Q

What type of pathway goes from the retina to the LGN?

A

One-way efferent.

103
Q

What type of pathway goes from the LGN to the cortex?

A

Two-way.

104
Q

Describe the two-way pathway between the LGN and the cortex.

A

For every afferent connection projecting to the cortex, the LGN receives ten different efferent connections from the cortex.

105
Q

It is thought that the cortex may use the LGN as a selective filter. What does this mean?

A

The cortex may turn on certain neurons in the LGN that is interested in, i.e. the brain is choosing what it attends to.

106
Q

How is the LGN arranged?

A

The LGN is arranged in six layers made up of three different cell types.

107
Q

Name the three cell types present in the LGN.

A

Parvocellular (P).
Magnocellular (M).
Koniocellular (K).

108
Q

What is the importance of the P pathway?

A

It is important in high spatial acuity (fine detail) and red-green colour vision.

109
Q

What is the importance of the M pathway?

A

It is important in sensitivity to light, darkness and motion.

110
Q

What are the functions of the K pathway believed to be?

A

It is a complementary pathway to the primary visual cortex.
It sends signals directly to the extrastriate visual cortex.
Its neurons integrate retinal visual information with non-retinal input.
It plays an early role in binocular convergence.

111
Q

The axons of which type of cells are projected to which regions of the LGN in the K pathway?

A

The axons of cells much larger than those in the P and M pathways project to very specific regions of the LGN.

112
Q

Temporal and nasal retinal subfields project to alternate layers of the LGN from contralateral and ipsilateral eyes. What does this mean?

A

The right half of the LGN receives visual input about the left half of the visual field from EACH eye.
The left half of the LGN receives visual input about the right half of the visual field from EACH eye.

113
Q

Where do the P and M pathways terminate?

A

In both ipsilateral and contralateral stripes in V1 of the primary visual system.

114
Q

Where is the visual cortex located?

A

In the occipital lobe.

115
Q

Name the different areas of the visual cortex.

A

V1-V5.

116
Q

What is the primary purpose of the visual cortex?

A

To receive, segment and integrate visual information.

117
Q

What happens to visual information as it is passed through areas V1 to V5 of the visual cortex?

A

The level of processing and complexity of the visual information increases.

118
Q

Axons from V1 project in two pathways to the V2 layer of the visual cortex. Describe these two pathways.

A

The dorsal pathway projects through several areas of the parietal lobe and into the frontal lobe.
The ventral pathway projects through the V4 layer of the visual cortex into regions of the inferior temporal cortex.

119
Q

How are the synaptic connections from the photoreceptors to the bipolar cells through to the ganglion cells modulated?

A

Lateral sideways inhibition by horizontal cells and certain types of amacrine cells.

120
Q

Describe OFF bipolar cells.

A

They respond to light off.
They depolarise in response to glutamate released from photoreceptors.

121
Q

Describe ON bipolar cells.

A

They respond to light on.
They hyperpolarise in response to glutamate released from photoreceptors.

122
Q

Name the two components of the receptive field of a cell.

A

The receptive field centre.
The receptive field surround.

123
Q

What is the receptive field centre?

A

A circular area of retina providing direct photoreceptor input.

124
Q

What is the receptive field surround?

A

A surrounding area providing input via horizontal cells (the indirect pathway).

125
Q

How does light affect an ON centre/OFF surround bipolar cell?

A

Light hitting the centre increases the cell’s response.
Light hitting the surround inhibits the cell’s response.

126
Q

How does light affect an OFF centre/ON surround bipolar cell?

A

Light hitting the centre inhibits the cell’s response.
Light hitting the surround increases the cell’s response.

127
Q

What are the two types of ganglion cells in terms of light response?

A

ON-centre and OFF-centre.

128
Q

What are the two major types of ganglion cells?

A

Smaller P-type ganglion cell.
Much larger M-type ganglion cell.

129
Q

Which are more prevalent: P-type or M-type ganglion cells?

A

90% of the ganglion cell population are P-type compared to 5% M-type.

130
Q

Describe the properties of M-type ganglion cells.

A

Larger receptive fields.
Conduct action potentials more rapidly in the optic nerve.
Respond to light with a transient burst of action potentials.
More sensitive to low-contrast stimuli.

131
Q

Describe the properties of P-type ganglion cells.

A

Smaller receptive fields.
Conduct action potentials more slowly in the optic nerve.
Continue to respond to light for as long as they are stimulated.

132
Q

What role are M-type ganglion cells thought to have?

A

Detection of stimulus movement.

133
Q

What role are P-type ganglion cells thought to have?

A

Sensitivity to shape and fine detail.

133
Q

Describe the ON-OFF response that occurs when light is shone onto the retina.

A

Light activate the ON bipolar cell.
This activates the centre of the ON-centre ganglion cell.
We see an ON response with an antagonistic OFF response in the OFF-centre or receptive field surround cell.

134
Q

Describe the ON-OFF response that occurs when a dark spot hits the centre of an ON-centre ganglion cell.

A

The centre is darker than the light in the receptive surround field.
There is no ON response in the ON-centre ganglion cell.
There is a response in the OFF-centre ganglion cell.

134
Q

Describe the ON-OFF response that occurs when the whole visual field is illuminated.

A

There is a drop in response firing rate in the ON-centre cell.
This is due to inhibition from the receptive field surround.

135
Q

How do horizontal cells contribute to a constant level of negative feedback?

A

Systems Neuroscience (4 decks)

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