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Y2S1 - Brain and Behaviour > Brain and Behaviour 3 > Flashcards

Brain and Behaviour 3 Flashcards

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1
Q

What is macular pigment?

A

Blue absorbing pigment

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2
Q

Where is macular pigment found?

A

The fovea and surrounding region

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3
Q

Why is macular pigment important for the fovea?

A

Reduces chromatic aberrations

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4
Q

Where does rod intensity peak?

A

20 degrees either side of the fovea

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5
Q

What is the parafoveal region?

A

Area of most sensitive vision under mesopic and scotopic conditions

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6
Q

Where is the blind spot?

A

On the optic disk where the optic nerve exits the retina

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7
Q

How closely are rods spaced in the parafoveal region?

A

As closely as cones in the fovea

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8
Q

Cones in the fovea and rods in the parafovea are spaced equally, but what differences are their in sensitivity?

A

Rod signal acuity is much reduced because they are summed or pooled

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9
Q

What is papilloedema?

A

Increased ICP leads to optic disk swelling

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10
Q

What are the two fundamental segments of photoreceptors?

A

Outer and inner segments, joined by cilium

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11
Q

What is the function of the outer segment of a photoreceptor?

A

Transduction

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12
Q

What is the function of the inner segment of a photoreceptor?

A

Normal cellular functions

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13
Q

What is the cilium?

A

Connect outer and inner segments of photoreceptors

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14
Q

What does ROS stand for?

A

Rod outer segment

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15
Q

What does the ROS consist of?

A

Stacked membranous discs containing visual pigment and enzymes of the transduction cascade

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16
Q

What does COS stand for?

A

Cone outer segment

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17
Q

What does the COS consist of?

A

Continuous folds of invaginating lamellae

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18
Q

What is rhodopsin?

A

Visual pigment

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19
Q

Describe the structure of rhodopsin.

A

Membrane protein, 7TM

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20
Q

What does rhodopsin bind?

A

Small chromophore molecule, 11-cis retinal

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21
Q

What are opsins?

A

Light sensitive GPCR

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22
Q

When is 11-cis retinal usually absorbed?

A

When in ultraviolet light

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23
Q

When is 11-cis retinal actually absorbed, and why?

A

Around 500nm due to opsin bonding (moiety)

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24
Q

What happens to the chromophore once a proton is absorbed?

A

Isomerisation from 11-cis to all-trans retinal

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25
Q

What is photoisomerisation?

A

Isomerisation in response to proton absorption

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26
Q

What does photoisomerisation of 11-cis retinal lead to?

A

The catalytically active form - metarhodopsin II (RH*)

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27
Q

What is Rh*?

A

Metarhodopsin II - catalytically active form of rhodopsin

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28
Q

What is metarhodopsin II?

A

Catalytically active form of rhodopsin

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29
Q

What happens after Rh* has played its role?

A

All-trans retinal dissociates from the protein slowly (100-1000 sec)

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30
Q

What does Rh stand for?

A

Rhodopsin

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31
Q

What form is Rh said to be in after all-trans retinal has dissocated?

A

Bleached form

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32
Q

Describe the bleached form of Rh.

A

All-trans retinal has dissociated and regeneration must occur before the Rh can be used again

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33
Q

What performs the regeneration of Rh?

A

Retinal pigment epithelium

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34
Q

What does RPE stand for?

A

Retinal pigment epithelium

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35
Q

What does RPE do?

A

Regenerates bleached Rh and dissociated all-trans retinal

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36
Q

What happens to all-trans retinal after dissociation?

A

Reduced to all-trans retinol and transported to RPE where it is converted back into 11-cis retinal

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37
Q

What happens to 11-cis retinal after being regenerated?

A

Transport back to the photoreceptor joining the bleached opsin to form Rh

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38
Q

How long can it take for complete regeneration of Rh after complete bleaching?

A

30 minutes

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39
Q

What retinal a derivative of?

A

Vitamin A

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40
Q

What can a deficiency of Vitamin A cause?

A

Night blindness

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41
Q

What is retinitis pigmentosa?

A

Progressive hereditary retinal degeneration

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42
Q

What does RP stand for?

A

Retinitis pigmentosa

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43
Q

What is the incidence of RP?

A

1 in 3000

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44
Q

Characterise RP.

A

Gradual onset of night blindness in adolescence leading to total loss of all periphery and in extreme cases, blindness

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45
Q

What is the cause of RP?

A

No single cause, but 5-10% caused by Rh mutations

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46
Q

Describe the general phototransduction pathway.

A

Rh* activates transducin (GPCR); this activates PDE; this hydrolyses cGMP into 5’GMP; reduction in [cGMP] results in channel closure, thus hyperpolarisation; GC resynthesises cGMP to terminate response

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47
Q

What does PDE stand for?

A

Phosphodiesterase

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48
Q

What does GC stand for?

A

Guanylate cyclase

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49
Q

How and why does cGMP affect the rod plasma membrane?

A

Membrane contains high density of cGMP gated cation channels; reduced cGMP closes them leading to negative hyperpolarisation

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50
Q

How is the phototransduction pathway amplified?

A

Each rhodopsin activates 100s of GPCR, each PDE hydrolyses 100s of cGMP

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51
Q

Describe the dark current

A

Open channels due to high [cGMP] causes Na and Ca ions to flow into the ROS causing depolarisation to -30mV

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52
Q

What completes the dark current circuit in the inner segment?

A

K channels and Na/K ATPase

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53
Q

At what point is the ROS saturated?

A

All cGMP channels are closed at -75mV

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54
Q

Define light adaptation

A

If flashes of indentical light intensity are superimposed ona background, photoreceptor responses get smaller

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55
Q

What mediates light adaptation?

A

Ca ions

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56
Q

How do Ca ions prevent saturation, and cause adaptation?

A

To prevent saturation, cGMP must be resynthesised by GC; GC is inhibited by Ca ions thus a decrease in [Ca] leads to the de-inhibition of GC

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57
Q

What is vision under photopic conditions mediated by?

A

Cones, exclusively

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58
Q

How are cone responses different to rod?

A

50x less sensitive, and are much faster

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59
Q

What type of vision do cones mediate?

A

Photopic colour vision

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60
Q

Define colour vision.

A

Ability to distinguish different objects on the basis of their spectral reflectance independently of their intensity

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61
Q

What are the three wavelengths cones are optimised to detect?

A

420, 534, 564nm

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62
Q

How is information on the wavelength of light extracted?

A

Comparison of the output of at least two cones tuned to different wavelengths

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63
Q

Why must two cones of different tuning be compared to detect wavelength?

A

If a green cone absorbs 10x less red photons, it will still output the same for 100 red photons as 10 green

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64
Q

What is a system with three differently tuned cones called?

A

Trichromatic

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65
Q

What does trichromatic mean?

A

System is based on three differently tuned cones

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66
Q

How is colour deriven from a trichromatic system?

A

Ratio of excitation in the three cones

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67
Q

What is the theory of how colour is deriven from a trichromatic system?

A

Young-Helmholz trichromacy theory

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68
Q

What is the Young-Helmholz trichromacy theory?

A

Theory of how colour is deriven from a trichromatic system

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69
Q

What is perceived when equal excitation of all three cone tunings occurs?

A

White light

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70
Q

What is the lack of red cones called?

A

Protanopia

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71
Q

What is protanopia?

A

Lack of red cones

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72
Q

What is the lack of green cones called?

A

Deuteranopia

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73
Q

What is deuteranopia?

A

Lack of green cones

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74
Q

What is the incidence of protanopia?

A

2% males

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75
Q

What is the incidence of deuteranopia?

A

2% males

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76
Q

What is the lack of blue cones called?

A

Tritanopia

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77
Q

What is tritanopia?

A

Lack of blue cones

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78
Q

What is the incidence of tritanopia?

A

<1%

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79
Q

What is a shifted red/green cone range called?

A

Anomalous trichromacy

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80
Q

What is anomalous trichromacy?

A

Shifted red/green cone ranges

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81
Q

What is the incidence of anomalous trichromacy?

A

2-6%

82
Q

What is the complete lack of cones called?

A

Rod monochromacy

83
Q

What is rod monochromacy?

A

Complete lack of cones

84
Q

What is the incidence of rod monochromacy?

A

1 in 30000

85
Q

What cones do non-primate mammals express?

A

Only blue and yellow

86
Q

Why is red/green colour blindness most common?

A

Both lie on the same arm of X-chromosome, thus UHR can easily occur to create anomalous hybrids

87
Q

What does UHR stand for?

A

Unequal homologous recombination

88
Q

What are the four main classes of interneurons?

A

bipolar, horizontal, amacrine and ganglion

89
Q

What other, non-neuronal cell type is present in the retina?

A

Glial Muller cells

90
Q

How many layers are in the retina?

A

5 - 3 nuclear and 2 plexiform

91
Q

What is meant by a nuclear cell layer?

A

Contains cell bodies

92
Q

What is meant by a plexiform cell layer?

A

Contains axons and neuronal processes

93
Q

What are the retinal layers, deep to superficial?

A

ONL, OPL, INL, IPL, GCL

94
Q

What does ONL stand for?

A

Outer nuclear layer

95
Q

What does OPL stand for?

A

Outer plexiform layer

96
Q

What does INL stand for?

A

Innel nuclear layer

97
Q

What does IPL stand for?

A

Inner plexiform layer

98
Q

What does GCL stand for?

A

Ganglion cell layer

99
Q

What is in the ONL?

A

Photoreceptor cell bodies

100
Q

What is in the OPL?

A

Synapses between photoreceptors, bipolars and horizontal cells

101
Q

What is in the INL?

A

Bipolar, horizonatl and amacrine cell bodies

102
Q

What is in the IPL?

A

Synapses between bipolar, amacrine and ganglion cells

103
Q

What is in the GCL?

A

Ganglion cell bodies

104
Q

What is the direct pathway of photoreception?

A

Photoreceptor - bipolar cell - ganglion cell; all using excitatory NT, glutamate

105
Q

What is lateral inhibition in the retina mediated by, and by what NT?

A

Horizontal cells - GABAergic

106
Q

What do horizontal cells do, and by what NT?

A

Lateral inhibition by GABA

107
Q

What do amacrine cells do, and by what NT?

A

Diverse interactions, many different NTs

108
Q

What are diverse interactions in the retina mediated by?

A

Amacrine cells

109
Q

What do photoreceptors synapse with?

A

Bipolar and horizontal

110
Q

What do ganglion cells receive inputs from?

A

Bipolar and amacrine cells

111
Q

What is the overall output from the retina carried by?

A

Ganglion cells, which become the optic nerve

112
Q

What is retinal processing characterised by?

A

Convergence and divergence

113
Q

What does retinal divergence result in?

A

Parallel processing

114
Q

What does retinal convergence result in?

A

Spatial summation

115
Q

What type of potentials do most retinal neurons used?

A

Graded potentials

116
Q

What is the only type of retinal neuron to use action potentials?

A

Ganglion cells

117
Q

What type of potential do ganglion cells use?

A

Action potentials

118
Q

What type of potentials do amacrine cells use?

A

Fire spikes in response to strong stimuli

119
Q

What is the only type of retinal neuron to use fire spikes?

A

Amacrine cells

120
Q

Why are graded potentials better?

A

More efficient means of transmitting information over short distances

121
Q

What is characteristic of photoreceptor synapses?

A

Presynaptic ribbon; postsynaptic targets always include processes from both bipolar and horizontal cells

122
Q

What do cone acons end with?

A

Cone pedicle - large synaptic swelling

123
Q

How many synapses can there be at each cone pedicle?

A

Up to 30

124
Q

What does the number of synapses at each cone pedicle convey?

A

Divergence to numerous bipolar cells

125
Q

What is the rod synaptic terminal called?

A

Spherules

126
Q

What is the difference between spherules and cone pedicles?

A

Spherules show no divergence - one spherule, one synapse

127
Q

Define a cell’s receptive field.

A

The area on the retina from which its activity can be influence by light

128
Q

What establishes the centre-surround receptive field?

A

Synaptic interaction at the conde pedicle

129
Q

What are the two general classes of centre-surround receptive field?

A

On-centre and off-centre

130
Q

What do off-centre bipolar cells do to a central stimulus?

A

Hyperpolarise

131
Q

What do off-centre bipolar cells do to a surround stimulus?

A

Depolarise

132
Q

What happens to an off-centre bipolar cell when in the light?

A

Photoreceptors hyperpolarise, glutamate release diminishes, bipolar cell also hyperpolarises

133
Q

What happens to an off-centre bipolar cell when in the dark?

A

Photoreceptors release glutamate, bipolar cell depolarises by opening of glutamate gated cation channels

134
Q

What do on-centre bipolar cells do to a central stimulus?

A

Depolarise

135
Q

What do on-centre bipolar cells do to a surround stimulus?

A

Hyperpolarise

136
Q

How do on-centre bipolar cells act in the opposite way to off-centre?

A

Use a metabotropic receptor; like rhodopsin it activates intracellular transduction cascade resulting in hydrolysis of cGMP and closure of ion channels

137
Q

What generates the antagonistic surround for on & off centre bipolar cells?

A

Lateral inhibition mediated by horizontal cells

138
Q

What two types of bipolar cells are there?

A

Midget and diffuse

139
Q

How are bipolar cells classified?

A

Receptive field size and dendritic morphology

140
Q

Characterise midget bipolar cells.

A

Most bipolar are midget, receive one input from a single cone to the centre of their receptive field

141
Q

Characterise diffuse midget bipolar cells.

A

Sum inputs from several cones - more sensitive but sacrifice spatial and chromatic detail

142
Q

How many bipolar cells does each foveal cone contact?

A

10 to 15

143
Q

What does the divergence of the fovea establish?

A

Parallel streams in the visual system

144
Q

What are parallel streams in the visual system?

A

Different aspects of the image are coded by different cells with overlapping visual fields

145
Q

Characterise the bipolar-ganglion cell synapse.

A

Excitatory

146
Q

What is the result of the excitatory nature of the bipolar-ganglion cell synapse?

A

Ganglion cells have a similar receptive field structure as bipolar cells

147
Q

What is the difference between the receptive field structure of bipolar cells and ganglion cells?

A

Ganglion cells are modified by lateral interaction from amacrine cells

148
Q

What is the most important subdivision of ganglion cells?

A

Magnocellular or parvocellular classes

149
Q

Characterise M ganglion cells.

A

PARASOL - large field, transient signal, fast conduction, high gain

150
Q

How are M ganglion cells different to P?

A

Respond more rapidly, generating transient responses to changes in intensity, more sensitive, larger diameter axons

151
Q

Why are ganglion cells classed as M or P?

A

Project to different layers in the LGN and cortex

152
Q

What does LGN stand for?

A

Lateral geniculate nucleus

153
Q

Characterise P ganglion cells.

A

MIDGET - the majority, small field, sustained response, slow conduction velocity, low gain

154
Q

What do M ganglion cells convey?

A

Motion detection

155
Q

What do P ganglion cells convey?

A

Form and colour

156
Q

How many distinct anatomical classes of amacrine cells are there?

A

20-30

157
Q

What is the role of amacrine cells?

A

Little is known - contribution to lateral inhibition, modulatory functions for adjusting the eye to different light sensitivities

158
Q

What is the distribution of amacrine cells?

A

M ganglion cells receive significantly more

159
Q

What is the only amacrine cell whose function we understand?

A

Rod, or AII amacrine cell - mediates signals from rods under scotopic conditions

160
Q

Outline the parvocellular stream.

A

Single cone > midget BP cell > midget ganglion > fine grain, colour

161
Q

Outline the magnocellular stream.

A

Many cones > diffuse BP cell > parasol ganglion > coarse grain, motion

162
Q

What is rod pooling?

A

Convergence of rod signals, resulting in extra sensitivity but lower spatial resolution

163
Q

What is the Purkinje shift?

A

Shift of light sensitivity in scotopic vision from 560nm to 500nm

164
Q

What type are ALL rod BP cells?

A

ON-centre

165
Q

What do rod BP cells connect to?

A

AII amacrine cells exclusively

166
Q

What outputs do AII amacrine cells make?

A

Inhibitory (glycine) to off-ganglion cells, excitatory to on-ganglion cells

167
Q

Outline the massive convergence of the rod pathway.

A

1500 rods > 100 rod-BP > 5 AII amacrine > 4 cone-BP > 1 ganglion cell

168
Q

What is the problem with the optic nerve?

A

Bottle neck in the visual system

169
Q

How is the bottle neck in the visual system overcome?

A

Convergence of 100 million photoreceptors into 1.5 million ganglion cells, followed by a massive divergence into 200 million neurons in the PVC alone

170
Q

What does PVC stand for?

A

Primary visual cortex

171
Q

What is the primary projection from the retina?

A

To the LGN

172
Q

Where do the two optic nerves converge before the LGN?

A

Optic chiasm

173
Q

What the two optic nerves do at the optic chiasm, and why?

A

Decussate so that axons representing the same half of the visual field ar combined

174
Q

Where does the optic radiation pass after the optic chiasm and LGN?

A

Fans out in a broad path through the internal capsule, ending in the PVC

175
Q

What is the PVC also known as?

A

Striate cortex or V1 or area 17

176
Q

Where is the PVC?

A

Buried in the medial aspect of the hemispheres - the calcarine sulcus

177
Q

What is cortical magnification?

A

Gross overrepresentation of the fovea in the PVC

178
Q

What does the LGN consist of?

A

4 parvo cellular layers and 2 magnocellular

179
Q

Describe the layers of the LGN, ventral to dorsal

A

1 - Magno, contra; 2 - Magno, ipsi; 3 - Parvo, ipsi; 4 - Parvo, contra; 5 - Parvo, ipsi; 6 - Parvo, contra

180
Q

Describe the outputs and inputs of the LGN

A

60% of input is cortical feedback, many local neuronal connections, direct excitatory projections to the cortex

181
Q

How thick is the grey matter of the visual cortex?

A

2mm

182
Q

Where do LGN fibres end, primarily, in the PVC?

A

Spiny stellate neurons in layer 4C-alpha and 4C-beta

183
Q

Where do outputs for higher visual areas exit the PVC?

A

Layers 2 & 3 - pyramidal neurons

184
Q

Where do PVC projections back to the thalamus exit?

A

Layer 6

185
Q

Where do PVC projections to deep brain structures leave?

A

Layer 5

186
Q

What are RFs?

A

Receptive fields

187
Q

How are the PVC RFs different to centre-surround organisation?

A

They are in bars or edges, with a particular orientation

188
Q

What are the two classes of PVC organisation?

A

Simple and complex

189
Q

Characterise simple PVC organisation.

A

Simple cells respond only to an edge of a particular orientation in a very well-defined position

190
Q

Characterise complex PVC organisation.

A

75% of cells - also respond like simple cells, but position is not so critical - they respond to directionally specific moving edges over a larger field

191
Q

What is ‘end-stopping’?

A

Some PVC cells become inhibited when an edge exceeds a critical length

192
Q

What theories have been designed to explain PVC organisation?

A

Hubel & Wiesel speculated simple RFs generated by combining centre-surround LGN inputs, and complex RFs generated by combination of simple RFs

193
Q

Briefly outline the architectural organisation of the PVC.

A

Ocular dominance columns - alternating slabs 0.5mm thick, one eye input then the other; Ortientation columns - cells with same orientation, adjacent columns slightly different; Blobs - highly selective areas for colour, rich in cytochrome oxidase

194
Q

What are ocular dominance columns?

A

Alternating slabs of 0.5mm thick cortex - receive dominant input first from one eye, then the other

195
Q

What are orientation columns?

A

Perpendicular to cortical surface with same preferred orientation; adjacent slabs have shifted preference

196
Q

What are PVC blobs?

A

Concentrations of cytochrome oxidase unresponsive to orientation by highly chromatically selective

197
Q

What are amblyopias?

A

Permanent defects in cortical function due to retinal problems in early infant life

198
Q

What are the two higher order visual pathways?

A

Dorsal - where; ventral - what

199
Q

What type of input projects into the dorsal visual pathway?

A

Magno

200
Q

Describe the dorsal where pathway.

A

V1 > V2 > V5 > Posterior parietal cortex

Y2S1 - Brain and Behaviour (4 decks)

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