Wijnen - Wiring of Brain Flashcards
(195 cards)
1
Q
Which brain regions do retinal ganglion cell axons project to?
A
Lateral geniculate nucleus (LGN)
Superior colliculus (SC)
2
Q
How is visual input from the eyes organized in the brain?
A
Left eye projects mostly to the right hemisphere and vice versa.
Some fibres remain on the ipsilateral side, while others cross at the optic chiasm.
3
Q
What is the optic chiasm?
A
The midline structure where optic nerves from both eyes meet and partially cross over.
4
Q
What do the terms ipsilateral and contralateral mean in neuroanatomy?
A
Ipsilateral: same side
Contralateral: opposite side
5
Q
How is spatial and cell-type information from the retina preserved in the brain?
A
Axon terminals from retinal ganglion cells terminate in specific layers and positions of their targets (LGN, SC)
Maintains information about retinal location and ganglion cell subtype
6
Q
How is the visual image processed in terms of orientation in the brain?
A
Like a pinhole camera, the retinal image is inverted (upside down and left-right flipped).
The brain, particularly at the superior colliculus, re-inverts the map to restore correct visual orientation.
7
Q
What are the two theoretical models for retinal ganglion cell (RGC) axon targeting?
A
Predetermined targeting: each neuron is genetically programmed to find its specific target.
Stochastic targeting: neurons form many initial connections, and inappropriate ones are pruned.
8
Q
What key experiment did Roger Sperry perform to test RGC axon targeting?
A
In amphibians, Sperry rotated the eye 180° and cut the optic nerve, allowing regeneration.
Then observed whether visual connections were restored properly or misrouted.
9
Q
What did Sperry’s rotated eye experiment reveal about RGC wiring?
A
Regenerated axons reconnected based on original retinal position, not new position.
Result: animals saw the world inverted, supporting predetermined mapping.
10
Q
Why are amphibians used in regeneration studies like Sperry’s?
A
Amphibians (e.g. frogs) have strong regenerative abilities, including neuron regeneration.
Allows researchers to test rewiring and functional recovery after injury.
11
Q
What would have happened if axon targeting were purely functional or stochastic?
A
The regenerated axons would have adjusted to the new orientation.
Visual responses would remain correct, even after eye rotation.
12
Q
What does Sperry’s work suggest about nervous system wiring?
A
Strong evidence for chemical cues and molecular guidance in axon targeting.
But doesn’t rule out some stochastic or activity-dependent refinement.
13
Q
What is Sperry’s chemoaffinity hypothesis?
A
The idea that neurons and axons carry unique chemical ‘identification tags’, allowing them to form specific synaptic connections.
Axons are guided by specific chemical affinities to their correct targets.
14
Q
What evidence supports the chemoaffinity hypothesis?
A
Sperry’s eye rotation experiments showed axon re-targeting followed original retinal position, not functional position.
Subsequent research found molecular guidance cues (e.g. gradients of attractants/repellents).
15
Q
How are retinal ganglion cell (RGC) axons mapped in the tectum?
A
Ventral retina → medial tectum
Dorsal retina → lateral tectum
Posterior retina → anterior tectum
Anterior retina → posterior tectum
16
Q
What kind of map does the retina form in the tectum?
A
A retinotopic map — spatial layout of the retina is preserved and inverted in the tectum (or superior colliculus in mammals).
17
Q
What experiment further confirmed retinal axon guidance specificity?
A
Half-retina ablations followed by axon regeneration still resulted in accurate targeting to correct tectal regions.
Indicates intrinsic wiring cues guide axons to their proper locations.
18
Q
What is the role of molecular guidance cues in brain wiring?
A
Axons use chemical gradients (e.g. ephrins, netrins, semaphorins) to navigate.
These cues are central to axonal guidance and synaptic specificity.
19
Q
What are chemical guidance cues in the nervous system?
A
Molecular signals that direct axon growth by mediating attraction or repulsion.
Can act over short or long distances.
20
Q
What is the purpose of axon bundling (fasciculation)?
A
Helps organise the nervous system efficiently.
Reduces misrouting by keeping axons together.
Analogous to cable management in computer systems.
21
Q
Which molecules can act as long-range attractants or repellents for axons?
A
Netrins – attract or repel
Semaphorins – typically repel
Ephrins – contact-mediated repulsion
IgCAMs and cadherins – promote adhesion/attraction
22
Q
What is special about ephrins and Eph receptors?
A
Ephrins (ligands) and Eph receptors are both membrane-bound.
Can mediate bidirectional signalling:
Eph acts as receptor in one cell
Ephrin can also signal in the opposite cell, depending on context
23
Q
What is bidirectional signalling in the context of ephrins?
A
Ephrin-A/EphA interaction can trigger signalling in either cell.
Forward signalling: Eph receptor transduces signal
Reverse signalling: Ephrin ligand acts as receptor
24
Q
How can netrins guide axons in developing nervous tissue?
A
Netrins can attract or repel axons depending on the neuron type and context.
Example: COS cells expressing netrin cause nearby axons in culture to divert toward them
25
What is the role of netrins in the vertebrate spinal cord?
Attract commissural neurons to the floor plate (midline).
Repel motor neurons from the floor plate.
Netrin acts as a bifunctional cue in the same tissue.
26
Are netrins a vertebrate-specific invention?
No — netrin signalling is also used in C. elegans, a simple invertebrate.
The gene UNC-6 (a netrin ortholog) attracts sensory neurons and repels motor neurons in C. elegans.
27
What does the presence of netrin signalling in C. elegans suggest about its evolutionary origin?
Netrin-based axon guidance is an ancient and conserved mechanism, predating vertebrates.
Suggests early nervous systems used similar guidance strategies.
28
What is the role of the growth cone in axon guidance?
The growth cone is the dynamic, motile tip of a growing axon.
It senses guidance cues and directs axon pathfinding by remodelling the cytoskeleton.
29
What cytoskeletal components are involved in growth cone movement?
Actin filaments (mainly in filopodia and lamellipodia)
Microtubules (support directional extension)
30
How do small GTPases (like Rho) influence growth cone behaviour?
Rho GTPases toggle between GTP-bound (active) and GDP-bound (inactive) states.
They regulate actin–myosin dynamics in response to guidance cues, driving local cytoskeletal changes.
31
How does the growth cone respond to attractive vs. repellent cues?
Attractive cues → local actin polymerisation and forward extension.
Repellent cues → actin depolymerisation and retraction, causing redirection of growth.
32
What was the goal of the tectal membrane stripe assay experiment?
To test the chemoaffinity hypothesis by examining whether retinal axons prefer specific tectal membrane regions based on chemical cues.
33
How was the stripe assay experiment designed?
Strips of posterior (P) and anterior (A) tectal membranes were placed in alternating lanes.
Retinal ganglion cells (RGCs) from the nasal and temporal retina were grown across them.
34
What were the key findings of the stripe assay experiment?
Temporal RGC axons avoided posterior (P) tectal membranes and grew only on anterior (A) strips.
Nasal RGC axons grew on both A and P, showing no preference.
35
What conclusion can be drawn from temporal RGCs avoiding posterior tectal membranes?
The posterior tectum expresses a repellent that selectively repels temporal axons, confirming chemoaffinity-based repulsion.
36
What enzyme was found to eliminate the repellent effect in the assay?
Phosphatidylinositol-specific phospholipase C removed the repellent effect.
37
What does phosphatidylinositol-specific phospholipase C do?
It cleaves GPI (glycosylphosphatidylinositol) anchors, which are used by extracellular membrane proteins to attach to the membrane.
38
How do researchers visualise the retina for mapping molecular gradients?
The retina is flattened from its natural curved shape to make 2D imaging possible.
Allows mapping of gradients from nasal to temporal regions.
39
What is the distribution of the EphA receptor in the retina?
High expression of EphA in the temporal retina.
Lower expression in the nasal retina.
Creates a temporal-to-nasal gradient.
40
What is the distribution of ephrin-A in the tectum?
High expression in the posterior tectum.
Lower expression in the anterior tectum.
Forms an anterior-to-posterior ephrin-A gradient.
41
How do EphA and ephrin-A gradients guide retinal axons?
EphA–ephrin-A interactions are repulsive.
Temporal axons (high EphA) are repelled by posterior tectum (high ephrin-A), so they grow into anterior tectum.
Nasal axons (low EphA) are less repelled, so they can grow into posterior tectum.
42
What do opposing gradients of EphA and ephrin-A achieve in visual system development?
Ensure retinotopic mapping: each region of the retina projects to a specific area of the tectum.
Maintains spatial organisation of visual input.
43
How can fluorescent dye be used to test retinotectal mapping?
Inject fluorescent dye into a specific region of the retina.
Trace labelled axons to their termination point in the superior colliculus (SC).
Reveals if axons reached their expected target location.
44
What does a wild-type mouse show when temporal retinal neurons are labelled with dye?
Axons project to the anterior superior colliculus on the contralateral side, confirming normal retinotopic mapping.
45
What happens when ephrin-A2 and ephrin-A5 are knocked out in mice?
Temporal axons misproject to posterior regions of the SC (where they would normally be repelled).
Indicates a loss of repellent guidance from the posterior SC.
46
How does ephrin-A dosage affect axon targeting?
Higher ephrin-A expression in the SC → accurate targeting.
Reduced ephrin-A → increased mistargeting of retinal axons.
Confirms a dose-dependent repellent effect.
47
What do these knockout experiments demonstrate about the EphA–ephrin-A system?
Provide genetic evidence supporting the chemoaffinity hypothesis.
Show that ephrin-A gradients are essential for establishing correct topographic maps in the visual system.
48
Besides removing ephrin-A, how else can researchers manipulate the EphA–ephrin-A system?
By increasing EphA expression in specific retinal ganglion cells (RGCs) using genetic knock-in methods.
49
What is Isl2, and how is it used in genetic manipulation?
Isl2 is a transcription factor expressed in a subset of RGCs.
It is used to drive misexpression of EphA3 in only Isl2-positive RGCs.
50
What happens when EphA3 is overexpressed in Isl2+ RGCs?
Isl2+ RGCs with extra EphA3 are displaced anteriorly in the superior colliculus.
Isl2– RGCs (no EphA3 overexpression) still project to the correct target.
51
What does the Isl2-EphA3 knock-in experiment demonstrate?
Shows that altering EphA levels alone can shift axonal targeting.
Supports the idea that axon targeting is graded and relative, not just absolute.
52
How are topographic maps in the retina–tectum system established?
Through complementary gradients of EphA (receptors) in the retina and ephrin-A (ligands) in the superior colliculus.
These gradients create a relative positional code for axon guidance.
53
How does EphA transmit intracellular signals?
EphA has a tyrosine kinase intracellular domain.
Activates second messengers that trigger repulsion or other responses in the axon.
54
Can ephrin-A also signal into the cell that expresses it?
Yes — ephrin-A can mediate reverse signalling when it interacts with other transmembrane proteins.
Even though it lacks its own intracellular domain, it can still act like a receptor.
55
What is reverse signalling in the EphA–ephrin-A system?
When ephrin-A acts as a signal receiver rather than just a ligand.
Signal flows into the ephrin-A–expressing cell via associated transmembrane proteins.
56
In which animals is retinal projection almost entirely contralateral?
Fish, pre-metamorphic frogs (tadpoles), and birds
Their retinal ganglion cells (RGCs) project almost entirely to the opposite hemisphere.
57
In which animals do some RGCs project ipsilaterally?
Mice, post-metamorphic frogs, ferrets, cats, marsupials, and especially primates (including humans)
Ipsilateral projections are associated with binocular vision.
58
What is binocular vision, and what advantage does it offer?
Binocular vision is when both eyes see overlapping fields of view.
It enables depth perception and distance estimation, useful for predators.
59
How does eye position influence the visual field?
Forward-facing eyes (e.g. primates, owls) → large binocular zone, better depth perception.
Side-facing eyes (e.g. rabbits, woodcocks) → wide peripheral vision, good for detecting predators.
60
What is the relationship between retinal wiring and binocular vision?
Ipsilateral RGC projections increase as binocular overlap increases.
Animals with greater binocular vision tend to have more RGCs projecting ipsilaterally.
61
What example highlights this difference in birds?
Owls have forward-facing eyes and more binocular vision (likely more ipsilateral projections).
Woodcocks have wide lateral vision and can see nearly 360°, with mostly contralateral wiring.
62
What do some retinal axons do at the optic chiasm in binocular animals?
A subset of axons stop at the chiasm, then turn back and project ipsilaterally instead of crossing.
63
What region of the retina gives rise to ipsilaterally projecting axons?
The very temporal (posterior) region of the retina.
64
What transcription factor is expressed in temporal RGCs that project ipsilaterally?
Zic2 — a transcription factor specifically expressed in temporal RGCs.
65
What receptor is expressed downstream of Zic2 in temporal RGCs?
EphB1 — a repellent receptor involved in axon guidance at the optic chiasm.
66
What guidance molecule at the optic chiasm repels EphB1-expressing axons?
EphrinB2, expressed by midline glial cells at the optic chiasm.
67
How does EphB1–EphrinB2 interaction affect axon guidance at the optic chiasm?
EphrinB2 repels axons that express EphB1, causing them to turn away and project ipsilaterally.
68
Why do temporal RGCs respond differently at the chiasm than other RGCs?
Only temporal RGCs express Zic2 → EphB1, making them responsive to repulsion by EphrinB2 at the midline.
Other RGCs lack EphB1 and continue contralaterally.
69
What part of the visual field corresponds to the temporal retina?
The binocular visual field — both eyes receive information from this area, requiring ipsilateral projections for integration.
70
What is experience-dependent plasticity in neuronal organisation?
Neural circuits are shaped by sensory experience, especially during critical developmental periods.
Deprivation of input can lead to long-term changes in connectivity.
71
What did Hubel and Wiesel study to investigate visual plasticity?
Monocular deprivation in kittens: one eye was sutured closed during early development.
Later measured neuronal responses in visual cortex to light stimuli from each eye.
72
What were the results of monocular deprivation in kittens?
Neurons in visual cortex responded only to the open eye.
The deprived eye failed to establish strong or useful cortical connections.
73
What happens to the ocular dominance distribution after early eye deprivation?
The distribution shifts toward dominance by the non-deprived eye.
Indicates the deprived eye fails to compete for cortical territory.
74
When is the critical period for visual plasticity in cats?
The first 2 months of postnatal life.
Deprivation during this time causes permanent wiring changes.
75
What do Hubel & Wiesel's findings imply about visual system development?
Neuronal wiring is activity-dependent.
Visual experience is essential for normal synaptic organisation in the cortex.
76
What method was used in monkeys to study eye-specific input to the brain?
Radioactive amino acids were injected into one eye’s retina, allowing axonal tracing to the lateral geniculate nucleus (LGN) and visual cortex.
77
How is radioactive label visualised in this experiment?
Appears as white bands in visual cortex (specifically in ocular dominance columns).
Indicates regions receiving input from the injected eye.
78
What is the normal pattern of input to the primary visual cortex in primates?
Alternating bands of input from the left and right eyes — seen as stripy patterns in layer 4 of the visual cortex.
79
What happens to the visual cortex pattern after monocular deprivation in monkeys?
Bands from the open eye expand, taking over more territory.
Bands from the deprived eye shrink, showing reduced input and connectivity.
80
What does the monkey radioactive tracing experiment show about visual development?
Neural connections are activity-dependent.
The eye with visual experience forms more cortical connections.
The deprived eye’s input is functionally and structurally weakened.
81
What unusual experiment was done in frogs to study visual competition?
A third eye was grafted onto a frog, leading to RGC axons competing for targets in the tectum.
82
What is the normal visual projection pattern in frogs?
Frogs normally have purely contralateral projections, with no binocular vision.
83
What happened when a third eye was added to a frog?
The new eye’s axons competed with native inputs for the same tectal territory, resulting in striped ocular dominance-like patterning, similar to cats and monkeys.
84
What does the third-eye frog experiment demonstrate?
Activity-dependent competition shapes visual wiring.
Even in species without binocular vision, competitive input can induce cortical-like segregation.
85
How are early connections formed in the visual system (e.g. cat LGN)?
Initial axons may innervate multiple layers regardless of eye origin.
Over time, non-correlated inputs are pruned, and eye-specific layers emerge.
86
What role does neuronal activity play in synaptic refinement?
Inputs that fire together are retained ("fire together, wire together").
Inputs that fire out of sync are weakened and pruned.
87
What kind of activity refines eye-specific layers before visual experience?
Spontaneous retinal waves — bursts of activity that travel across the retina before birth or eye opening.
88
What technique revealed spontaneous activity in the newborn ferret retina?
Multi-electrode arrays (MEAs) recorded electrical activity at multiple locations across the retina
89
What was observed in the retina using MEAs and calcium imaging?
Spontaneous cholinergic waves of activity travelled across the retina
Nearby neurons showed highly correlated firing, forming activity-based networks
90
What neurotransmitter is involved in spontaneous retinal waves in ferrets?
Acetylcholine (cholinergic signalling)
91
How is activity correlation affected by distance in the retina?
Close neurons have highly correlated activity
Distant neurons have less synchronous activity
92
What technique was used to track RGC projections in developing ferrets?
Fluorescent tracers injected into each eye:
Right eye = red
Left eye = green
Allowed visualisation of axonal targeting in the lateral geniculate nucleus (LGN).
93
What is observed in LGN of normal ferrets from P1 to P10?
At P1: diffuse, overlapping projections from both eyes.
At P10: clear eye-specific segregation into distinct binocular zones in the LGN.
94
What is epibatidine and how was it used in this experiment?
A nicotinic acetylcholine receptor agonist.
When injected into the eyes, it disrupts spontaneous retinal waves by inducing tonic activation.
95
What is the effect of epibatidine on LGN development?
Retinal wave disruption results in failure of eye-specific segregation.
LGN projections remain diffuse and immature, resembling the P1 state.
96
What happens if only one eye receives epibatidine?
The eye lacking retinal waves (e.g. right eye) shows poor projection refinement.
The eye with normal activity forms sharper, better-defined projections in the LGN.
97
How does enhancing retinal activity affect projection patterning?
Using a cyclic AMP agonist to enhance waves in one eye results in stronger, expanded LGN projections from that eye.
98
What does this experiment demonstrate about spontaneous retinal waves?
Retinal activity patterns are instructive — they guide the refinement of visual projections to the LGN.
Experience-independent, spontaneous activity is necessary for proper wiring.
99
What was Donald Hebb’s key contribution to neuroscience in 1949?
Proposed that when cell A repeatedly helps fire cell B, the synaptic connection between them is strengthened.
The principle is often paraphrased as: “neurons that fire together, wire together.”
100
What is the mechanism behind Hebbian plasticity in development?
Repeated synchronous activity between neurons leads to:
- Growth processes or
-Metabolic changes that strengthen synapses.
101
What happens to synapses that are not used synchronously?
They become weakened and are more likely to be eliminated.
This supports activity-dependent pruning of neural circuits.
102
How does Hebbian plasticity relate to developmental synaptic wiring?
Early neural circuits may form with many initial connections, some random.
Through activity, only meaningful, synchronously active connections are reinforced, shaping functional neural networks.
103
What cycle reinforces Hebbian synapses?
A virtuous cycle: used synapses become stronger, leading to more activity, which further strengthens them.
104
How does Hebbian plasticity apply to patterning in the lateral geniculate nucleus (LGN)?
LGN neurons initially receive input from both eyes.
Over time, asymmetric activity (e.g. stronger input from one eye) causes LGN neurons to favour one eye.
105
What happens when one eye provides stronger input to an LGN neuron?
According to Hebb’s rule, inputs from the stronger eye fire more synchronously with the LGN neuron.
These synapses are strengthened, while weaker, asynchronous inputs are pruned.
106
What is the outcome of Hebbian refinement in the LGN?
Due to activity-dependent competition and Hebbian reinforcement, inputs from one eye dominate, and others are eliminated.
107
What receptor plays a key role in molecularly implementing Hebb’s rule?
The NMDA receptor, a subtype of glutamate receptor.
108
Why is the NMDA receptor considered a coincidence detector?
It is normally blocked by Mg²⁺ ions.
The block is only relieved when the postsynaptic neuron is already depolarised, allowing Ca²⁺ influx if glutamate is also present.
109
What is required to unblock the NMDA receptor?
Postsynaptic depolarisation (e.g. recent firing) to remove the Mg²⁺ block
Glutamate release from the presynaptic terminal
110
How do NMDA receptors contribute to synaptic strengthening?
When coincident firing occurs, NMDA receptors open and allow Ca²⁺ influx, triggering signalling cascades that lead to synaptic potentiation.
111
What do AMPAR receptors do?
AMPA receptors mediate initial fast depolarisation via Na⁺ influx.
This depolarisation helps remove the Mg²⁺ block from NMDA receptors, enabling coincidence detection.
112
How does NMDA receptor activity support Hebbian learning in development?
Strengthens synapses where presynaptic and postsynaptic neurons are active together, aligning with “fire together, wire together.”
113
What is the whisker-barrel system?
A somatosensory pathway in rodents where each whisker on the face has a mapped representation in the brain.
Organised into barrelettes (brainstem), barreloids (thalamus), and barrels (cortex).
114
How is the whisker input mapped in the rodent brain?
Topographically: Each whisker corresponds to a distinct, spatially organised region at all three processing levels.
115
Where are the barrels located in the rodent brain?
In the primary somatosensory cortex (S1) — visible as discrete, stained clusters that match the physical whisker layout.
116
What is the developmental timeline of barrel formation?
Barrels form in postnatal days 0–7 (P0–P7).
Sensory input and activity guide this precise wiring.
117
Is the whisker-barrel projection contralateral or ipsilateral?
Contralateral — whisker inputs from the right side of the face are mapped in the left hemisphere, and vice versa.
118
What happens to thalamocortical axons in newborn rodents before whisker use?
Axons are diffusely distributed across stellate cells in somatosensory cortex; no clear barrel formation yet.
119
120
How does whisker use affect axonal projections in the first postnatal week?
Whisker activity drives thalamocortical axons to cluster into barrels, forming a precise spatial map in the cortex.
121
What happens if whiskers are plucked during barrel formation?
The affected row of barrels becomes fused or elongated rather than distinct, showing the map is activity-dependent.
122
How does the NMDA receptor contribute to barrel formation?
The NMDA receptor (specifically the GluN1 subunit) acts as a coincidence detector.
Mutations in GluN1 result in a loss of barrel structure, showing NMDA is essential for Hebbian patterning.
123
What does the fused-barrel phenotype in NMDA receptor mutants show?
Strong evidence that NMDA receptor signalling is necessary for activity-dependent synaptic segregation.
124
How do somatosensory neurons behave in a normal barrel cortex?
Each neuron limits its dendritic connections to axons within one barrel, reflecting input from one specific whisker.
125
What happens to single-cell connectivity in NMDA receptor mutants?
Neurons extend dendrites across multiple barrels, losing whisker-specific organisation.
Indicates failure of synaptic pruning and refinement.
126
Why is barrel-specific wiring advantageous?
Promotes precise sensory mapping and efficient local processing.
Prevents confusion from broad, overlapping input.
127
What is the developmental sequence of barrel-specific neuron refinement?
Initial broad connectivity
NMDA receptor–dependent activity
Pruning of weak inputs
Selective strengthening of whisker-specific synapses
128
What is the final outcome of NMDA-guided synaptic refinement in S1 cortex?
Somatosensory neurons become specialised carriers of information from a single whisker, ensuring high-resolution tactile representation.
129
What are the two main mechanisms that guide retinal axon mapping in the superior colliculus?
Chemoaffinity gradients (e.g. ephrin-A/Eph-A)
Activity-dependent retinal waves (e.g. cholinergic signalling)
130
What is the role of ephrins in the superior colliculus?
Ephrin-A gradients provide spatial guidance along anterior–posterior and medial–lateral axes for retinal axons.
131
What happens when the β2 subunit of the nicotinic acetylcholine receptor is knocked out?
Cholinergic retinal waves are abolished.
Leads to dispersed and less refined axonal targeting in the superior colliculus, even with intact ephrin gradients.
132
What does the β2 knockout experiment show about retinal wave function?
Spontaneous activity is required for refining and focusing retinal projections beyond what chemoaffinity alone can achieve.
133
What happens if both ephrin signalling and cholinergic waves are removed?
Retinal projections become completely disorganised in the superior colliculus.
Shows that both systems are required for correct map formation.
134
How do ephrin gradients and retinal activity complement each other?
Ephrins provide initial spatial targeting.
Activity (retinal waves) refines and stabilises the map through Hebbian mechanisms.
135
Where are EphA and ephrinA gradients found in the visual system?
Retina (temporal–nasal axis)
Lateral geniculate nucleus (LGN)
Superior colliculus (SC)
Primary visual cortex (V1)
136
What is the relationship between EphA expression in RGCs and their targets?
RGCs with high EphA project to regions of high ephrinA in LGN and SC, where repulsion guides them to precise termination zones.
137
How do retinal axons navigate through the visual system using these gradients?
Axons follow complementary chemoaffinity cues through retina → LGN → SC → visual cortex, maintaining topographic fidelity.
138
What is the molecular mechanism guiding EphA/ephrinA interactions?
EphA–ephrinA binding typically results in repulsion, steering axons away from regions of high ligand-receptor interaction.
139
Why do gradients of EphA and ephrinA persist throughout the visual pathway?
They ensure consistent spatial mapping and preservation of visual topography from retina to cortex.
140
How can we visualise retinal topographic maps in the visual cortex?
By colour-coding axons from specific LGN regions, revealing orderly mapping onto the primary visual cortex (V1).
141
What happens to cortical topographic maps when both EphrinA2/A5 and retinal activity are disrupted?
The map becomes severely disorganised, showing the necessity of both for proper wiring.
142
What is the effect of disrupting only EphrinA2/A5 or only β2 subunit of the nicotinic ACh receptor?
The map becomes partially disorganised, but some structure remains, due to compensation by the remaining system.
143
What does this imply about visual system development?
There is functional redundancy and resilience:
- Activity can partially compensate for missing guidance cues
- Guidance cues can help organise maps even in the absence of retinal activity
144
What conclusion can be drawn from combining Ephrin and activity mutants in visual cortex mapping?
Both EphrinA gradients and retinal activity are required for optimal map formation, but each can partially substitute for the other.
145
Why is lamina-specific targeting important in the retina?
Different cell types (e.g. ON vs OFF bipolar cells) must connect to specific layers of the inner plexiform layer to ensure correct signal processing.
146
What are ON and OFF layers in the retina?
ON bipolar cells synapse in the inner sublayers of the inner plexiform layer (e.g. S4).
OFF bipolar cells synapse in the outer sublayers (e.g. S1, S2).
147
What molecules help mediate lamina-specific targeting in the retina?
SDK1: targets S4
DscamL: targets S1
Dscam: targets S5
SDK2: targets S2
148
How do SDK and Dscam molecules influence retinal connectivity?
They act as cell adhesion molecules, promoting selective attraction or repulsion to specific sublayers.
149
What does the presence of different guidance molecules in the inner plexiform layer suggest?
Local molecular interactions, not just long-range cues, are essential for fine-scale synaptic organisation within a single structure.
150
What is the role of Sema6A and PlexA4 in retinal lamination?
Sema6A and PlexA4 mediate repulsion between axons targeting different inner plexiform sublayers, maintaining layer separation.
151
Where do Sema6A-positive neurons normally project?
Below layers S1 and S2.
152
Where are PlexA4-positive neurons located in the retina?
Layers S1 and S2 of the inner plexiform layer.
153
What happens when the Sema6A gene is knocked out?
Neurons that would normally target S1 (e.g. dopaminergic amacrine cells) now mis-target to upper layers (S4, S5).
Indicates Sema6A provides repulsive guidance to prevent misprojection.
154
What cell type is used to study Sema6A-mediated targeting in the retina?
Dopaminergic amacrine cells, identified by their expression of tyrosine hydroxylase, a dopamine synthesis enzyme
155
Why are dopaminergic amacrine cells a good model for studying retinal lamination?
They have precise laminar targeting (to S1) and rely on Sema6A for correct positioning.
156
What is the effect of knocking out the β2 subunit of the nicotinic acetylcholine receptor on retinal ganglion cells (RGCs)?
Columnar organisation of RGC projections is disrupted, but laminar targeting remains intact.
157
What does preserved layer targeting in β2 knockouts suggest?
Molecular cues (e.g. Sema6A) are sufficient to guide laminar targeting even without retinal waves.
158
What does disrupted columnar organisation in β2 knockouts indicate?
Retinal cholinergic waves are required for proper column formation in RGC projections.
159
What two processes guide RGC axonal organisation?
Chemoaffinity cues (e.g. Sema6A) → laminar targeting
Activity-dependent signals (e.g. cholinergic retinal waves) → columnar organisation
160
What conclusion can be drawn about the roles of guidance cues vs. activity in visual wiring?
Chemoaffinity cues determine where axons project (laminae)
Neuronal activity shapes how they organise spatially (columns and refinement)
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What type of eye does Drosophila melanogaster have?
A compound eye, made up of ~800 ommatidia (retinulas), each containing 8 photoreceptor cells (R1–R8).
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How are the photoreceptor cells in a Drosophila ommatidium organised?
R1–6: Surround the central core, project to the lamina
R7: Terminates early and is replaced by R8, both project to the medulla
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How does visual information travel from Drosophila photoreceptors to the brain?
R1–6: Synapse in the lamina, which relays info to the medulla
R7 and R8: Project directly to distinct layers of the medulla
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Which layers of the Drosophila medulla do R1–R6 connect to?
Layers 1, 2, and 5 (via lamina neurons)
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Which layers of the Drosophila medulla do R7 and R8 photoreceptors target?
R7 → Layer 6
R8 → Layer 3
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What does the Drosophila visual system reveal about layer-specific targeting?
It shows that laminar organisation is a conserved principle across species, not exclusive to vertebrates.
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What is the 'sevenless' gene in Drosophila and what happens when it is mutated?
It encodes a receptor tyrosine kinase required for R7 photoreceptor specification.
In sevenless mutants, the R7 neuron fails to develop.
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Is sevenless required in the R7 cell itself or another cell?
It functions cell-autonomously in the R7 photoreceptor.
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What is 'boss' and where does it function?
Boss (Bride of Sevenless) is the ligand for the Sevenless receptor.
It functions in the R8 photoreceptor, which lies directly beneath R7.
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What kind of genetic tool was used to determine sevenless and boss function?
Genetic mosaics: creating mutant clones within a wildtype background, using cell-type specific rescue to localise function.
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What intracellular signalling pathway does Sevenless activate?
Sevenless → Drk → Sos → Ras → Raf → MAPK (MAP kinase) → transcription factor activation
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What is the ultimate effect of Sevenless/Boss signalling?
Induces expression of R7-specific genes, allowing proper cell fate specification and survival of the R7 photoreceptor neuron.
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What does this model illustrate about neuronal development?
Cell fate can be specified through direct contact and inductive signalling between neighbouring cells.
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Which photoreceptor cells in Drosophila mediate colour vision, and what Rhodopsins do they express?
R7 and R8 cells mediate colour vision
R7: Express Rh3 (UV) or Rh4 (UV-blue)
R8: Express Rh5 (blue) or Rh6 (green-red)
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Which rhodopsin do Drosophila R1–R6 photoreceptors express, and what is their function?
Rh1, with broad spectral sensitivity
Function primarily in motion detection and brightness, not colour vision
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How are Rh3 and Rh4 expression distributed among R7 cells?
Stochastically distributed across the compound eye
Approx. 30% Rh3, 70% Rh4
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How is rhodopsin expression in R8 linked to that in R7?
R7 Rh3 → R8 Rh5
R7 Rh4 → R8 Rh6
Tightly coupled, always matching
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What transcription factor governs the stochastic specification of R7 cell fate in Drosophila?
Spineless
When expressed in R7: activates Rh4, represses Rh3
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What is the role of spineless in coordinating R7–R8 rhodopsin expression?
Cell-autonomously: promotes Rh4, represses Rh3 in R7
Non-cell-autonomously: suppresses R7-to-R8 signal, allowing Rh6 in R8
No spineless: R7 becomes Rh3, R8 becomes Rh5
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What kind of regulatory mechanism does the R7–R8 pairing illustrate in Drosophila?
A combination of stochastic gene expression and intercellular signalling that ensures paired fate determination.
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To which layer do R7 and R8 photoreceptors project in the Drosophila medulla?
R7 → M6 layer
R8 → M3 layer
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What happens when R7 photoreceptors lack N-cadherin?
They mis-target the M3 layer instead of M6
This mimics R8 cell targeting
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What is the function of N-cadherin in R7 cells?
Acts as a cell adhesion molecule guiding R7 axons to the correct M6 layer
Ensures layer-specific wiring crucial for proper colour vision processing
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Why is proper layer targeting of R7 and R8 important in Drosophila?
Each layer corresponds to specific spectral inputs from different rhodopsins
Correct targeting preserves the colour discrimination circuitry
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What two molecules are required for R8 photoreceptors to target the M3 layer in Drosophila?
Netrin in M3
Frazzled receptor on R8 axons
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What happens if netrin or frazzled is removed?
R8 cells mistarget, failing to reach M3
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What happens if netrin is misexpressed in M1 or M2 layers?
R8 cells mistarget to M1 or M2 instead of M3
Demonstrates netrin’s sufficiency for layer targeting
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What is the role of capricious in R8 layer targeting?
Required for R8 targeting to M3
Loss of capricious → mistargeting (often to M2)
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What happens if capricious is misexpressed in R7 cells?
R7 cells adopt R8-like targeting, misprojecting to M3 layer
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What does the ability to reprogram targeting by manipulating single genes suggest about nervous system evolution?
Small genetic changes in guidance cue expression can produce new circuits
These changes may be co-opted by evolution for novel functions
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Which transcription factor specifies R8 fate in Drosophila photoreceptors?
Senseless
Promotes R8-specific rhodopsin expression
Suppresses R7-specific rhodopsin
Promotes expression of capricious (layer-targeting cue for R8)
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Which transcription factors define R7 identity in Drosophila?
NF-YC: suppresses Senseless
Prospero: suppresses R8 rhodopsin, may promote R7-specific targeting cues
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How is the correct layer-specific targeting of R7 and R8 ensured at the transcriptional level?
Senseless in R8 promotes capricious → targets M3 layer
Prospero (likely) promotes unidentified R7 cue → targets M6 layer
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What is the overall effect of R7 and R8 transcriptional regulation?
Coordinates sensory receptor expression (rhodopsins)
Coordinates axon guidance molecule expression
Ensures neurones connect to the correct lamina with correct function
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