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CBNS 121 LECTURE 11 TARGET SELECTION, Exams of Social Sciences

United States International UniversitySocial Sciences

CBNS 121 LECTURE 11 TARGET SELECTION

Typology: Exams

2025/2026

Available from 01/22/2026

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CBNS 121 LECTURE 11 TARGET SELECTION
What is target selection? - Answers - The final step in axon guidance; growth cone
gets closer to target
Axon and target recognize each other and promote formation of synapses
Describe stages of axon formation - Answers - Axon guidance, axon targeting,
synapse formation
Describe stages of targeting - Answers - Defasciculation (de-bundle axons)
โ†’ enter target and slow axon
โ†’ promote branching
โ†’ find topographic location
โ†’ find layer
โ†’ connect with target cell
Which factors affect general target selection? - Answers - Chemoattractants (netrin)
Chemorepellents (semaphorins, ephrins)
Growth factors (FGF2, neurotrophins); concentration differential between pathway and
target (e.g. FGF2)
Changes in levels of above factors
What is an example of a chemoattractant? - Answers - Netrin
What is an example of chemorepellent? - Answers - Semaphorins, ephrins
What is an example of growth factors? - Answers - FGF2, neurotrophins (survival
factors for neurons)
Describe innervation of Drosophila muscle fibers by motor neurons - Answers - Each
muscle fiber is innervated by one motor neuron
Motor neuron targets appropriate cell via adhesive, attractive, and repulsive cues that
allow motor neurons to target correct cell
Describe netrin mutants in Drosophila motorneurons - Answers - Acts as
chemoattractant
Netrin/frazzled (netrin receptor) mutants show specific targeting defects
Describe connectin. What is the effect of loss of connectin in Drosophila motorneurons?
- Answers - Homophilic adhesion molecule
Loss yields minimal defects
Ectopic expression yields more serious
Describe the effect of fasII on muscles - Answers - Homophilic adhesion molecule
(binds to fasII in growth cone)
pf3
pf4
pf5

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CBNS 121 LECTURE 11 TARGET SELECTION

What is target selection? - Answers - The final step in axon guidance; growth cone gets closer to target Axon and target recognize each other and promote formation of synapses Describe stages of axon formation - Answers - Axon guidance, axon targeting, synapse formation Describe stages of targeting - Answers - Defasciculation (de-bundle axons) โ†’ enter target and slow axon โ†’ promote branching โ†’ find topographic location โ†’ find layer โ†’ connect with target cell Which factors affect general target selection? - Answers - Chemoattractants (netrin) Chemorepellents (semaphorins, ephrins) Growth factors (FGF2, neurotrophins); concentration differential between pathway and target (e.g. FGF2) Changes in levels of above factors What is an example of a chemoattractant? - Answers - Netrin What is an example of chemorepellent? - Answers - Semaphorins, ephrins What is an example of growth factors? - Answers - FGF2, neurotrophins (survival factors for neurons) Describe innervation of Drosophila muscle fibers by motor neurons - Answers - Each muscle fiber is innervated by one motor neuron Motor neuron targets appropriate cell via adhesive, attractive, and repulsive cues that allow motor neurons to target correct cell Describe netrin mutants in Drosophila motorneurons - Answers - Acts as chemoattractant Netrin/frazzled (netrin receptor) mutants show specific targeting defects Describe connectin. What is the effect of loss of connectin in Drosophila motorneurons?

  • Answers - Homophilic adhesion molecule Loss yields minimal defects Ectopic expression yields more serious Describe the effect of fasII on muscles - Answers - Homophilic adhesion molecule (binds to fasII in growth cone)

Muscles with higher relative levels tend to receive extra synapses What does TN motorneuron avoid? What happens when adhesion is added? What happens if repulsion is lowered? - Answers - Avoid muscle 6 and 7 Increase adhesion: increase in FasII on muscle 6 โ†’ forms synapse on muscle 6 Lower repulsion: semaphorin lowered What is the effect of increasing attraction OR reducing repulsion in TN? What is the effect of increasing attraction AND reducing repulsion in TN? - Answers - OR: Promotes innervation AND: promotes balance (TN neurons goes back to avoiding muscles 6 and 7) What does RP3 motorneuron target? What is the effect of increasing attraction OR reducing repulsion? What is the effect of increasing attraction AND reducing repulsion? - Answers - Targets muscle 6 and 7 OR: neuron does not innervate muscle AND: promotes balance (RP3 neurons goes back to innervating muscles 6 and 7) How do motor neurons interact with muscle fibers? - Answers - Combinatorial coding Patterns of activity can modulate responses to molecules What factors affect targeting to individual muscle fibers? How does this work? - Answers - Factors that influence axonal guidance: netrins, semaphorins, FasII Alterations in relative levels of these factors are sufficient to induce changes in patterns of targeting What is the target of retinal axon? - Answers - Optic tectum Superior colliculus What is the function of FGF? How does FGF act on the tectum? - Answers - Growth factor, can act as a morphogen; no directionality FGF lowers when it reaches the tectum โ†’ axons slow down settle โ†’ facilitate attraction between growth cone and target What happens if FGF signaling is increased in tectum? What happens if FGF signaling is blocked in tectum? - Answers - Increased: axons keep going until they find a drop in FGF Blocked: axons grow towards the tectum (via other guidance cues i.e. semaphorin) but go around tectum (cannot locate) What are neurotrophins? - Answers - A family of neurotrophic factors (survival factors for neurons) Promote growth of axon Facilitate innervation

Which ephrins are expressed in the LGN/MGN boundary? - Answers - EphrinA2 acts as repellant EphrinA How do cells target to the correct layer in the spinal cord? - Answers - Via semaphorins and repulsion SEMA-III repels NGF-dependent axon NT-3 dependent axon not repelled What is the function of adhesion molecules and how do they work? - Answers - Homophillic Adhesion molecules constrain axons through contact-mediated interactions Promote axons stay in the appropriate target What is the effect of SEMA-IIIa KO in spinal cord? - Answers - Nociceptor axons enter ventral spinal cord because removed boundary What is the function of NGF? - Answers - Growth factor that promotes survival of nociceptors Which factors combine to achieve target selection? - Answers - Repulsive cues, adhesion molecules, and branching/defasciculation factors How does branching work? How does defasciculaiton work? - Answers - Branching slows down the axon โ†’ forces axon to settle Defasciculation: axons leave highway that they are following to get to the proper target What is the function of adhesion molecules in target selection? - Answers - Promote that axons stay in the appropriate target What are examples of topographic maps? - Answers - Somatotopic representation in the neocortex: cortical "homunculus" Olfactory mapping What is topographic mapping? - Answers - Orderly representation within the nervous system, of some particular physical aspect of the outside world How does olfactory mapping differ from retinal mapping? - Answers - In visual system mapping is by anatomical axes (one retina to tectum) In olfactory system mapping is by odorant specificity (olfactory neurons can only sense one smell; only express one odorant receptor) Which olfactory neurons converge to the same location? - Answers - Neurons expressing the same type of odorant receptor converge to the same location

Where are topographic maps for the visual system established and how? - Answers - Guidance cues establish topographic maps in the superior colliculus Does the visual system maintain axonal associations in optic nerve and tract to build point-to-point specificity in the visual system? Explain. - Answers - No "Double-ventral" retina: retinotectal mapping is not due to order in the optic tract Does timing of axonal growth impact point-to-point specificity in the visual system? Explain. - Answers - No. Heterchronic retina transplant finds that retinotectal mapping is not due to timing of RGC axon entry What was the method for Sperry's experiment? - Answers - Rotate the dorsal retina of frog to ventral retina In normal retina, dorsal axon project to ventral tectum and ventral retina projects to dorsal tectum What were the predictions for Sperry's experiment? - Answers - Non-specific targeting and chemoaffinity Axons will terminate as if retina was not rotated If axons are cross linked, transplanting retina will result in axons finding their proper targets (will terminate in opposite position from normal) Describe the result of Sperry's experiment - Answers - Inversion of mapping โ†’ frog's tongue in opposite direction With two ventral retinas, all axons end up on dorsal tectum โ†’ axons from the retina detect something specific in tectum Describe non-specific targeting - Answers - Axons will terminate as if the retina was not rotated Describe chemoaffinity - Answers - Axons find their proper target in the tectum i.e. they will terminate in the opposite position from normal Describe how ventral RGCs project and what they activate - Answers - First axons that arrive to retina come from dorsal retina (signal to ventral tectum); second axon settles further in tectum Dorsal retina โ†’ ventral tectum; ventral retina โ†’ dorsal tectum What is the effect of maladaptive responses? - Answers - Suggest chemospecificity Eye rotation: fly is detected in the bottom visual field (original dorsal retina/ventral tectum) Describe topographic mapping by complementary gradients - Answers - Axons project to correct targets, but detects stimulus in the wrong place