| Column 1 | Column 2 (memorize)
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| Explain the CNS midline guidnce cues. | (1) Repellents from the dorsal spinal cord initially direct commissural axons ventrally. (2) Attraction cues emanate from the ventral spinal cord guiding the axons ventrally. (3) Repulsive cues from the ventral spinal cord then guide the commissural axons across the midline and prevent recrossing. (4) Attractants then guide the axons post-crossing.
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| What happens if a frog's retina is rotated in a young frog? | Frogs miss flies, indicating the the wiring still procedes to go to the same place.
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| What happens if the anterior retinal field of a goldfish is ablated? | Normal innervation of the anterior tectum by posterior retinal axons, but little to no innervation of the posterior tectum. This indicates that a fixed set of cues directs the topographic connections in the visual system.
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| What did laser ablation of P neurons show in the grasshoper's CNS? | It showed that the P neuron established the axon pathway for the G neuron, since the G neuron was not able to join with the axon of the P neuron and its growth cone stalled were they were supposed to meet.
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| What are the general classes of guidance? | 1. Contact attraction 2. Contact repulsion 3. chemoattraction and 4. chemorepulsion
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| Netrin-1 | Chemoattractive. Required for crossing the midline.
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| UNC5 | The Netrin-1/UNC6 repulsive receptor.
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| UNC40 | The Netrin-1/UNC6 attractive receptor. (UNC40/DCC)
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| What did the DCC experiment show? | DCC(/UNC40) receptor expressing cell was attracted to Netrin. The DCC+UNC5 receptor expressing cell was repulsed by Netrin. Inhibition of cAMP-mediated signaling on DCC+ cell using a protein kinase A (PKA) inhibitor repulsed by Netrin.
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| Semaphorin | Potent repellents (eg. Sema3A and Collapsin-1).
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| Neuropilin-1 | The receptor for the semaphorin Sema3A.
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| What happens with Sema3A knockouts? | The axons of the mouse proliferate into regions that they shouldn't. Same thing happens when its receptor neuropilin-1 is knocked out.
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| CAMs | Cell adhesion molecules. Act as contact-mediated cues.
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| What happens in FasII mutants? | Fasciclin II (FasII) is part of the Ig superfamily of CAMs. When it is knocked out, the cells MP1, dMP2, and pCC do not make contact with each other, as they normally do.
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| Give an example where an Ig superfamily members counter repulsive cues to maintain a balance of attraction and repulsion for regulating fasciculation. | In Drosophia, motor neurons extend from the CNS into subsets of axons at different choice points. If Sema1a is under expressed or FasII is overexpressed, then they fail to counter the repulsive cues and fail to defasciculate from the main bundle. However, if Sema1a is under expressed AND FasII is underexpressed then the balance of attraction and repulsion is restored.
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| Give an example where an Ig superfamily member signals repulsion following homophilic interactions. | Dscam is required for axons to break the homophilic interactions and branch out. Dscam double knockouts fail to sufficiently branch out.
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| How do cells cross the CNS midline instead of just getting stuck at the floor plate? | Before crossing the midline Robo-1 and -3 are bound, inactivating each other. Upon crossing the midline Robo-3 is no longer expressed, and Robo-1 is repulsed by the midline repellent Slit.
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| What cues do axons use after crossing the CNS midline? | They use a gradient of the morphogen Wnt to project along the anterior-posterior axis.
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| What morphogens are involved in projecting axons to the midline? | BMPs give an initial "push" ventrally, whereas Shh pulls the axons ventrally. Wnt is then used for anterior-posterior projections.
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