Axon Guidance Mechanisms and Neural Map Formation

What is the midline in the context of axon guidance?

The midline is the median line or plane of bilateral symmetry where commissural axons cross.

Where do commissural axons cross in the spinal cord?

At the floor plate, which is the most ventral part of the spinal cord.

What experimental method was used to test how commissural axons cross the midline?

An explant of the dorsal spinal cord was taken to observe axon behavior.

What was observed when neurons close to the roof plate were tested?

They deviated away to the right.

What role do secreted factors play in axon guidance?

Secreted factors can act as repulsive or attractive cues for axon migration.

What was the effect of BMP-7 on axon guidance in the experiments?

Axons next to BMP-7 expressed explants deviated, indicating a repulsive cue.

What gradient do Netrins form in the spinal cord?

A dorso-ventral gradient that attracts axons to the floor plate.

Why are Netrins considered necessary for axon guidance?

In knockout (KO) mice lacking Netrins, axons did not reach the floor plate.

What additional role does Shh play in axon guidance?

Shh acts as an attractive guidance cue in the floor plate.

What happens to axons in the absence of both Netrin and Shh?

No axons reach the floor plate.

What regulates the crossing of axons at the midline?

Slits regulate the crossing of axons.

What is the purpose of the 'open book' preparation in experiments?

It allows researchers to trace axons with dye to see if they cross the floor plate.

What happens to axons in slit mutants?

Axons can enter the floor plate but get stuck and travel anteriorly on the wrong side.

What is the significance of the DCC gene in congenital mirror disorder?

Mutations in DCC cause approximately 35% of cases of congenital mirror disorder.

What is a retinotopic map?

A spatial organization of neural connections that maintains the relationships between cell bodies of neurons and their corresponding axons.

What is a neural map?

The organization of neural connections that reflects spatial or functional relationships between neurons.

How can a neural map be characterized?

It can be both spatial and functional, based on the arrangement of axons.

What is the effect of ephrin-A5 on axon guidance?

Ephrin-A5 repels posterior axons with no effect on anterior axons.

What was observed in the experiments with ephrin-A5 mutants?

Without ephrin-A5, anterior axons were more spread out but did not fully reach the posterior side.

What is the relationship between the orientation of axons and their target regions in an inverted retina?

In an inverted retina, anterior axons connect to posterior brain regions.

What is the significance of the stripe assays in axon guidance studies?

They demonstrate the repulsive effects of ephrin-A5 on posterior axons.