Fig. 9. How fasciculation between commissural axons alters their midline kinetics. A schematic drawing shows a leader axon (pink) and a number of follower axons (blue and black) growing through the midline. The leading axon being the first, is completely exposed to the guidance cues in the environment. Its growth cone must sense all the positive and negative midline cues and interpret them accordingly, which results in slow kinetics of leader axons at the midline where these cues are found. By growing along the leader, follower axons are less exposed to midline cues. This can happen because their growth cones are shaped differently, which limits their exposure to conflicting midline signals and/or because the substrate that the leader axon provides contributes an extra signal that allows them to grow across the midline swiftly.