Neuron I

and even convert repulsion to attraction if there are high levelsmediated by second messengers. This leads to polarization of the growth cone, and a turn toward (attraction) or away (repul-sion) from the source of the guidance cue. cGMP. Decreasing the baseline calcium level in the growth cone converts attraction to repulsion, implying an interaction between the baseline calcium level and the amount of calciumCalcium signals mediate both growth cone turning and outgrowth (Cohan et al., 1987). Binding of the guidance cue to influx in determining the sign of the response (Zheng, 2000). Furthermore, increasing cAMP on one side of the growth coneThe ability of growing axons to accurately locate targets during development or regeneration is critical for the formation of correct neural circuits. Growing axons are tipped with growth cones, which detect distributions of molecular guidance cues in their environment. A key mechanism for guidance is chemo-taxis, whereby growth cones detect and respond to concentra-tion gradients of these cues (Tessier-Lavigne and Goodman