Growing axons have a highly motile structure at the growing tip called the growth cone, which "sniffs out" the extracellular activities in the environment for signals that instruct the axon which direction to grow. These signals, called guidance cues, can be fixed in place or diffusible; they can attract or repel axons. Growth cones contain receptors that recognize these guidance cues and interpret the signal into a chemotropic response. The general theoretical framework is that when a growth cone "senses" a guidance cue, the receptors activate various signaling molecules in the growth cone that eventually affect the cytoskeleton. If the growth cone senses a gradient of guidance cue, the intracellular signaling in the growth cone happens asymmetrically, so that cytoskeletal changes happen asymmetrically and the growth cone turns toward or away from the guidance cue.