One kind of molecule, for example, guides nerve axons toward the belly of a worm. Some guidance molecules have other functions. Some guidance molecules attract certain axons while repelling others. Scientists believe an axon's response to different molecules is determined by proteins called receptors on the surface of the growth cone that the molecules fit into much as a key fits a lock. When a molecule attaches to these receptors, it causes the growth cone to grow or stop or turn.
Cells can change the receptors and other molecules that are active at a given time.
Thus, growth cones can respond to different guidance molecules at different stages during their development and change direction. Scientists are rapidly identifying new guidance molecules and learning how they function. They have described several major families of guidance molecules, and believe more than a hundred other molecules may be found. Many researchers believe repulsive guidance molecules may prevent regeneration of axons in the spinal cord and brain after injury, stroke or neurodegenerative diseases. If so, drugs that interfere with these molecules might be developed to make nerve regeneration possible.