So far we have seen that components of the extracellular matrix and certain cell-adhesion molecules participate in neuronal outgrowth. Growth cones can also respond to soluble factors. Small molecules such as neurotransmitters and various soluble peptides have been shown to modulate growth cone motility in vitro. These studies, as well as the observation that axons often travel long distances toward their targets, support the long-held notion that specific secreted molecules can act as long-range attractants.