Guidance of motor axons: where do we stand?

Motor neurons are among the longest projection neurons and their axons extend into the periphery following highly stereotypical pathways. Motor neurons share similar guidance molecules with other projection neurons. Hence, one challenge in the field has been to understand how the vast complexity of connections can be regulated by a relatively small number of factors. In this review, we describe the transcriptional programs and signaling pathways that guide motor axons as they navigate toward their muscle targets. In particular, we underscore how signals triggered by these pathways are integrated in time and space to increase the diversity of the steering mechanisms and improve the accuracy of axon path finding.