Motor neurons (MNs) are a diverse class of cells that originate in the central
nervous system and project to muscles in the periphery. The development of all MNs
subtypes begins with Sonic Hedgehog (Shh) signaling in the spinal cord and then
progresses to increasingly specific guidance signaling systems. MN subtypes are
classified by the combined activities of transcription factors and molecular signaling
systems that determine axon trajectory. Molecular cues have been attributed to the
organization of MNs into columns within the spinal cord and then into specific motor
pools. These cues have also been shown to be responsible for axon guidance into the
periphery and important for axons to innervate the correct targets. Despite advances in
research on specific guidance cues used by different subdivisions of MNs, it is still not
fully understood how signaling systems work with one another in the same environment
to guide different classes of MNs to their respective muscle targets
