Abstract
Motoneurons are known to be an essential component of central pattern generators in invertebrates, but it is only recently that they have been shown to play a similar role in vertebrate locomotor circuits. Here, we review early experiments implicating motoneurons in the genesis of spontaneous motor activity in development and more recent experiments identifying motoneurons as important regulators of locomotor activity in the adult zebrafish and in the neonatal mouse spinal cord. We discuss the mechanisms responsible for these actions, the experimental challenges in studying the role of motoneurons in the mammalian spinal cord and the functional significance of the excitatory influence of motoneuron activity on locomotor behavior.