A new class of spinal interneurons: the origin and function of C boutons is solved.

C boutons, a major synaptic input to motor neurons, were first described years ago, but their origin and functional significance were unknown. In this issue of Neuron, a group of cholinergic interneurons located near the central canal is identified as the source of C boutons. The rhythmic activity in these neurons during locomotion increases motor neuronal excitability, thereby potentiating the strength of muscle contraction.