The role of a trigeminal sensory nucleus in the initiation of locomotion

Key points •  Rhythmic activity is a feature of many regions of the CNS, but surprisingly, no precise pathway for the initiation of locomotion is yet known for any vertebrate. •  Using a well‐proven, simple vertebrate system, the hatchling Xenopus tadpole, we report here our discovery of a detailed neuron‐by‐neuron pathway for initiating locomotor activity on one side of the CNS. •  We describe a small population of brainstem neurons (trigeminal interneurons) that are directly excited by trigeminal sensory neurons when the head skin is touched. •  These neurons amplify brief sensory signals and relay excitation to an electrically coupled population of hindbrain reticulospinal neurons (descending interneurons), whose firing initiates swimming locomotion. •  We believe that our discovery of this primitive, direct pathway, which appears simpler than initiation pathways so far defined in invertebrates, is of evolutionary interest and raises the important possibility of equivalent pathways in more complex vertebrates including mammals.