Figure 1.

Summary of principles of organization of spinal neurons. (A) Spinal neurons of particular transmitter phenotypes and morphologies are specified in a dorso-ventral fashion by the combination of transcription factors activated by gradients of diffusible morphogens. V0 interneurons are a mixture of excitatory and inhibitory cells, all with commissural axons; V1 cells are inhibitory with a primary ascending ipsilateral axon; V2 interneurons are a mixture of excitatory and inhibitory cells with a primary descending axon; V3 cells are excitatory and have either a commissural or ipsilateral descending axon. (B) A summary schematic of the developmental order of spinal circuitry and the associated movement in larval zebrafish. Neurons responsible for progressively slower movements in larvae are added as zebrafish develop. (C) Schematic summarizing the recruitment order of interneurons and motoneurons, which occurs from the bottom of spinal cord up. (D) Schematic summarizing the switch in premotor interneuron activity responsible for driving the progressive dorso-ventral activation of motoneurons as larvae swim faster and faster.