Neural networks in the spinal cord can generate locomotion in the absence of rhythmic input from higher brain structures or sensory feedback because they contain an intrinsic source of excitation. However, the molecular identity of the spinal interneurons underlying the excitatory drive within the locomotor circuit has remained unclear. Using optogenetics, we show that activation of a molecularly defined class of ipsilateral premotor interneurons elicits locomotion. These interneurons represent the excitatory module of the locomotor networks and are sufficient to produce a coordinated swimming pattern in zebrafish. They correspond to the V2a interneuron class and express the transcription factor Chx10. They produce sufficient excitatory drive within the spinal networks to generate coordinated locomotor activity. Therefore, our results define the V2a interneurons as the excitatory module within the spinal locomotor networks that is sufficient to initiate and maintain locomotor activity.

A biochemical model of the receptor, G-protein and effector (RGE) interactions during transduction in the cilia of vertebrate olfactory receptor neurons (ORNs) was developed and calibrated to experimental recordings of cAMP levels and the receptor current (RC). The model describes the steps from odorant binding to activation of the effector enzyme which catalyzes the conversion of ATP to cAMP, and shows how odorant stimulation is amplified and delayed by the RGE transduction cascade. A time-dependent sensitivity analysis was performed on the model. The model output-the cAMP production rate-is particularly sensitive to a few, dominant parameters. During odorant stimulation it depends mainly on the initial density of G-proteins and the catalytic constant for cAMP production.