New modules are added to vibrissal premotor circuitry with the emergence of exploratory whisking

Rodents begin to use bilaterally coordinated, rhythmic sweeping of their vibrissae ("whisking") for environmental exploration around 2 weeks after birth. Whether (and how) the vibrissal control circuitry changes after birth is unknown, and the relevant premotor circuitry remains poorly characterized. Using a modified rabies virus transsynaptic tracing strategy, we labeled neurons synapsing directly onto vibrissa facial motor neurons (vFMNs). Sources of potential excitatory, inhibitory, and modulatory vFMN premotor neurons, and differences between the premotor circuitry for vFMNs innervating intrinsic versus extrinsic vibrissal muscles were systematically characterized. The emergence of whisking is accompanied by the addition of new sets of bilateral excitatory inputs to vFMNs from neurons in the lateral paragigantocellularis (LPGi). Furthermore, descending axons from the motor cortex directly innervate LPGi premotor neurons. Thus, neural modules that are well suited to facilitate the bilateral coordination and cortical control of whisking are added to the premotor circuitry in parallel with the emergence of this exploratory behavior.