Resurgent Na+ currents promote ultrafast spiking in projection neurons that drive fine motor control

The underlying mechanisms that promote precise spiking in upper motor neurons controlling fine motor skills are not well understood. Here we report that projection neurons in the adult zebra finch song nucleus RA display: 1) robust high-frequency firing, 2) ultra-short half-width spike waveforms, 3) superfast Na+ current inactivation kinetics and 4) large resurgent Na+ currents (INaR). These spiking properties closely resemble those of specialized pyramidal neurons in mammalian motor cortex and are well suited for precise temporal coding. They emerge during the critical period for vocal learning in males but not females, coinciding with a complete switch of modulatory Na+ channel subunit expression from Nav{beta}3 to Nav{beta}4. Dynamic clamping and dialysis of Nav{beta}4s C-terminal peptide into juvenile RA neurons provide evidence that this subunit, and its associated INaR, promote neuronal excitability. We propose that Nav{beta}4 underpins INaR that facilitates precise, prolonged, and reliable high-frequency firing in upper motor neurons.