Sparse HVC sequences and a hypothesis for their control of respiratory patterns.

A) HVC neurons generate a sparse sequence of activity. Each neuron is active only once per song rendition, and it has been suggested that a sub-population of these neurons is active at each moment in the song [34], [57]. Here we propose that some IPs are generated by a mechanism in which HVC initiates and terminates IPs (I/T model). In this model, HVC ‘triggers’ downstream circuitry (perhaps in the brainstem) that controls the activation and time course of inspiratory pulses. Early in the IP, HVC exerts little control on the time course the pressure waveform. At the end of the IP, HVC again takes over control of respiratory circuitry to generate the EP for the next syllable. B) This model predicts the non-uniform stretch of most IPs observed in our experiments. Slowing of the HVC chain by cooling HVC increases the interval between IP initiation and termination, thus increasing the IP duration without changing the shape of the early part of the IP. Thus, the temperature-dependent stretch of the IP waveform occurs only in the later parts of the IP. Changes in the IP waveform can be described as the earlier or later truncation of an underlying IP waveform.