The central pathway for motor control of learned birdsong has been the subject of intensive investigation in the three decades since its discovery (Nottebohm et al., 1976
). The song motor circuit includes feedforward projections between the telencephalic premotor and motor nuclei, HVC(used as a proper name) and the robust nucleus of the arcopallium (RA), respectively, with RA innervating brainstem vocal-respiratory networks directly (see ) (Vicario, 1991
; Wild, 1993b
; Suthers and Margoliash, 2002
). RA projection neurons (PNs) project topographically onto their brainstem targets: neurons in the dorsal third of RA project to expiratory-related [nucleus retroambigualis (RAm)] or inspiratory-related [nucleus parambigualis (PAm)] premotor neurons in the medullary ventral respiratory column, whereas neurons in the ventral two-thirds of RA project directly to the tracheosyringeal portion of the hypoglossal motor nucleus (XIIts), which innervates the muscles of the syrinx, the bird’s vocal organ (Vicario, 1991
; Wild, 1993a
). Robust connections between RAm, PAm, and XIIts are thought to mediate the coordination of vocal and respiratory muscle activity during singing, under the control of the telencephalon (Sturdy et al., 2003
; Kubke et al., 2005
Figure 1 Lentivirus-based pathway tracing in the song motor circuit. A, A schematic of the zebra finch brain, cut parasagittally, highlighting relevant song-related brain circuits. The anterior forebrain pathway, which drives motor variability, originates in HVC (more ...)
The current understanding that all RA PNs send their axons out of the telencephalon serves as an organizing principle of song motor circuit anatomy. Emergent from this anatomy is the view that song patterning mechanisms involve the unidirectional flow of information from HVC to RA. Indeed, a hierarchical model of song, in which HVC activity codes for song syllables and RA activity codes for subsyllable notes (Yu and Margoliash, 1996
), as well as a feedforward model, in which HVC functions as an autonomous pattern generator, coding for all song components (Hahnloser et al., 2002
), presume that HVC functions without motor-related feedback during song production. A more recent recursive model posits that motor-related feedback from the brainstem plays an important role in song production by providing timing signals to HVC (Ashmore et al., 2005
). Electrical stimulation of PAm elicits disruption of ongoing song, supporting the idea that an indirect respiratory-related motor feedback from the brainstem can influence HVC activity. Because this recursive pathway relays information from PAm to HVC via a thalamic intermediary (nucleus uvaeformis) (Striedter and Vu, 1998
), it may affect HVC activity over relatively long timescales. A direct RA–HVC pathway, however, such as the one suggested by Wild (2004)
, could provide shorter-latency feedback to HVC that might be of special functional importance, given the precise and rapid vocal and respiratory modulations typical of birdsong.
We examined in greater detail axonal projections from RA using a variety of viral, traditional, and physiological tracing methods. After injections of a sensitive viral-mediated tracer into RA, labeled fibers that resemble axon terminals were found in HVC. Injection of a viral-mediated tracer that selectively labels presynaptic terminals showed that a population of RA PNs form synapses in HVC. Consistent with these findings, conventional retrograde tracer injections in HVC confirmed the presence of a subset of neurons in dorsocaudal regions of RA, and some of these neurons were found by dual tracer injections into RAm and HVC also to project to RAm in the lateral medulla. Therefore, a previously unknown pathway may enable a subset of RA neurons to provide song-related signals to the respiratory brainstem but also transmit a copy of this information to song patterning networks in HVC.