Figure 2: Placing intrinsic properties of HVC neurons in the context of the song system and singing behavior.

Schematic of a male zebra finch along with a sample spectrogram of a song showing frequency of sound versus time are shown in the lower panel. Reasonably complete song system and auditory system pathways are shown in the middle panel (the cross-hemisphere connections and pathways to SNc/VTA and VP are not illustrated). The vocal motor pathway (VMP, blue color) contains circuits that directly pattern song output. Incoming sensory information is processed by HVC and NIf (green color), and HVC and RA shape motor sequences that project out to the peripheral vocal organs, the syrinx and respiratory muscles, via the hindbrain nucleus nXIIts and brainstem respiratory nuclei RAm and PAm (purple color). In particular, the dorsal part of RA (purple) sends its afferent axons to nuclei DMP, DM, RAm and PAm, while the ventral part of RA (blue) send its axons down to nXIIts. The anterior forebrain loop (AFP, red color) pathway contains circuits that are important for song learning and song variability. HVC sends projections to a basal ganglia loop (striatal-thalamic-cortical-striatal) which has an important output projection to the song motor pathway at nucleus RA. The auditory pathway (light brown color) contains circuits that process sounds, including feedback during singing. Auditory signals enter the brain at the cochlear nucleus (CN) eventually reaching forebrain nuclei such as NCM and CM. Two nuclei that are known to be connected to the song system but are not well understood are medial MAN, which sends its output to HVC, and VTA (orange color). VTA sends dopaminergic inputs to area X. The upper panel shows a schematic of HVC’s internal circuitry. HVC includes multiple types of HVC_X neurons that project to area X, HVC_RA neurons that projects to nucleus RA, HVC interneurons, and a very sparse population of HVC_Av that projects to nucleus Avalanche. The nucleus is bathed with a wide set of hormones and neuromodulators (colored circles). X-projecting and RA-projecting neurons are known to excite interneurons via NMDA and AMPA synapses, while interneurons in their turn inhibit both classes of projecting neurons via GABA synapses. Moreover, HVC_RA neurons send excitatory projections onto HVC_AV neurons. See Ashmore et al. 2005 for nomenclature and further discussion.