Figure 1.

Simplified schematic of the avian song-control system, including steroid receptor distribution. HVC and RA comprise the descending telencephalic motor circuit. RA projects to the brainstem motoneurons (including nXIIts) that control the muscles involved with respiration and the sound production organ, the syrinx. RA also projects back to HVC (Roberts et al., 2008). HVC, X, DLM, and LMAN comprise the anterior forebrain pathway, which is necessary for song learning and some forms of adult song plasticity. DLM, Medial portion of the dorsolateral nucleus of the anterior thalamus; nXIIts, the tracheosyringeal portion of the hypoglossal nucleus; RA, robust nucleus of the arcopallium; X, Area X, a subdivision of the medial striatum. Nomenclature used here follows Reiner et al. (2004). We note that only androgen receptor mRNA and estrogen-concentrating cells have ben detected in Area X.