Neurogenesis in the adult avian song-control system.

New neurons are added throughout the forebrain of adult birds. The song-control system is a model to investigate the addition of new long-projection neurons to a cortical circuit that regulates song, a learned sensorimotor behavior. Neuroblasts destined for the song nucleus HVC arise in the walls of the lateral ventricle, and wander through the pallium to reach HVC. The survival of new HVC neurons is supported by gonadally secreted testosterone and its downstream effectors including neurotrophins, vascularization, and electrical activity of postsynaptic neurons in nucleus RA (robust nucleus of the arcopallium). In seasonal species, the HVC→RA circuit degenerates in nonbreeding birds, and is reconstructed by the incorporation of new projection neurons in breeding birds. There is a functional linkage between the death of mature HVC neurons and the birth of new neurons. Various hypotheses for the function of adult neurogenesis in the song system can be proposed, but this remains an open question.