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Evidence for "direct" and "indirect" pathways through the song system basal ganglia

TitleEvidence for "direct" and "indirect" pathways through the song system basal ganglia
Publication TypeJournal Article
Year of Publication2005
AuthorsFarries MA, Ding L, Perkel DJ
JournalJ Comp Neurol
Date Published2005
ISBN Number0021-9967 (Print)0021-9967 (Linking)
KeywordsAnimals, Basal Ganglia/*physiology, Electric Stimulation, Efferent Pathways/*physiology, Electrodes, Implanted, Electrophysiology, Male, Finches/*physiology, Fluorescent Dyes, Glutamic Acid/physiology, Receptors, GABA-A/physiology, Receptors, N-Methyl-D-Aspartate/physiology, Terminology as Topic, Washington

Song learning in oscine birds relies on a circuit known as the "anterior forebrain pathway," which includes a specialized region of the avian basal ganglia. This region, area X, is embedded within a telencephalic structure considered homologous to the striatum, the input structure of the mammalian basal ganglia. Area X has many features in common with the mammalian striatum, yet has distinctive traits, including largely aspiny projection neurons that directly innervate the thalamus and a cell type that physiologically resembles neurons recorded in the mammalian globus pallidus. We have proposed that area X is a mixture of striatum and globus pallidus and has the same functional organization as circuits in the mammalian basal ganglia. Using electrophysiological and anatomical approaches, we found that area X contains a functional analog of the "direct" striatopallidothalamic pathway of mammals: axons of the striatal spiny neurons make close contacts on the somata and dendrites of pallidal cells. A subset of pallidal neurons project directly to the thalamus. Surprisingly, we found evidence that many pallidal cells may not project to the thalamus, but rather participate in a functional analog of the mammalian "indirect" pathway, which may oppose the effects of the direct pathway. Our results deepen our understanding of how information flows through area X and provide more support for the notion that song learning in oscines employs physiological mechanisms similar to basal ganglia-dependent forms of motor learning in mammals.