[A thalamo-cerebral connection in vocal center of canary]

Canary song is controlled by two groups of thalamo-cerebral nuclei. One, the hyperstriatum ventrale pars caudale (HVc) and the robust nucleus of the archistriatum (RA), is a motor driving system for vocalization. The other group, which includes the HVc, the nucleus magnocellularis of neostriatum (MAN), Area X and the nucleus dorsointermedius posterior thalami (DIP), modulates the driving system. The HVc receives synaptic projections from the MAN and sends fibers to Area X. Axons of Area X monosynaptically innervate the thalamic nucleus, the DIP, from which neurons extend axons back to the cerebral nucleus, the MAN. DIP neurons relay incoming impulses by way of Area X to the MAN. Double labeling of DIP neurons with HRP and Fast Blue shows that axonal terminals from Area X connect directly with DIP neurons which send fibers to the MAN. The axon formed a bulge from which multiple branches extended to the postsynaptic cell bodies covering most of the surface. The structure of the DIP synapse may be related to a characteristic pattern of discharge of the DIP neuron, which is transmitted over thalamic projection to cerebral vocal nuclei.