FUNCTIONAL-ANATOMY OF NEURAL PATHWAYS CONTRIBUTING TO THE CONTROL OF SONG PRODUCTION IN BIRDS

In birds, as in humans, vocal control involves the intricate coordinat ion of three major groups of muscles, namely, those of the vocal organ , the respiratory apparatus, and the vocal tract, including the jaw an d tongue. The neural pathways involved in the control of each of these groups of muscles are described for songbirds and compared with those in non-oscine birds and mammals. The pathway in songbirds that contro ls the syrinx, the bird's vocal organ, originates in the telencephalon and projects via the occipito-mesencephalic tract directly upon vocal motoneurons in the medulla. Activity in this pathway configures the s yrinx into phonatory positions for the production of species typical v ocalizations. Another component of this pathway mediates control of re spiration during vocalization, since it projects upon both expiratory and inspiratory groups of premotor neurons in the ventrolateral medull a, as well as upon several other nuclei en route. This pathway appears to be primarily involved with the control of the temporal pattern of song, but is also importantly involved in the control of vocal intensi ty, mediated via air sac pressure. There are extensive interconnection s between the vocal and respiratory pathways, especially at brainstem levels, and it may be these that ensure the necessary temporal coordin ation of syringeal and respiratory activity. The pathway mediating con trol of the jaw appears to be different from those mediating control o f the syrinx and respiratory muscles. It originates in a different par t of the telencephalon and projects upon premotor neurons in the medul la that, on preliminary analysis, appear to be separate from those pro jecting upon the syringeal motor nucleus. The separateness of this pat hway may reflect the imperfect correlation of jaw movements with the d ynamic and acoustic features of song. The brainstem pathways mediating control of vocalization and respiration in songbirds have distinct si milarities to those in non-oscine birds and in mammals such as cats an d monkeys. However, songbirds and parrots, like humans, but unlike oth er non-songbirds, have developed a special telencephalic vocal control system for the production of learned vocalizations.