Functional anatomy of neural pathways contributing to the control of song production in birds

In birds, as in humans, vocal control involves the intricate coordination of three major groups of muscles, namely, those of the vocal organ, the respiratory apparatus, and the vocal tract, including the jaw and 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 controls 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 syrinx into phonatory positions for the production of species typical vocalizations. Another component of this pathway mediates control of respiration during vocalization, since it projects upon both expiratory and inspiratory groups of premotor neurons in the ventrolateral medulla, 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 intensity, mediated via air sac pressure. There are extensive interconnections between the vocal and respiratory pathways, especially at brainstem levels, and it may be these that ensure the necessary temporal coordination of syringeal and respiratory activity. The pathway mediating control of the jaw appears to be different from those mediating control of the syrinx and respiratory muscles. It originates in a different part of the telencephalon and projects upon premotor neurons in the medulla that, on preliminary analysis, appear to be separate from those projecting upon the syringeal motor nucleus. The separateness of this pathway may reflect the imperfect correlation of jaw movements with the dynamic and acoustic features of song. The brainstem pathways mediating control of vocalization and respiration in songbirds have distinct similarities to those in non-oscine birds and in mammals such as cats and monkeys. However, songbirds and parrots, like humans, but unlike other non-songbirds, have developed a special telencephalic vocal control system for the production of learned vocalizations.