The neural substrate for 'learned' and 'nonlearned' activities in birds: Adiscussion of the organization of bulbar reticular premotor systems with side-lights on the mammalian situation

The reticular formation of the brainstem contains premotor systems for vari ous musculomotor systems. In this paper, the bulbar premotor systems for ja w and tongue movements, head and neck movements, locomotion, and respiratio n and vocalization in birds are reviewed and compared to premotor systems i n mammals. Roughly, the bulbar reticular formation can be subdivided in thr ee longitudinal zones: a dorsolateral (RPcdl) and a ventromedial (RPcvm) pa rvocellular zone and a gigantocellular zone (RGc), RPcdl contains premotor neurons for the jaw and neck system, RPcvm for the jaw, tongue and neck sys tem, and RGc for the tongue and locomotory system. RPcdl receives input fro m the descending sensory trigeminal system, parts of RPcvm and RGc from ves tibular nuclei, whereas the tectum has a projection to the contralateral RG c. RPcdl and RPcvm receive substantial telencephalic input through the occi pitomesencephalic tract. The bulbar part of the respiratory system consists of a series of cell groups in the ventrolateral reticular formation and ha s connections with motor centers of the vocalization system. The similariti es and differences between the avian and mammalian situation are discussed. Musculomotor systems participate in various activities. It is argued that a premotor system should possess sufficient flexibility to control the part icipation of a motor system in the different activities. This flexibility m ay permit the occurrence of learning processes in terms of refining basical ly existing motor patterns. The emergence of new and more complex motor pat terns as in vocalization requires the involvement of hierarchically higher brain centers.