A review of systems and networks of the limbic forebrain/limbic midbrain.

Evolutionarily older brain systems, such as the limbic system, appear to serve fundamental aspects of emotional processing and provide relevant and motivational information for phylogenetically more recent brain systems to regulate complex behaviors. Overall, overt behavior is, in part, determined by the interactions of multiple learning and memory systems, some seemingly complementary and some actually competitive. An understanding of limbic system function in emotion and motivation requires that these subsystems be recognized and characterized as extended components of a distributed limbic network. Behavioral neuroscientists face the challenge of teasing apart the contributions of multiple overlapping neuronal systems in order to begin to elucidate the neural mechanisms of the limbic system and their contributions to behavior. One major consideration is to bring together conceptually the functions of individual components of the limbic forebrain and the related limbic midbrain systems. For example, in the rat the heterogeneous regions of the prefrontal cortex (e.g., prelimbic, anterior cingulate, subgenual cortices and orbito-frontal areas) make distinct contributions to emotional and motivational influences on behavior and each needs consideration in its own right. Major interacting structures of the limbic system include the prefrontal cortex, cingulate cortex, amygdaloid nuclear complex, limbic thalamus, hippocampal formation, nucleus accumbens (limbic striatum), anterior hypothalamus, ventral tegmental area and midbrain raphe nuclei; the latter comprising largely serotonergic components of the limbic midbrain system projecting to the forebrain. The posterior limbic midbrain complex comprising the stria medullaris, central gray and dorsal and ventral nuclei of Gudden are also key elements in the limbic midbrain. Some of these formations will be discussed in terms of the neurochemical connectivity between them. We put forward a systems approach in order to build a network model of the limbic forebrain/limbic midbrain system, and the interactions of its major components. In this regard, it is important to keep in mind that the limbic system is both an anatomical entity as well as a physiological concept. We have considered this issue in detail in the introduction to this review. The components of these systems have usually been considered as functional units or 'centers' rather than being components of a larger, interacting, and distributed functional system. In that context, we are oriented toward considerations of distributed neural systems themselves as functional entities in the brain.