(from Latin, limbus: border) The limbic system consists of a ring of structures that line the inner border of the cerebral hemispheres, as well as the diencephalic (see DIENCEPHALON) structures closely associated with them. These include the PARAHIPPOCAMPAL GYRUS, CINGULATE CORTEX, SUBCALLOSAL GYRUS, HIPPOCAMPUS, AMYGDALA, MAMMILLARY BODIES of the HYPOTHALAMUS, and anterior thalamic (see THALAMUS) nuclei. Other structures that are often included in the limbic system are the entire hypothalamus, the SEPTAL NUCLEI and the HABENULA. The limbic system governs emotional aspects of behaviour, memory, and pleasure and pain.
The concept of the limbic system derives from the term grand lobe limbique that was used by Paul Broca (1824–1880) in 1878 to refer to the inner cortical structures. He did not discuss its function. In 1937, James Papez (1883–1958) proposed that a closed neuronal circuit through these structures underlies the expression of emotion. This circuit, known as PAPEZ CIRCUIT, starts with the hippocampus, and is relayed through the mammillary body, anterior thalamic nuclei, and cingulate cortex, and back to the hippocampus. In 1937, Klüver & Bucy reported that bilateral damage to the temporal lobes (see TEMPORAL LOBE), including the hippocampus and amygdala, produces dramatic changes in emotional behaviour in monkeys (see KLÜVER-BUCY SYNDROME). In the 1950s, Paul MacLean suggested the term limbic system which refers to all of the structures mentioned as well as the hypothalamus, septal nuclei, NUCLEUS ACCUMBENS, and ORBITOFRONTAL CORTEX. McLean also suggested that the limbic system represents a phylogenetically old part of the brain. These early studies provided the basis for studying the neurobiological basis of EMOTION.
Although the general scheme of the limbic pathways conceived by the early investigators was validated by modern anatomical studies, many corrections were also made in the details, and many connections have been added to the old scheme. The major input to the hippocampus is now known to come from the ENTORHINAL CORTEX via the PERFORANT PATH, not from the cingulate cortex, which instead projects to the SUBICULUM. Because the entorhinal cortex receives input from the adjacent ASSOCIATION CORTEX in the temporal lobe, it provides a link between the NEOCORTEX and the limbic system. The projection to the mammillary body via the FORNIX has been found to arise in the subiculum, and not in the hippocampus as originally thought. The mammillothalamic tract has been confirmed, but the anterior thalamic nuclei have been found to project to the subiculum, rather than to the hippocampus proper. The subiculum in turn projects to the entorhinal cortex, the origin of the perforant path to the hippocampus. The subiculum also has reciprocal connections with association areas of the neocortex. Thus, like the entorhinal cortex, the subiculum is an important link between the neocortex and the limbic system.
The only major output of the hippocampus, originating from its PYRAMIDAL NEURONS, is directed to the lateral septum. Different hippocampal regions map in an orderly way onto the lateral septum, and from there onto the hypothalamus. These topographic (see TOPOGRAPHY) or orderly projections may have a role in mediating the expression of different classes of goal-oriented behaviour. The lateral septum also projects to the medial septum, which in turn projects back to the hippocampus through CHOLINERGIC and GABAERGIC pathways. These pathways are important in the generation of the HIPPOCAMPAL THETA waves, a characteristic rhythmical activity of the hippocampal ELECTROENCEPHALOGRAM in animals. The amygdala consists of three regions, the corticomedial nuclei which receive massive olfactory input, the basolateral nuclei which are interconnected with certain neocortical regions, and the central nucleus which projects directly to the hypothalamus and certain autonomic nuclei in the BRAINSTEM. The amygdala and the hippocampal formation are also interconnected. Thus, the limbic system may be viewed as an interconnected network interfacing between the neocortex and the hypothalamus, thus functionally associating MOTIVATION and EMOTION with cognitive functions.
The scope and the function of the limbic system have expanded considerably since its original conception. Much of what is now recognized as the limbic system was originally known as the RHINENCEPHALON or ‘smell brain’. However, the early studies had already made it clear that the limbic system governs not only OLFACTION, but also emotional behaviour. Furthermore, more recent studies have convincingly shown that some parts of the limbic system are involved in cognitive functions which are not traditionally associated with the limbic system. For example, the amygdala, while having a role in olfaction through its extensive olfactory input, is involved in memory associated with emotion. Similarly, the hippocampal formation is clearly involved not only in olfaction and emotion, but also in LEARNING and MEMORY. These advances in our understanding of the function of limbic structures have made the term limbic system somewhat less useful than it was in the past. As suggested by several authors, most recently Blessing, time may have come to abandon the term ‘limbic system’.
KAZUE SEMBA
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