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7Limbic System
The limbic system is composed of both neocortical and phylogenetically older cortical areas (portions of the archicortex and paleocortex) and a number of nuclei. The cellular architecture of the archicortex and paleocortex differs from that of the neocortex. The major structures of the limbic system are the hippocampal formation, the parahippocampal gyrus and entorhinal area, the cingulate gyrus, the mamillary body, and the amygdala. These structures are interconnected in the Papez circuit and also make extensive connections with other regions of the brain (neocortex, thalamus, brainstem). The limbic system thereby enables communication between mesencephalic, diencephalic, and neocortical structures.
Through its connection with the hypothalamus, and thus with the autonomic nervous system, the limbic system participates in the regulation of drive and affective behavior. Its main function, teleologically speaking, is said to be the generation of behavior that promotes the survival of the individual and of the species. Moreover, the hippocampus plays a very important role in learning and memory. Lesions of the hippocampal formation, or of other structures that are functionally associated with it, produce an amnestic syndrome. Different disturbances of memory can arise, depending on the site of the lesion.
Anatomical Overview
Broca, in 1878, described the ring of brain convolutions surrounding the corpus callosum, diencephalon, and basal ganglia, naming it the “grand lobe limbique” (great limbic lobe, from the Latin limbus, ring). In some respects, this complex of structures can be considered a zone of transition between the brainstem and the neocortex. The cortical areas within it are composed of archicortex (hippocampus and dentate gyrus), paleocortex (piriform cortex), and mesocortex (cingulate gyrus). Further limbic structures are the entorhinal and septal areas, the indusium griseum, the amygdala, and the mamillary bodies (Fig. 7.1). The extensive fiber connections linking all of these structures led Papez, in 1937, to propose that a loop, or circuit, of neural activation (the Papez circuit, see Fig. 7.2) might be the anatomical substrate of emotional feeling and expression and of affective states corresponding to instinctual drive. This theory received support from the studies of Klüver and Bucy (KlüverBucy syndrome). Growing
Baehr, Duus' Topical Diagnosis in Neurology © 2005 Thieme
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Anatomical Overview · 313 7
Cingulate gyrus
Commissure
of the fornices Indusium griseum (medial and lateral longitudinal striae)
Anterior commissure
Fornix
Septal area
Amygdala
Mamillary body
Hippocampus |
Entorhinal area |
Fig. 7.1 The limbic cortex
evidence of the anatomical and functional linkage of the various limbic structures led MacLean to coin the term “limbic system.”
More recently, however, the concept of the limbic system as a discrete functional unit has come into question, as further studies have shown that the limbic structures possess important neural connections not just with each other but with outside structures as well. Thus, the limbic system cannot be regarded as a closed system in either an anatomical or a functional sense. The functions associated with the limbic system, such as instinctual and affective behavior, motivation, and drive, as well as learning and memory (see below), should not be thought of as the preserve of the limbic system alone. These functions depend on an intact cooperation of the limbic system with many other areas of the brain.
Once this has been understood, there is no further objection to the use of the term“limbicsystem,”particularlybecausetheanatomicalconnectionsbetween the various limbic structures, which originally motivated this term, are indeed present,robust,andfunctionallyimportant.Nouniformalternativeterminology has yet come into general use. Pathological changes of the limbic structures are still described, in the clinical setting, as lesions of the limbic system.
Baehr, Duus' Topical Diagnosis in Neurology © 2005 Thieme
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7 314 · 7 Limbic System
Thalamocingulate |
|
tract (radiation) |
Cingulum |
Anterior nucleus of the thalamus
Mamillothalamic tract
Fornix, from the hippocampus to the mamillary body
Cingulum, back to the hippocampus
Fig. 7.2 The Papez circuit (hippocampus − fornix − mamillary body − anterior nucleus of the thalamus − cingulate gyrus − cingulum − hippocampus)
Internal and External Connections
Papez Circuit
A group of limbic structures, including the hippocampus, are connected to one another in the so-called Papez circuit, which contains a number of neural relay stations arranged in a circuit or loop. Beyond the basic wiring diagram of the
Baehr, Duus' Topical Diagnosis in Neurology © 2005 Thieme
All rights reserved. Usage subject to terms and conditions of license.