The occurrence of abnormal emotional reactions in the course of disease is associated with lesions that preferentially involve certain parts of the nervous system. These structures are grouped under the term limbic and are among the most complex and least understood parts of the nervous system. The Latin word limbus means “border” or “margin.” Credit for introducing the term limbic to neurology is usually given to Broca, who used it to describe the ring of gray matter formed primarily by the cingulate and parahippocampal gyri that encircles the corpus callosum and underlying upper brainstem. Actually, Thomas Willis had pictured this region of the brain and referred to it as the limbus in 1664. Broca preferred his term, le grand lobe limbique, to rhinencephalon, which was the term then in vogue and referred more specifically to structures having an olfactory function. Neuroanatomists have extended the boundaries of the limbic lobe to include not only the cingulate and parahippocampal gyri but also the underlying hippocampal formation, the subcallosal gyrus, and the paraolfactory area. The terms visceral brain and limbic system, introduced by MacLean, have an even wider designation and more completely describe the structures involved in emotion and its expression; in addition to all parts of the limbic lobe, they include a number of associated subcortical nuclei such as those of the amygdaloid complex, septal region, preoptic area, hypothalamus, anterior thalamus, habenula, and central midbrain tegmentum, including the raphe nuclei and interpeduncular nucleus. The major structures that constitute the limbic system and their relationships are illustrated in Figs. 24-1 and 24-2.
The cytoarchitectonic arrangements of the limbic cortex clearly distinguish it from the surrounding neocortex. The latter, as stated in Chap. 21, differentiates into a characteristic six-layer structure. In contrast, the inner part of the limbic cortex, including the hippocampus, is referred to as archior allocortex and is composed of irregularly arranged aggregates of nerve cells that tend to be in a trilaminate configuration. The cortex of the cingulate gyrus, which forms the outer ring of the limbic lobe, is transitional between neocortex and allocortex—hence, it is called mesocortex. The entorhinal cortex adjacent to the anterior hippocampus has a similar transitional architecture. Information from a wide array of cortical neurons is funneled into the dentate gyrus and then to the CA (cornu ammonis) pyramidal cells of the hippocampus. Output from the hippocampus is mainly from the pyramidal cells of the CA1 segment and subiculum, whose axons form the fibria and fornix. The amygdaloid complex, a subcortical nuclear component of the limbic system, also has a unique composition, consisting of several separable nuclei, each with connections to other limbic structures.
The connections between the orbitofrontal neocortex and limbic lobes, between the individual components of the limbic lobes, and between the limbic lobes and the hypothalamus and midbrain reflect their many functional relationships in regard to emotion. At the core of this system lies the medial forebrain bundle, a complex set of ascending and descending fibers that connect the orbitomesiofrontal cortex, septal nuclei, amygdala, and hippocampus rostrally, and certain nuclei in the midbrain and caudal pons. This system, of which the hypothalamus is the central part, was designated by Nauta as the septohypothalamo–mesencephalic continuum.