The Limbic System

Abstract

The term “limbic” was first used in 1664 by Thomas Willis to describe the cortical structures on the medial side of the cerebral hemisphere, surrounding the brain stem. Two centuries later, Paul Broca noticed that the cingulate gyrus and the parahippocampal gyrus form a border (limbus) around the corpus callosum and the brain stem. Broca subdivided his grand lobe limbique into inner (the hippocampus) and outer (the cingulate and parahippocampal gyri) rings. During the last decades of the nineteenth century and the first decades of the twentieth century, it was generally believed that most if not all structures of Broca’s limbic lobe were dominated by olfactory input and therefore form part of the rhinencephalon. In 1937, James Papez proposed that these structures are involved in a closed circuit. The circuit of Papez includes projections from the hippocampus via the fornix to the mamillary body and then via the mamillothalamic tract of Vicq d’Azyr to the anterior thalamic nucleus, from here to the cingulate gyrus, and as last step from the cingulate gyrus back to the hippocampus. Papez suggested that his circuit formed the anatomical basis for emotions. In 1952, Paul MacLean included the circuit of Papez with the amygdala and the hypothalamus into his limbic system, supposed to be responsible for emotional behaviour (the “visceral” or “emotional” brain). Lennart Heimer promoted an expanded version of the classic limbic lobe of Broca, which contains all non-isocortical parts of the cerebral hemisphere together with the laterobasal-cortical amygdaloid complex, with several output channels in the basal forebrain. Thus defined, the limbic lobe contains all of the major cortical and amygdaloid structures known to be especially important for emotional and behavioural functions. Experimental studies in the early 1970s identified the output channels of the limbic lobe in the basal forebrain as the ventral striatopallidal system, the extended amygdala and the basal nucleus of Meynert.

Both the amygdala and the hippocampal formation are involved in memory processing and memory disorders. Almost all severe amnesias occur after bilateral involvement of limbic structures. Association and limbic structures suffer the blunt of damage in dementia. In this chapter, following a brief discussion of the concept of the limbic system (► Sect. 14.2); the olfactory system (► Sect. 14.3); the basal forebrain (► Sect. 14.4); the amygdala (► Sect. 14.5); neuropsychiatric disorders such as schizophrenia (► Sect. 14.6) in which the amygdala, among other structures, is involved; the hippocampus (► Sect. 14.7); and memory and memory disorders (► Sect. 14.8) will be discussed. ► Section 14.9 includes a survey of the neuroanatomical basis of memory impairment in Alzheimer’s disease. The English terms of the Terminologia Neuroanatomica are used throughout.