Limbic System

The limbic system is a loosely defined collection of structures at the junction of the diencephalon and the cerebral hemisphere. Some authors consider the limbic system a discrete lobe of the brain, but because of its extensive thalamic and hypothalamic connections, we will regard the limbic system as a transitional region between the diencephalon and the hemisphere. Its two most important structures—the amygdala and the hippocampus—are found within the temporal lobe. The name ''limbic'' derives from the Latin limbus, meaning ''border,'' and the limbic system was originally named by the French neurologist Paul Broca in 1878 to reflect the location of a number of cortical areas at the margin of the hemisphere. These cortical regions—the cingulate gyrus, the parahippocampal gyrus, and the hippocampus—are typified by phylo-

genetically primitive cortex, and it was long thought that they were devoted to the sense of smell because of their connections with the olfactory system at the base of the frontal lobe (Fig. 12). However, later studies provided ample evidence that, whereas the limbic system in animals is indeed strongly linked with olfaction, the limbic system in humans is dedicated to other, more important functions. It is now widely accepted that the human limbic system has a prominent role in both emotion and memory.

In 1937, an influential paper by James Papez proposed that an interconnected series of structures —the cingulate gyrus, the parahippocampal gyrus, the hippocampus, the fornix, the mammillary bodies, the mammillothalamic tract, and the anterior nucleus of the thalamus—comprised the cerebral basis of emotion. This group of limbic structures came to be known as the Papez circuit (Fig. 13), and despite decades of debate, this region endures as a central concept in the neuroanatomy of emotion. Studies in the past decade have also demonstrated that the amygdala, located adjacent to the hippocampus (Fig. 13), has an important role in emotional learning, and this nucleus has joined the Papez circuit as a core constituent of the limbic system. Sensory input to the limbic system undergoes processing in the amygdala, and the emotional significance of a stimulus is thereby determined. In this way, the experience of emotion is mediated by limbic structures. There is also an effector component of the limbic system; connections with the hypothalamus account for the important autonomic and endocrine reactions to powerful emotions that have been referred to as the ''fight or flight'' response.

Figure 12 Illustration of the relationship of the limbic system to the cerebral hemisphere. Reprinted with permission from Kandel, E. R., Schwartz, J. H., and Jessell, T. M., Eds. (2000). Principles of Neural Science, 4th ed., p. 987. McGraw-Hill, New York.

Figure 13 The Papez circuit shown schematically. The arrows indicate connections between components of the circuit. Abbreviations: A, amygdala; H, hippocampus; PHG, parahippocampal gyrus; CG, cingulate gyrus; AN, anterior nucleus of the thalamus; MB, mammillary body; T, thalamus. Reprinted with permission from Filley, C. M. (2001). Neurobehavioral Anatomy, 2nd ed., p. 27. University Press of Colorado, Boulder, CO.

Figure 13 The Papez circuit shown schematically. The arrows indicate connections between components of the circuit. Abbreviations: A, amygdala; H, hippocampus; PHG, parahippocampal gyrus; CG, cingulate gyrus; AN, anterior nucleus of the thalamus; MB, mammillary body; T, thalamus. Reprinted with permission from Filley, C. M. (2001). Neurobehavioral Anatomy, 2nd ed., p. 27. University Press of Colorado, Boulder, CO.

In a parallel development, the hippocampus came to be understood as a key area for the processes of memory formation. The acquisition of declarative memory, which refers to facts and events as opposed to the skills that are associated with procedural memory, of course is vital for successful human existence. This capacity is apparently dependent on the hippocampus, a three-layered cortical region tucked deep within the medial temporal lobe and connected with other regions also concerned with recent memory, including the dorsal medial nucleus of the thalamus and the basal forebrain (Fig. 14). Whereas lesions in these latter regions have also been associated with memory dysfunction, the most dramatic demonstrations of recent memory disturbance have been observed in individuals with bilateral destruction of the hippocampus. The overlap of brain systems mediating emotions and memory is an intriguing neuroanatomic feature and suggests that those events with the most emotional significance are most likely to be encoded in memory; this aspect of mental life generally can be confirmed by common experience.

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