The limbic system comprises large areas of the forebrain where the emotions are generated and the responses to emotional stimuli are coordinated. Understanding its functions is particularly challenging because it is a complex system of numerous and disparate elements, most of which have not been fully characterized. A compelling reason for studying the limbic system is that the major psychiatric disorders—including bipolar disorder, major depression, schizophrenia, and dementia—involve malfunctions in the limbic system.
Anatomy of the Limbic System. The limbic system comprises specific areas of the cortex and subcortical structures interconnected via circuitous pathways that link the cerebrum with the diencephalon and brainstem (Fig. 7.9). Originally the limbic system was considered to be restricted to a ring of structures surrounding the corpus callosum, including the olfactory system, the cingulate gyrus, parahip-pocampal gyrus, and hippocampus, together with the fiber tracts that interconnect them with the diencephalic components of the limbic system, the hypothalamus and anterior thalamus. Current descriptions of the limbic system also include the amygdala (deep in the temporal lobe), nucleus accumbens (the limbic portion of the basal ganglia), septal nuclei (at the base of the forebrain), the prefrontal cortex (anterior and inferior components of the frontal lobe) and the habenula (in the diencephalon).
Circuitous loops of fiber tracts interconnect the limbic structures. The main circuit links the hippocampus to the mammillary body of the hypothalamus by way of the fornix, the hypothalamus to the anterior thalamic nuclei via the mammillothalamic tract, and the anterior thalamus to the cingulate gyrus by widespread, anterior thalamic projections (Fig. 7.10). To complete the circuit, the cingulate gyrus connects with the hippocampus, to enter the circuit again. Other structures of the limbic system form smaller loops within this major circuit, forming the basis for a wide range of emotional behaviors.
The fornix also connects the hippocampus to the base of the forebrain where the septal nuclei and nucleus accumbens reside. Prefrontal cortex and other areas of association cortex provide the limbic system with information based on previous learning and currently perceived needs. Inputs from the brainstem provide visceral and somatic sensory signals, including tactile, pressure, pain, and temperature information from the skin and sexual organs and pain information from the visceral organs.
At the caudal end of the limbic system, the brainstem has reciprocal connections with the hypothalamus (see Fig. 7.10). As noted above, all ascending sensory systems in the brainstem send axon collaterals to the reticular formation, which, in turn, innervates the limbic system, particularly via monoaminergic pathways. The reticular formation also forms the ascending reticular activating system, which serves not only to arouse the cortex but also to impart an emotional tone to the sensory information transmitted nonspecifically to the cerebral cortex.
Monoaminergic Innervation. Monoaminergic neurons innervate all parts of the CNS via widespread, divergent pathways starting from cell groups in the reticular formation. The limbic system and basal ganglia are richly innervated by catecholaminergic (noradrenergic and dopamin-
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