Neural Circuitry of Anxiety and Stress Disorders

Many data now indicate that the amygdala, along with its many efferent connections, is critically involved in emotion. Although the amygdala complex is generally defined by several distinct groups of cells, including the lateral, basal, accessory basal central, medial, and cortical nuclei, new data indicate that it is more useful to think of the amygdala as the basolateral amygdala (Bla) and to think of its several target areas as parts of a broader network that subserve more specialized functions (Fig. 64.1). The Bla receives sensory information from the thalamus, cortex (169), and ventral hippocampus (54) and then activates or modulates synaptic transmission in target areas appropriate for the reinforcement signal with which the sensory information has been associated. This involves both positive and negative associations. However, because most of the literature on the amygdala has analyzed the role of the Bla and its adjacent target, the central nucleus of the amygdala (CeA), in aversive conditioning, this work serves as the main focus of this chapter. Brief summaries of the role of Bla outputs to other targets shown in Fig. 64.1 follow. Because the periaqueductal gray (PAG) has received considerable attention in the study of defensive behavior and the hippocampus in the study of contextual fear conditioning, these data are reviewed next. Finally, brain systems and neurotransmitters involved in the inhibition of fear are reviewed, given the clinical significance of this information.