Is There an Amygdala and How Far Does It Extend?

Abstract

Burdach is credited with first using the term "amygdala" to describe an almond-shaped mass of gray matter in the anterior portion of the human temporal lobe. With the subsequent development of histological techniques in the latter part of the nineteenth century it became readily apparent that there were areas dorsal and medial to Burdach's amygdala that seemed to be anatomically related to it. These regions include the nuclei that are now called the central, medial, and cortical amygdalar nuclei. Johnston considered the amygdala to have two major portions: (1) the centromedial nuclei, which are closely associated with the striatum, and (2) the cortical and basolateral nuclei, which are closely associated with the cerebral cortex. Johnston also importantly observed that the centromedial nuclei appear to have extensions that project rostralward to become continuous with the bed nucleus of the stria terminalis. Johnston's concept of a multicompartmental amygdala, with striatal-like and cortical-like components, dominated thought about the structure and function of the amygdala for most of the twentieth century. Recently, it was suggested that the amygdala is not a single neurobiological entity, but is better conceived as a region where several distinct structures, each with affiliations with different systems, are in close proximity to each other. These current concepts and issues concerning the configuration of the amygdala are discussed in light of recent cytological, immunohistochemical, and connectional studies.