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Sea Horses and Almonds

Everyone knows the amygdalae and their location in the nervous system, or so it seemed until a few years ago. First called the amygdala by Burdach in 1819 because of their resemblance to almonds (Prunus amygdalus), the origin of today's discontent seems to have been laid down by J.B. Johnston in 1923 he described the amygdaloid complex. This refers to the collation of nuclei, which are divided into two main groups: the centromedial and basolateral. Johnston commented on the essentially basal ganglia associations of the former, with apparent extensions to the nucleus referred to as the bed nucleus of the stria terminalis. In the past half-century, the development and progress of neuroanatomy is at once revealing and rewarding, the more so for neuropsychiatry. For example, in the 1960s, 75 hypothalamic connections were discovered. With newer staining techniques, this has now risen to over 2,000. This has led to wiping aside older ideas and concepts, with accumulation of knowledge. In turn, different ways of trying to understand the structure and function of the limbic forebrain, and correlate this with behavioral and clinical observations have unfolded. This nexus for the fields of neuropsychiatry and neuroanatomy includes a number of structures, discussed by Johnston, which crucially involve connections between the limbic forebrain and medial temporal structures. Hence, the renewed interest in the amygdalae. As with all disciplines, there seems to be lumpers and splitters. One camp seems to disavow the very existence of the amygdala, rather holding the view that there are only lots of nuclei, divided anatomically and functionally, which have no coherence. According to them, it is only a mistaken almond shape to the crude eye of the early neuroanatomists. Others have taken a different line, developing the concept of the extended amygdala—a continuous bridge of cells—flowing from the centromedial amygdala to the bed nucleus of the stria terminalis and onward to the ventral striatum, especially involving the area of the accumbens. This implies an enlarging (rather than a shrinking) concept of the amygdala, which has led to speculation about some commonality of function, especially with regard to emotional and neuropsychiatric disorders. Heimer, one of the protagonists of the concept of the extended amygdala (with Alheid and de Olmos), writes, "The extended amygdala, by virtue of its downstream connections to the hypothalamus and brainstem, constitutes an important emotion-motor relay." These ideas are very exciting for their potential to illuminate the role of these structures for neuropsychiatric disorders. Recent brain imaging techniques have allowed visual and computerized reconstructions of Burdach's almonds to be viewed in several settings. They appear to be key structures in further regulating aspects of both normal and pathological human social behaviors. What is more, there is a consensus developing that amygdala lesions lead to impaired social interactions. The amygdalae are involved in the very early phase of facial recognition and cues to fear; perhaps centrally, in such pathologies as social phobias and posttraumatic stress disorder. They are part of a circuitry of cerebral abnormalities in depression and schizophrenia. Equally relevant is the work suggesting that the amygdala and its connections to the ventral striatum (accumbens) are the substrate for the development of drug addictions. In this case, their normal role of establishing Pavlovian conditioning and conditioned reinforcement for effective social integration of appetitive behaviors becomes usurped by powerful neurochemical hijackers. Unlike the delicate, sea horse-shaped hippocampus, which has preoccupied the neuroscience world for over a hundred years, the amygdalae only recently have become the intense focus of attention. This stems in part from the regular, brittle structure of the hippocampus, with its easily visible, neatly folded layers, so available for experimental manipulation. It is an ideal model system for observing neuronal plasticity and behavior. Now, the amygdalae—almonds with fragmented nuclei—have become a source of a heated discourse in neuroanatomical circles, with lumpers and splitters outstaining each other. This, therefore, has stimulated an interest in these structures and their connections for neuropsychiatry, and once again illustrates the relevance of a sound comprehension of neuroanatomy for clinical neuropsychiatry.