Trends in Neurosciences
What is the amygdala?
Section snippets
Historical background
Meynert's claim in 1867 that the amygdala of Burdach is a ventral, temporal lobe extension of the claustrum (according to Meynert the deepest layer of cortex)[3] sparked a more than 50 year controversy about how to classify the amygdala in terms of basic parts of the cerebral hemisphere (telencephalon, endbrain). Equally distinguished neuroanatomists soon proposed, instead, that the amygdala is part of the lenticular nucleus (a gross anatomical term for the globus pallidus and putamen – two
Neurotransmitter evidence: the central and medial nuclei
Immunohistochemistry for GABA (7, 8) and in situ hybridization for glutamic acid decarboxylase[9] (GAD), the enzyme converting glutamate to GABA, reveals a characteristic, very dense band of labeled neurons that extends ventrally and uninterrupted through the caudoputamen, the central amygdalar nucleus (CEA) and then the medial amygdalar nucleus (MEA), where it ends along the ventromedial edge of the cerebral hemisphere (Fig. 2). In contrast, other parts of the amygdala contain only scattered
Olfactory cortex of the caudal piriform lobe
It now seems clear that the cortical amygdalar `nucleus' (COA) and the `nucleus' of the lateral olfactory tract (NLOT) are in fact distinct areas of the olfactory cortex (forming the caudal end of the piriform lobe), partly because they lie on the surface of the hemispheres ventral to the rhinal sulcus and display a laminated organization with radially oriented pyramidal cells[18], and partly because they lie caudally adjacent to the piriform area and receive differential inputs from the main
The basolateral complex and the claustrum
This leaves us with the basolateral complex, which is the most problematic in terms of classification; it corresponds to the region originally called amygdala by Burdach, and identified as a temporal extension of the claustrum by Meynert and others in the last century. Based on embryological considerations and adult topographic relations, we suggest that Meynert was correct in his assignment of what are now referred to as the lateral and basal nuclei to the deepest layer of cortex, along with
The medially and dorsally extended amygdala
Alheid and Heimer[16] have argued for extending the amygdala both medially and dorsally. First, they suggest that because the amygdala innervates the bed nuclei of the stria terminalis (BST) and intervening regions of the substantia innominata (ventral pallidum), and because the latter two regions share with the amygdala similar patterns of descending projections, the BST and caudodorsal regions of the substantia innominata belong to the (extended) amygdala as well (but see [24]). Somewhat
Organization of major amygdalar connections
The evidence reviewed thus far, together with the connections we shall now review, suggest the arrangements of amygdalar cell groups illustrated in Fig. 4 and Fig. 5. Structurally, these cell groups are differentiated parts of the striatum, cerebral cortex and claustral complex, whereas functionally they belong to the olfactory, autonomic and frontotemporal cortical systems.
The literature on amygdalar connections is vast, complex, contradictory and incomplete, and cannot be reviewed thoroughly
What is the amygdala?
However one chooses to define the precise borders of the amygdala, it is a structurally and functionally heterogeneous region of the cerebral hemispheres. We have attempted to classify the various parts of the amygdala (as currently understood) in terms of larger cerebral hemisphere divisions, to provide a reasonable list of parts in each division, and to review the major neural inputs and outputs of the various parts. Overall, the evidence suggests that it is necessary to ask whether the
Acknowledgements
We would especially like to acknowledge the collaboration of Newton Canteras on the critical, early stages of the original experimental and conceptual work outlined here. Original work from our laboratory has been funded in part by NS-16686 from the NIH.
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