Recent anterograde and retrograde studies in the rat have provided detailed information on the origin and termination of the interconnections between the amygdaloid complex and the hippocampal formation and parahippocampal areas (including areas 35 and 36 of the perirhinal cortex and the postrhinal cortex). The most substantial inputs to the amygdala originate in the rostral half of the entorhinal cortex, the temporal end of the CA1 subfield and subiculum, and areas 35 and 36 of the perirhinal cortex. The amygdaloid nuclei receiving the heaviest inputs are the lateral, basal, accessory basal, and central nuclei as well as the amygdalohippocampal area. The heaviest projections from the amygdala to the hippocampal formation and the parahippocampal areas originate in the lateral, basal, accessory basal, and posterior cortical nuclei. These pathways terminate in the rostral half of the entorhinal cortex, the temporal end of the CA3 and CA1 subfields or the subiculum, the parasubiculum, areas 35 and 36 of the perirhinal cortex, and the postrhinal cortex. The connectional data are summarized and the underlying principles of organization of these projections are discussed.

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The present study is part of an ongoing project aimed at understanding the electrophysiologic properties of single amygdaloid neurons and their correlations with the morphology of the somata as well as axonal and dendritic trees. The axonal morphology of 14 three‐dimensional, reconstructed spiny neurons (4 in the lateral and 10 in the basal nucleus) that were filled in vivo with intracellular injections of biocytin is described. Three‐dimensional reconstruction was performed using Neurolucida software (MicroBrightField). Sholl analysis was used to assess the axonal length as well as the number of axonal varicosities and endings within concentric spherical shells placed at 50‐μm intervals from the soma. These data indicate that the same neuron can innervate several amygdaloid nuclear divisions or nuclei and extra‐amygdaloid regions. This finding suggests that the same neuron can modulate various brain areas in parallel. Both the presumed intra‐amygdaloid (all axonal branches within the amygdala) and extra‐amygdaloid (axons also outside the amygdala) projection neurons have dense perisomatic axonal arborizations, and consequently, the intra‐amygdaloid and extra‐amygdaloid projection neurons are difficult to differentiate based on the analysis of perisomatic axonal morphology. Furthermore, the same extra‐amygdaloid neuron can drive many neurons both locally as well as at extra‐amygdaloid projection areas within a relatively short time. Finally, the axonal morphology of spiny neurons located in the lateral or basal nuclei was similar. These data provide baseline quantitative information about the axonal dimensions of amygdaloid neurons and can form the anatomic basis for modeling amygdaloid neuronal circuits when more quantitative data regarding neuronal numbers, size, and dendritic morphology become available.