Afferent and efferent fiber systems of the human amygdala: anatomical, pathophysiological, and clinical significance

OBJECTIVE: The amygdaloid body, also known as the amygdaloid nuclear complex or amygdala, is a collection of multiple interconnected diencephalic gray matter nuclei and is in turn part of the larger limbic nuclear complex. Important commissural and projection fibers are interconnected with the amygdala. The aim of this study was to present original anatomical and radiological studies of the amygdala and a thorough review of its anatomical connections, spatial topography, functional roles, and associated clinical syndromes, specifically tailored for relevance to neuroclinicians. METHODS: In this study, the authors investigated the afferent and efferent fiber pathways of the amygdala with white matter fiber dissection and diffusion tensor tractography. Functional and clinical syndromes associated with these fiber pathways were also reviewed in this study. RESULTS: The amygdala can be classified into three main nuclei groups: laterobasal (deep), corticomedial, and superficial (cortical) nuclei. The ansa peduncularis, ansa lenticularis, stria terminalis, and stria medullaris thalami are the main amygdalofugal pathways. In this study, the authors showcase these fiber pathways via the white matter fiber dissection technique and diffusion tensor tractography. The functional roles and clinical significance of these pathways are also reviewed. The amygdala has a role in cognitive functions (i.e., processing of unpleasant and frightful stimuli, emotion regulation, classic conditioning, reward processing, social cognition, and memory formation). It has been reported that positive responses are obtained by targeting the amygdala with deep brain stimulation in conditions such as autism, anxiety, and depression. CONCLUSIONS: Comprehensive neuroanatomical knowledge of the amygdala and its afferent and efferent pathways will avoid iatrogenic damage to these delicate structures.