The human medial amygdala: structure, diversity, and complexity of dendritic spines

What is it about?

The medial nucleus of the amygdala (Me) is a component of the neural circuit for the interpretation of multimodal sensory stimuli and the elaboration of emotions and social behaviors in primates. We studied the presence, distribution, diverse shape, and connectivity of dendritic spines (major postsynaptic neuronal elements) in the human Me of adult men. Data were obtained from the five types of multipolar neurons found in the Me using an adapted Golgi method and light microscopy, the carbocyanine DiI fluorescent dye and confocal microscopy, and transmission electron microscopy. Three-dimensional reconstruction of spines showed a continuum of shapes and sizes, with the spines either lying isolated or forming clusters. These dendritic spines were classified as stubby/wide, thin, mushroom-like, ramified or with an atypical morphology including intermediate shapes, double spines, and thorny excrescences. Pleomorphic spines were found from proximal to distal dendritic branches suggesting potential differences for synaptic processing and strength in these Me neurons. Furthermore, the human Me has large and thin spines with a gemmule appearance, spinules, and filopodium, indicative of local neural plasticity. The ultrastructural data showed dendritic spines forming monosynaptic or complex multisynaptic contacts at the spine with asymmetric (excitatory) or symmetric (inhibitory) characteristics. Additional findings included en passant, reciprocal, and serial synapses in the Me. Complex long-necked thin spines were observed in this subcortical area, typical of our species. These new data reveal the diversity of the dendritic spines in the human Me likely involved with the integration and processing of local synaptic inputs and with functional implications in physiological and various neuropathological conditions. Key words: 3D reconstruction; DiI dye; extended amygdala; Golgi method; neuronal morphology; spine shape; ultrastructure.

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Why is it important?

For the first time the dendritic spines of this human brain area were described using three different techniques and revealed shapes ranging from simple to very complex features. This indicates that our brain is able to compute very different inputs and process the information in a way that establish the link between perception and interpetration of sensorial stimuli, emotion and the proper social behavior display.

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