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The human insular cortex is considered the “fifth lobe” of the brain–and although it represents less than 2% of the total cortical surface area–, it is involved in an remarkably large number of different functions. Previous in vivo neuroimaging studies in humans have identified three distinct functional subdivisions in the insular cortex: the dorsal anterior, ventral anterior, and posterior insula. These three regions have different cytoarchitectonic characteristics. The anterior aspects of the insula are involved in the integration of complex autonomic, cognitive, and emotional process thought to be important in interoceptive awareness, while the posterior aspects are essential for integrating autonomic and interoceptive signals. However, the synaptic organization of this brain region is still unknown due to the difficulties involved in studying the human brain. The current study analyzes at the ultrastructural level the synaptic …