The amygdala is an almond shaped subcortical structure located deep within the temporal lobes, medial to the hypothalamus and adjacent to the hippocampus. The amygdala has long been linked with a person's mental and emotional state. The regions described as amygdalae comprises several nuclei with distinct functional traits. Among these nuclei are the basolateral complex, the centromedial nucleus and the cortical nucleus. The basolateral complex can be further subdivided into the lateral, the basal and the accessory basal nuclei.
The amygdalae is connected to the hypothalamus, to the thalamic reticular nucleus for increased reflexes, to the nuclei of the trigeminal nerve and facial nerve which implements facial muscle movements, and to the ventral tegmental area, locus coeruleus, and laterodorsal tegmental nucleus for activation of dopamine, norepinephrine and epinephrine. The amygdala plays a critical role in the expression of emotions and the learning of new emotional responses. Much evidence suggests that human anxiety disorders result from anomalies in amygdala function.
Phylogenetically, the amygdala is a very old structure; probably very early on in Phylogeny. It was primarily involved in protecting organisms, moving them away from obnoxious chemical milieu. As organisms evolved the amygdala got different kinds of sensory information in to evaluate stimuli in the environment, and that is one of the reasons why it is more highly connected with the neocortex as organisms evolved. It's getting more and more high-level information to do an interpretation of what is going on in the environment.