THE CIRCUITS OF THE BODY'S ALARM SYSTEM

The amygdala is a complex structure of the brain and has about twelve sub-regions. Though not all of the regions are involved in fear reactions, many of them maintain connections that participate in such reactions actively. From experiments with lesions and tracing of neural pathways, we now recognize the lateral nucleus as the gateway into the amygdala. It is through the lateral nucleus that the amygdala receives information from the outside world. For example, in experiments with fear conditioned by sounds, the stimulus that exits the auditory thalamus enters the amygdala via the lateral nucleus.

The structure through which information exits the amygdala has also been clearly identified. It is the central nucleus, which controls the bodily reactions associated with fear.

Within the amygdala, a number of internal pathways have been identified by which information is routed from the lateral nucleus to the central nucleus. There is one pathway that goes directly from the lateral nucleus to the central nucleus.

The neurons of the lateral nucleus also project their axons to several other, intermediate nuclei: the basal nucleus (with its parvicellular and magnocellular divisions), the accessory basal nucleus, and the medial nucleus. These structures then relay the information to the central nucleus.

Each of these nuclei can be modulated by brain structures that can influence the emotions, such as the hippocampus, the frontal cortex, and the hypothalamus. Most of them can also be subdivided into sub-regions whose connectivity also appears to be very specific.