One of the
basal ganglia, located in the
temporal lobe in each side of the
brain, and part of the
limbic system. They are shaped very much like
almonds (thus the name given to them, as it means almond) and are about the size of a
fingernail. They are connected very strongly to other parts of the brain, especially areas processing
sensory input, through such channels as the
stria terminalis. It appeared to have first
evolved in early fishes.
It is involved in interpreting the sensory input, and relating them to survival and emotional needs, helping to initiate responses. It is especially involved in both interpreting and producing many non-verbal signs, especially those related to anger, fear, anxiety, and defensiveness. In fact, surgical removal of the amygdala causes the lack of understanding of negative signs, like growls, screams, and angry voices. There is the possibility that people who exhibit fearlessness may have a damaged or less functional amygdala.
Releases of hormones during "fight or flight" impulses are also caused by the amygdala, such as adrenaline.
Most of the reactions caused by the amygdala are innate, and not learned. You don't need to be taught to cause a release of adrenaline, or to make your hair stand on end. Actions such as freezing during danger are controlled here. Learning does affect the part, however. A single highly negative experience will often be remembered, and will be enough to cause the amygdala to kick in should the experience happen again.
While the amygdala learns quickly, usually taking only a single experience to cause a fear or avoidance reaction, it is nearly impossible to unlearn, because of the high connectivity. The cortex, where our higher thought mostly occurs, can influence the amygdala, but not nearly as much as the other way around, which is why emotions can often take over, and why a phobia takes a lot of effort to overcome.
The amygdala is a very common locus for temporal lobe seizures.
Physically, it can be subdivided into a number of distinct nuclei, each anatomically distinct.
- The lateral nucleus receives input from parts of the thalamus, along with a number of sensory areas in the cortex. Output goes mainly to the basolateral nucleus, but also to the central nucleus, and many parts of the cortex.
- The basolateral nucleus receives it's input mainly from the lateral nucleus. It actually consists of three distinct cell types, and there is a large variety of neurotransmitters in this nucleus.
- The central nucleus, which is connected to the lateral and basolateral nuclei, and sends it's output mainly to the hypothalamus and brain stem.
- The medial nucleus.
The connection between the basolateral nucleus and the central nucleus has been shown to be essential for the formation of emotional memories.
Sources:
The Brain -- Lesson 2, Amygdala, http://www.ascd.org/pdi/brain/amygdala.html
Amygdala, http://members.aol.com/nonverbal2/amygdala.htm
The New Brain: Joseph LeDoux on the Amygdala, http://www.feedmag.com/brain/parts/ledoux.html
Introduction to the amygdala, http://marlin.utmb.edu/~nkeele/intro.html