Location of the amygdala in the human brain

Located in the brain's medial temporal lobe, the almond-shaped amygdala (in Latin, corpus amygdaloideum) is believed to play a key role in the emotions. It forms part of the limbic system. In humans and other animals, it is linked to both fear responses and pleasure. Conditions such as anxiety , autism, depression, narcolepsy, post-traumatic stress disorder, and phobias are suspected of being linked to abnormal functioning of the amygdala owing to damage, developmental problems, or neurotransmitter imbalance.

The amygdala is actually several separately functioning nuclei that have been grouped together by their anatomical proximity. Key among these nuclei are the basolateral complex, the centromedial nucleus, and the cortical nucleus. The basolateral complex receives input from the sensory systems and is necessary for fear conditioning. The centromedial nucleus is the main output for the basolateral complex and is involved in emotional arousal. It sends outputs to the hypothalamus for activation of the sympathetic nervous system, the reticular nucleus for increased reflexes, the trigeminal nerve and facial nerve for facial expressions of fear, and the ventral tegmental area, locus ceruleus, and laterodorsal tegmental nucleus for activation of dopamine, norepinephrine and epinephrine. The cortical nucleus is involved in olfaction and pheremone processing. It receives input from the olfactory bulb and olfactory cortex.

Fear conditioning, which trains animals to associate fear with other (previously neutral) stimuli, alters the information stored in the amygdala, as shown by experiments from Joseph LeDoux 's lab and others. In this regard the amygdala serves as a simple Pavlovian learning machine that associates aversive events with neutral events, helping animals react to their world.

If the amygdala is injected with a drug that blocks protein synthesis shortly after fear conditioning, it does not acquire long-term memory of the fear.

In language learning, some hypothesize that second language learning for adults may not make ready use of the amygdala in procedural memory usage and so emotional links to words are slower to form.

References

• Eric Kandel, James H. Schwartz, and Thomas M. Jessell. 2000. Principles of Neural Science: 4th ed. McGraw-Hill: New York.