The Limbic System - Hippocampus/Amygdala
The hippocampus does three things. It processes information to form memories. It coordinates those memories by time and the senses of vision and smell. It then stores these memories and allows retrieval of them. It has a high number of receptors for the stress hormone, cortisol, making it vulnerable to stress. Research shows that the hippocampus is actually smaller and less well-developed in individuals exposed to high amounts of stress. (1) Alzheimer’s Disease affects the hippocampus first. This area allows us to remember the names of things and to keep up with what just happened.
The hippocampus is actually one structure that has a foot in both hemispheres of the brain. This is crucial to the understanding of how trauma affects behavior. The corpus callosum connects the two hemispheres, but is found to be less well developed in those who have experienced trauma. (1) This leads to abrupt shifts between the two hemispheres. Whereas non-traumatized patients show memory retrieval to be harmonized, the abused patients show that they use predominantly the right hemisphere to remember bad memories. We will go into this more in next week’s essay.
The amygdala is an almond-shaped structure at the head of the hippocampus in the temporal lobes. It receives sensory information through complex connections. Some of these connections are related to whether one would freeze or flee during “fight or flight” response. PTSD is directly related to a situation in which neither solution is feasible. The amygdala is what tells the hippocampus whether a memory is a bad one or a good one. It is larger in people with bipolar disorder and is genetically different in those with panic disorder. (2) It mediates the emotions love, mood, rage, fear, and anxiety, as well as dealing with appetite and attention. Research into those with damage to the temporal lobe from seizures has shown physical changes, but important to the understanding of PTSD, it also can show dissociative symptoms. (1) Those individuals with heightened states of arousal are working from this area of the brain. The amygdala is also critical in reward cycles and motivation that can relate to drug addiction. (3)
Both of these structures are associated strongly with varying mental disorders. PTSD is one that we will continue to examine further in this four- part series.
(2) Neuroanatomical hypothesis of panic disorder, revised.
Gorman JM, Kent JM, Sullivan GM, Coplan JD
Am J Psychiatry 2000 Apr;157(4):493-505
(3) The amygdala
Current Biology, Volume 17, Issue 20, Pages R868-R874
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