“How our bodies respond to stressors to a large effect determines our overall health.” (Frodle & O´Keane, in press).
The seahorse is a delicate little thing. So is its cerebral namesake. The hippocampus, which is the scientific name of the seahorse, rests in the temporal lobes on both sides of the brain. It is a critical structure for learning and memory and is deleteriously affected by various diseases and other sources of stress. Studies have shown loss of hippocampal volume in depression, bipolar disorders, schizophrenia and other psychiatric disorders. Chronic insomnia, sleep apnea, hypertension, diabetes, Alzheimer´s dementia and mild cognitive impairment (MCI), which can be the precursor of dementia, are also associated with smaller hippocampi (plural of hippocampus).
But earlier life experiences also contribute to hippocampal health and this should not be overlooked. It has, for example, been shown that people with low self-esteem have smaller hippocampi than those who have high self-esteem. Self-esteem has even been found to explain 20-50% of the variability in hippocampal size between individuals! Maternal care and attachment as well as child abuse and traumatization have also been linked to less hippocampal volume, particularly on the left side of the brain. In particular, parental nurturing prior to age 4 seems to predict hippocampal size. Childhood abuse is a major risk factor for psychiatric disorders and probably also for low self-esteem. And having a small hippocampus might make individuals more susceptible to stress later on in life and therefore less resilient to all the later developing diseases which can affect hippocampal volume. Getting a good start in life is of major importance, both for emotional and physical health.
But how is it possible that so many things are associated with smaller hippocampi as the hippocampus is mostly associated with memory and learning?
Why is the hippocampus so vulnerable?
The brain is the major organ in regulating the stress response and several areas are of importance. Psychological stress triggers activity in the HPA (hypothalamic-pituitary axis) which in turn, among other things, leads to the secretion of cortisol from the adrenal cortex (adrenal glands). The hippocampus has a high concentration of glucocorticoid (cortisol) receptors and rat studies have shown that chronic stress and excessive exposure to cortisol ultimately damages the neurons in the hippocampus. The sensitivity of hippocampal neurons to stress has not only been demonstrated in rats but also in nonhuman primates. We cannot study damage to the hippocampus at the neuronal level in living humans. However, the previously mentioned studies of human clinical populations extend the animal studies, by showing that the hippocampus is sensitive to stress and adverse life circumstances in early childhood. Hippocampal damage caused by stress could also lead to problems with memory and learning and would also sustain the cortisol levels as the hippocampus would not be able to inhibit the HPA activity as it should normally do.
Can we eliminate stress?
Stress is an unvoidable part of life and it can be a great energizer. But chronic stress is noxious. We can, and should, attempt to reduce it and the way we deal with it as high levels of chronic stress can have a huge impact on our health. Previously I discussed the benefits of meditation on stressed-out brains and the fact that the hippocampi appear particularly vulnerable to the effects of stress. Go ahead, read that post again and start meditating!
Fotuhi, M., Do, D. & Jack, C. (2012). Modifiable factors that alter the size of the hippocampus with ageing. Nature Reviews Neurology, 8, 189-202.
Frodl, T. & O´Keane, V. (in press). How does the brain deal with cumulative stress? A review with focus on developmental stress, HPA axis function and hippocampal structure in humans. Neurobiology of Disease,http://dx.doi.org/10.1016/j.nbd.2012.03.012
Heide, K. M. & Solomon, E. P. (2012). Biology, childhood trauma, and murder: Rethinking justice. International Journal of Law and Psychiatry, 29, 220-233.
Pruessner, J. C., Dedovic, K., Pruessner, M., Lord, C., Buss, C., Collins, L., Dagher, A. & Lupien, S. J. (2010). Stress regulation in the central nervous system: evidence from structural and functional neuroimaging studies in human populations – 2008 Curt Richter Award Winner. Psychoneuroendocrinology, 35, 179-191.
Teicher, M. H., Anderson, C. M. & Polcari, A. (2012). Childhood maltreatment is associated with reduced volume in the hippocampal subfields CA3, dentate gyrus, and subiculum. Proceedings of the National Academy of Sciene, 109(9), 563-572.