The term hippocampus is derived from the Greek word meaning "sea-horse," which might somehow describe the shape of each hippocampal nucleus, although frankly, the resemblance is not seen . In the illustration to the left, pink color is added to the hippocampus for clarity. You can see that it adheres to the curve of nerve fibers that curve once again to become the body of the fornix nerve pathway. This image links to its source, which includes a YouTube video on the anatomy of the hippocampus and surrounding structures.
The hippocampus, memory, and depression:
The hippocampi—one in each hemisphere, extending to meet the amygdalae within the temporal lobes—are crucial for "forming, storing, and processing memory," according to the MedlinePlus Dictionary. It should be noted here that the hippocampi are two of the first regions of the brain to suffer atrophy in Alzheimer's disease. In the illustration to the right from the Journal of Neuroscience (links to source), the amygdala is colored red and the hippocampus is colored blue.
The MRI sagittal view to the left (links to source) shows the amygdala (labeled "A") and the hippocampus (labeled "H"). In Brainscapes: An Introduction to What Neuroscience Has Learned about the Structure, Function, and Abilities of the Brain (1995), Richard M. Restak explains that "Fibers from all four lobes, along with association fibers uniting these separate connections into one unified experience, converge into the hippocampal region." Restak writes: "Thanks to extensive two-way connections, with other brain areas the hippocampus and its immediate connecting structures integrate and coordinate both our outer- and inner-world experiences into a unity." The hippocampi and other structures—such as the amygdalae—that process and integrate stimuli provide input to the autonomic nervous system (ANS; see ANS—the autonomic nervous system). So one can experience all the autonomic consequences of fear at the mere memory of a traumatic event.
Restak writes: "Damage to the hippocampus on both sides of the brain deprives the victim of the ability to learn new things and thus suspends the person in a time warp composed of the distant past, a present as thin and sharply etched as a knife blade, and an uncertain and fearful future. This happens because under normal circumstances we are able to maintain our sense of identity—who we are—only by forming new memories from moment to moment and accessing old ones at a leisurely command."
Another example of kindling, which we discuss above, is the effects of stress on the hippocampi. In his 1995 New York Times article titled, "Severe Trauma May Damage the Brain as Well as the Psyche," Daniel Goleman explains that studies in rats and primates suggest that glucocorticoids are the culprit. Goleman quotes Robert Sapolsky, who explains that glucocorticoids "may be neurotoxic to the hippocampus at the massive levels that are released under extreme stress or during trauma. I'm talking about the levels you would see in a zebra running from a lion, or a person fleeing a mugger—a real physical life-and-death crisis—if it is repeated again and again as time goes on."
If the glucocorticoids released during extreme stress and trauma damage the hippocampi, it is no wonder that, according to Sapolsky in Why Zebras Don't Get Ulcers, "there is atrophy of the hippocampus in long-term depression. The atrophy emerges as a result of the depression (rather than precedes it), and the longer the depressive history, the more atrophy and the more memory problems."
Sapolsky points to the work of psychologists Martin Seligman and Steven Maier who exposed animals to "pathological amounts" of stress. "The result is a condition strikingly similar to a human depression." Sapolsky explains that it is "repeated" stress that generates depressive symptoms combined with "a complete absence of control on the part of the animal." In other words, the animal has no outlets that can be used to vent frustration. "When it comes to what makes for psychological stress, a lack of predictability and control are at the top of the list of things you want to avoid," Sapolsky writes.
Sapolsky calls attention to the work of Joseph LeDoux of New York University, "who pretty much put the amygdala on the map when it comes to anxiety." In a way that only he can do, Sapolsky sums up the paradox between severe, traumatic stress and its effect on the hippocampi versus the amygdalae. "Suppose a major traumatic stressor occurs, of a sufficient magnitude to disrupt hippocampal function while enhancing amygdaloid function. At some later point, in a similar setting, you have an anxious, autonomic state, agitated and fearful, and you haven't a clue why—this is because you never consolidated memories of the event via your hippocampus while your amygdala-mediated autonomic pathways sure as hell remember."
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