HIPPOCAMPUS   [ back to Glossary Index ]
Hippocampus is the brain structure most involved in short term memory and recent learning. It is located in the medial temporal lobe and shaped in a closed loop resembling a seahorse. The  hippocampus receives input from afferent fibers in the entorhinal cortex and transmits output through efferent fibers in the fornix and mammilary bodies. Proper functioning of the hippocampus and its pathways is essential for encoding and retrieval of recently learned verbal and visual data. Chronic alcoholics with a severe dietary deficiency of thiamine may suffer complete destruction of their mammilary bodies and have no capacity to retrieve new information, a problem known as Wernicke-Korsakoff's syndrome. The hippocampus proper, which includes the dentate gyrus, is divided topographically into sectors CA1, CA2, CA3 and CA4. CA stands for cornu ammonis referring to the ram's head shape of the hippocampus when seen in cross section. The brain cells in the hippocampus show a very high degree of morphological plasticity, and will regrow synapses by new axonal sprouting far more quickly than in other parts of the brain. Autistic children show just the opposite. Their hippocampal cells are small and have much fewer, more stunted connection with adjacent cells. The normal hippocampus is a highly dynamic encoding device which uses these different subfields to process different kinds of input.  The neurons in CA1 (Sommer's sector) are exquisitely sensitive to any deprivation of oxygen or glucose and begin to die off before other brain cells, beginning after just a few minutes of an anoxic episode, such as a drowning or inhalation of CO. Death of hippocampal brain cells will lead to varying degrees of permanent memory deficit. Complete removal of the hippocampi on both side of the brain during surgery for intractable epilepsy results in the complete destruction of the capacity to form new memories of new faces, names or experiences. Yet even a patient with this degree of amnesia for facts or events, can be trained in new procedural skills (like tennis) without any recall of having taken lessons or of his tennis teacher's name or face. The cells of the hippocampus have an extremely low threshold for seizure activity, which is why they must be removed, at least partially during many surgeries to control temporal lobe epilepsy. The hippocampus lies just below a small, almond shaped structure called the amygdala. The amygdala assigns emotional valences to everything we perceive, and cues the "rational" part of the brain as to who or what to fear, to love, to like, to dislike, to approach or avoid. Just the sight of a feared object (be it a snake, a doberman, a suspension bridge or a long drop from a balcony) can send the observer into instant, sweaty panic. This occurs in part because the amygdala has close, strong connections with the hippocampus, and can rapidly activate the brain's fear alarm. Restoring "rational control" through deep breathing and mantra like repetition of phrases like "its going to be OK," tends to counter-act this association. Although adult macquac monkeys appear to grow new hippocampal cells throughout their life span, the same may not be true of humans. Neurologists tell us that the human capacity for memory begins to fall off gradually starting sometime between age 30-35, as hippocampal cells age; and that it is normal for humans in their 70s to experience memory retrieval problems, for example forgetting  where they left their keys or the name of the movie they just saw.