The hippocampus: organizational patterns in health and senescence.

The human hippocampal-dentate complex and the adjacent entorhinal cortex undergo a sequence of histological changes during the process of aging and senescence. Such alterations show a loose correlation with the age of the individual, and a more precise fit with the degree of clinical and psychosocial deterioration demonstrated prior to the individual's death. The structural changes are more or less idiosyncratic to the specific area involved but, in general, include loss of dendrite spines. distortion and swelling of the cell-body dendrite complex, and progressive destruction of the dendrite domain. Since one-third to two-thirds of the complement of archicortical neurons may be involved, a very significant fraction of synaptic connections are lost, and with them, much of the computational power of these areas. Psychophysiological studies strongly suggest a relationship between the human hippocampus-entorhinal complex with memory formation and/or retrieval. Animal studies indicate a possible role for the hippocampus as a cognitive mapping system which generates working models of the environment. The utility of such models might lie in part in facilitating selection of context-dependent behavior in a changing spatio-temporal setting. Progressive alteration of hippocampal function with aging and senescence could thus contribute significantly to patterns of increasing confusion and disorientation so common in the senile individual.