E.A. Nimchinsky et al.
A neuronal morphologic type unique to humans and great apes
Proceedings of the National Academy of Science 96 (27 Apr 99): 5268
The human neocortex, a two-millimeter thick rind of around 1.6 m2, contains approximately 1010 nerve cells in more or less discrete layers, usually six in most regions of the cortex. Compared to the neocortex of prosimians (lower primates such as lemurs, lorises, and tarsiers), the evolutionary line leading to humans has experienced a great expansion of cortical areas, with a several-hundredfold increase in volume. Nevertheless, the neuronal types that populate the neocortex have apparently remained remarkably constant and are morphologically recognizable across primate species.
An exception is the so-called "spindle neuron", found in the anterior cingulate cortex, a region located deep within the central divide between the two hemispheres. Spindle neurons are characterized by an elongate, gradually tapering, large-sized cell body that is virtually symmetrical about its vertical and horizontal axes. It has been found in the human cortex and in the cortex of the common chimpanzee, and recent studies in humans indicate that spindle cells are especially vulnerable to degeneration in Alzheimer's disease, which is associated with a loss of approximately sixty percent of these particular nerve cells.
E.A. Nimchinsky et al. "A neuronal morphologic type unique to humans and great apes" in the Proceedings of the National Academy of Science 96 (27 Apr 99): 5268, now report a study of 28 primate species representing all superfamilies of prosimian and anthropoid primates. The results indicate that spindle neurons are a feature of the anterior cingulate cortex of all humans and great apes (Pongidae: orang-utan, gorilla, chimpanzee), but not of any other primate species.
The authors suggest these observations are of particular interest when considering primate neocortical evolution, revealing possible adaptive changes and functional modifications over the last 15 to 20 million years in the anterior cingulate cortex, a region that plays a major role in the regulation of many aspects of autonomic function and of certain cognitive processes. The fact that these unique neurons are apparently severely affected in the degenerative process of Alzheimer's disease indicates that some of the differential neuronal susceptibility that occurs in the human brain in the course of age-related dementing illnesses may have appeared only recently during primate evolution.
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