Tuesday, March 13, 2007

Brain cells and arrangements unique to human cerebral cortex

A New Focus article in the March 2 issue of Science by Michael Balter reviews work on several brain neuronal types and arrangements that are distinctive to humans and great apes.

Spindle neurons (also called Von Economo or VEN neurons after their Austrain discoverer) are provide one example (credit J. Allman):

...these neurons are located in only two parts of the brain: the anterior cingulate cortex, deep in the center of the brain, and the frontoinsular cortex, located inside the frontal lobes. In humans, both of these structures appear to be involved in aspects of social cognition such as trust, empathy, and feelings of guilt and embarrassment. Not only were VENs unique to great apes, but humans had many more VENs than other apes. And the human VENs were markedly larger.
John Allman of Cal Tech suggests that
...the large VENs might relay information rapidly from the anterior cingulate and frontoinsular cortices to other parts of the brain....They are really stripped-down, high-performance kinds of cells...the big VENs might help humans adjust behavior swiftly in response to rapidly changing social situations....
New data on dementia seem to fit that notion.
Last December, a team led by William Seeley at UC San Francisco reported in Annals of Neurology that subjects afflicted with a type of dementia that causes inappropriate and impulsive social behavior had 74% fewer VENs in their anterior cingulate cortex compared to normal controls.
The article goes on to discuss the fact that minicolumns of the, groups of 80 to 100 nerve cells bundled together vertically in the cerebral cortex are much wider in humans than in chimps and monkeys (average of 51 versus 36 micrometers) due to an increase in the space taken up by neuropil (the axons, dendrites, and synapses that make neural connections). That is, there are many more connections.

Astrocyte Cells: (Credit: Oberheim et al. Univ. Rochester)

It also turns out that that levels of the messenger RNA that makes thrombospondins - large proteins released by astrocytes which trigger synapse formation - are six times higher in human cerebral cortex than in chimps or monkeys. The differences were seen in the cerebral cortex but not in the cerebellum and nonbrain tissues. (Astrocytes are support cells that make up nearly half the cells in the human brain, but their functions have remained a mystery.)

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