Deep core of brain gains respect.

Deep core of brain gains respect

The cerebral cortex, the outer shell of the brain, has long been considered the nervous system's crowning achievement. What could compare to the sophistication of our speech and thought centers? Certainly not the deep "reptilian' brain structures known as the basal ganglia.

This view of the brain is all wrong, says Ann Graybiel of Massachusetts Institute of Technology. The deep brain structures have a sophistication all their own that rivals that of the cerebral cortex.

The basal ganglia have been considered primitive despite what is already known about their important functions. They play a role in the control of movement, influencing not just how an animal moves but also its decision whether or not to move. They also seem to be required for spatial memory. The basal ganglia receive input from almost all areas of the cortex. Some cells in this area have quite sophisticated characteristics; for example, in trained monkeys one type of cell responds to a click only when the sound signals a reward.

Part of the basal ganglion's underappreciation may stem from the much more homogeneous appearance of its cellular architecture compared with the dramatic layers of the cerebral cortex. "The basal ganglia structure looks more like liver,' Graybiel says. But in her recent experiments she has distinguished cells by their neurotransmitters. The regions of the basal ganglia called the striatum are very rich in neurotransmitter chemicals. Almost all neurotransmitters and neuromodulators found anywhere in the nervous system are present in the striatum. Graybiel reports there is a clear pattern of organization in the striatum when the cells are distinguished by these signal chemicals.

"There is a logic to it,' Graybiel says. Instead of layers as in the cortex, there are patches of cells, which Graybiel calls striasomes. Each neurotransmitter is especially dense either within or outside the striasomes. The striasomes also delineate areas that innervate different brain areas.

Graybiel finds evidence also for other patches within the striatum that represent input from different regions of the cortex. "Think of a whole mosaic of input compartments interleaving among the striasomes,' she says. This mosaic resembles the organization of the cortex where interdigitating sets of columns handle different functions. Graybiel says, "The basal ganglion brings together different modalities, associated in a special chemical environment.'