Glial cells under physiologic and pathologic conditions.

Glial cells have long been considered to play roles in the nervous system that are unexciting compared with those of neurons. They provide neurons with nutrients, guide migrating neurons and their precursors during development, and dispose of the brain's "waste." Recent evidence, however, suggests that glial cells play more sophisticated, neuronlike roles. They integrate neuronal input, modulate synaptic activity, and process signals related to learning and memory. These findings have significant implications for humans with neurodegenerative diseases. In addition to activation on nervous system injury and during neuronal degeneration, glial cells also degenerate in several neurodegenerative diseases. Therefore, glial cell loss may contribute to the impairment of learning and memory. Therapeutic approaches to combat human neurodegenerative diseases thus need to restore the function of both neurons and glial cells.