Extrinsic control of intrinsic neuronal plasticity: an hypothesis from work on simple systems.

All advanced nervous systems use plastic properties of neurons for learning, remembering and achieving other information processing feats. Recent findings on a variety of relatively simple nervous systems or reduced parts of more complex ones suggest the notion that, in addition to having neurons with intrinsically plastic properties, nervous systems also have mechanisms for controlling the development, expression, and maintenance of changes in their inherently malleable neurons. This hypothesis has important implications for understanding both the brain's learning machinery and also aspects of nervous system organization not obviously related to learning.