The mammalian brain is capable of a substantial functional reorganization,
manifesting on a cortical somatotopical and on a behavioral level. Possible
mechanisms are reviewed based on the work by others and ourselves on somat
osensory reorganization in humans. The somatosensory system is characterize
d by divergent projections from the periphery to the cerebral cortex. Chang
es in synaptic weights allow for reorganization of sensory processing: On o
ne side, limb amputation will result in a representational "invasion" of th
e deafferentated cortex from neighboring regions with concomitant perceptua
l changes. On the other side, sensorimotor training can increase the repres
entational cortical zone of a limb. Plastic changes can be temporary or per
sistent. Modulating factors like pain and certain drugs seem to induce a pe
rmissive state in the cortex resulting in enhanced reorganization. Thus, sp
ecific physical training combined with pharmacoceutical modulation holds pr
omise to improve functional recovery after brain lesions.