TROPHISM, TROPISM, AND SPECIFICITY IN NERVE REGENERATION

Target-derived neurotrophic factors are of basic importance for surviv al of neurons. In the normal state, such neurotrophic factors, synthes ized by the target tissues, are taken up by nerve terminals and transp orted by retrograde axonal transport in axons to the nerve-cell bodies to maintain their viability. After nerve injury, neurotrophic factors are synthesized by non-neuronal cells (Schwann cells and fibroblasts) in the nerve trunk, thereby supporting the outgrowth of axons. Neurit e-outgrowth-promoting factors on cell surfaces (cell adhesion molecule s, ''recognition molecules'') or in the extracellular matrix promote e xtension of the axons by providing an appropriate ''adhesiveness'' in the substrate. Both neurotrophic and neurite-outgrowth-promoting facto rs are essential for axonal growth after injury. Specificity in end-or gan reinnervation is a complex phenomenon which may be based on physic al factors at the zone of injury, as well as on molecular interaction between axons and substrate cells along the pathways and at the target level. Such processes may include molecular recognition of appropriat e axons and maintenance of such axons by trophic mechanisms, as well a s the pruning of inappropriate axons. The ultimate errors in target re innervation are reflected in a cortical re-organization in the somatos ensory cortex. The capacity of the brain to ''reprogram'' itself and a dapt to this functional re-organization is critical for the ultimate r ecovery of functional sensory/motor function after nerve injuries.