The spinal motoneurone is under the permanent influence of peripheral afferent fibers, interneurons, and numerous descending projections from supraspinal structures. Motoneuronal activity summarizes these different and convergent modulations at one moment. Spasticity corresponds to exageration of monosynpatic reflex, from IA fiber to motoneuron alpha, associated with spinal hyperexitability. Various lesions of central nervous system give rise to spasticity, specially if they affect supra spinal descending controls, mainly reticulo-spinal tracts. The role of neuronal plasticity to explain the progressive time course of spasticity is also discussed.