[Pathophysiology of spasticity].

Lance's definition of spasticity focuses on the exaggeration of the tonic stretch reflex as one component of the upper motor neuron syndrome. In daily practice, many different symptoms are referred to as spasticity. Experimental studies stress the particular role of the premotor cortex and the medial reticular formation for the genesis of spasticity. Physiological studies clearly demonstrate the two components, phasic and tonic, of the stretch reflex. Whatever the pathology, the clinical picture of spasticity seems to depend less upon the etiology of the lesion and more upon its location in the neuraxis. There is a regional organisation of the spinal circuitry according to the function of the segmental nerves and, therefore, a particular clinical presentation related to each spinal segment. The muscular efferents are also heterogeneous and linked to function. This is the fundamental base of focal treatment of spasticity.