The long-neglected field of neurological diseases affecting the peripheral motor sj'stem is now being investigated with growing interest. Improved methods (19) have increased substantially the amount of information that can be extracted from biopsies of the neuromuscular junction in patients. On the otlier hand, biophysical research on vertebrates has recently harvested quite significant results about the elementary processes of nerve conduction (53), of neuromuscular transmission (16) and of muscular contraction (55). AU thèse and other advances undoubtedly call for more critical analyses of disturbed mechanisms in patients with motor impairment. It can be surmised that doser investigations are going to lead to a reconsideration of current neurological entities and to the isolation of new neuromuscular syndromes (e.g., 66). Significant progress along thèse lines will be promoted if the physiopathological and histopathological approaches are intimateh' correlated and if the functional significance of structural anomalies can be established. Indeed it is only by confronting observations coUected by either method on the same material that some of the pending problems can eventually be solved. PHYSI0-P.4TH0L0GY OF MOTOR INNERVATION Mere eommon sensé indicates that interruption of motor nerve fibers at any level will make the attached muscle fibers useless. More interesting for the purpose of the présent discussion arc those clinieal cases in which the anatomical continuity of the lower motor neuron is preserved. Such cases raise the problem of localizing the defective link in the chain of excitatory processes which enable the nerve impulse to elicit the contractile response.