We describe a case of continuous motor unit potential (MUP) activity of central origin (unlike stiff man syndrome and progressive encephalomyelitis) characterized clinically by rigidity, painful muscle spasms, abnormal postures and spinal myoclonus. The topography of the manifestations, the subacute and benign course, the presence of stable sequels 2 years after onset and a searching process of differential diagnosis lead us to attribute the condition to an inflammation of the cord, which makes the case of particular clinical interest.