MOTOR-NEURON DISEASE STUDIES OF THE CORTICOSPINAL EXCITATION OF SINGLE MOTOR-NEURONS BY MAGNETIC BRAIN-STIMULATION

The responses to magnetic brain stimulation of 57 motor units (MUs) in the first dorsal interosseous muscle of 33 patients with sporadic mot or neuron disease (MND), and 44 MUs of 15 healthy control subjects hav e been compared. Thresholds for MU excitation both in the resting stat e and during tonic activity have been measured and the modulation of M U discharge by series of magnetic stimuli has been determined from per istimulus time histograms (PSTHs). Abnormalities in the MND MUs have b een correlated with the physical signs of impaired hand muscle functio n. Motor units from hands showing pure lower motor neuron (LMN) signs fired at increased frequency when recruited, had normal or reduced thr esholds for activation by magnetic scalp stimuli and had normal or lar ge primary excitatory peaks with normal inter-subpeak intervals on PST Hs. Occasionally muscles with pure LMN signs also had fasciculations t hat could be driven by cortical stimuli. In contrast, MUs from hands s howing pure upper motor neuron (UMN) signs fired at reduced frequency when recruited, had raised thresholds for excitation by cortical stimu li, had prolonged primary peaks with wide inter-subpeak intervals and had large secondary peaks on PSTHs. The rise times and amplitudes of t he underlying post-synaptic potentials released by cortical stimuli in spinal motor neurons have been deduced from measurements of the peaks in PSTHs. This analysis suggests that in some MUs from muscles showin g pure LMN signs, stimuli evoke larger than normal excitatory post-syn aptic potentials (EPSPs), probably resulting from abnormalities in cen tral motor fibres. It is proposed that a major cause of the changes fo und in MND MUs from hands showing either pure UMN or pure LMN signs ma y be an abnormality in the motor cortex leading to faulty initiation o f the descending corticospinal volley from magnetic stimuli.