THE SITE OF ACTION OF MAGNETIC STIMULATION OF HUMAN MOTOR CORTEX IN APATIENT WITH MOTOR-NEURON DISEASE

Cortical and spinal root (C-8-T1) magnetic stimulation evoked single m otor unit potentials in the abductor pollicis brevis and abductor digi ti minimi muscles in a patient with chronic motor neuron disease. This patient was unique in that there were few surviving motoneurons in th ese muscles making it possible to record single motor units. Central m otor conduction time was within normal limits. Cortical mapping of mot or evoked potentials (MEPs) was carried out using a twin magnetic coil stimulating over the motor cortex at intervals of 1 cm along the coro nal axis and 1-2 cm along the sagittal axis. As the site of cortical s timulation was moved from the centre, the latency of the MEPs increase d by 0.7-0.8 ms suggesting one synaptic delay. This study provides fur ther data that magnetic stimulation of the human cortex indirectly act ivates pyramidal cells via interneurons.