[Motor evoked potentials. Technic and results in the normal subject].

The transcranial stimulation of the motor cortex and spinal cord was introduced in clinical neurology by Merton and Morton (1980). This reproducible and non invasive technique gives the possibility to elicit unilaterally or bilaterally evoked motor potentials. It is possible to define a Total Conduction Time (TCT) from cortex to muscle and a Peripheral Conduction Time (PCT) from spinal cord to muscle. The central conduction time is given by the latency difference between TCT and PCT. This delay represents the duration of the descending corticospinal volley plus the delay necessary to depolarize spinal motor neurons. For central pathways supplying the upper limbs TCT is 4.3 milliseconds and for central pathways supplying the lower limbs TCT is 9.3 milliseconds. This technique of central motorpathway assessment can be used to define the magnitude of a central motor deficit in neurological diseases affecting the corticospinal tract, during surgical procedures involving the spinal cord and in comatose patients. The new magnetic stimulation is painless and will probably widen the utilization of this method.