[Possibilities and limits of determining central motor conduction latencies in the human]

It was the aim of the present study to define the normal range of latencies to muscle compound action potentials of different muscles in the upper and lower extremity elicited by transcranial cortex stimulation (see Tab. 1). It was demonstrated that parameters, such as voluntary background innervation (see Fig. 1) and stimulation strength (see Fig. 2) have great influence on both the latencies and the amplitudes of these responses (see Figs. 3 and 4). Thus, as a prerequisite for an interpretation of latencies to cortically evoked responses as being normal or abnormal is a detailed description of the conditions under which the responses have been obtained. Additionally the individual conduction distances along the central and peripheral motor pathways have to be taken into account (see Fig. 5). Furthermore it is shown that the combination of cortex stimulation and stimulation on spinal level with the aim of a fractionated determination of central motor conduction velocities suffers from several limitations. The main limitation arises from the observation that the excited parts of the motor system are non-congruent (see Fig. 6). Isolated high-cervical tract stimulation exciting motoneurones of the upper extremity frequently could not be performed since the excitation threshold of the motor roots is much lower than that of the descending motor tracts (see Fig. 6 and 7).