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Department of Medical Physiology, Panum Institute, University of Copenhagen, Denmark.

1. The latency of effects in the tibialis anterior (TA) and soleus (Sol) muscles evoked by electrical and magnetic stimulation of the motor cortex was evaluated in human subjects by H reflex testing. Post-stimulus time histograms (PSTHs) were established for the discharge of single voluntarily activated motor units and motor-evoked potentials (MEPs) in the surface electromyogram. 2. At rest both electrical and magnetic stimulation evoked an inhibition of the Sol H reflex at the lowest intensities of stimulation. In some subjects a facilitation with an earlier onset was seen when increasing the stimulation strength. When the anode for the electrical stimulation was placed at the vertex directly above the leg motor area, the inhibition or facilitation often had the same latency as when evoked by magnetic stimulation. However, when the anode was placed 2-3 cm lateral to the vertex, effects evoked by the electrical stimulus often occurred 1-2 ms earlier. 3. Short-latency peaks in the PSTH of the discharges of single TA motor units also tended to occur earlier when evoked by electrical stimulation with the anode lateral to the vertex than when evoked by magnetic stimulation or electrical stimulation with the anode at the vertex. 4. In one subject, near-maximal electrical stimulation evoked MEPs with a latency corresponding to that seen following stimulation of the brainstem by electrodes placed bilaterally over the mastoid processes approximately 16 cm more distal. Maximal magnetic stimulation, in contrast, never resulted in responses with a latency shorter than that seen with the weakest electrical stimuli at the vertex. 5. The initial facilitation of the Sol H reflex evoked by magnetic stimulation and by electrical anodal stimulation at the vertex increased when the subject performed a voluntary plantarflexion. In contrast, the earlier facilitation evoked by electrical anodal stimulation 2-3 cm lateral to the vertex had the same size both at rest and during contraction. 6. We suggest that magnetic stimulation and electrical anodal stimulation at the vertex may preferentially activate descending cortical cells at, or close to, the cell soma. The initial responses evoked by these two stimuli may therefore be influenced by the excitability of the cortical cells. On the other hand, electrical stimulation with the anode 2-3 cm lateral to the vertex seems to often activate the axons at a deeper level. The initial responses evoked by this type of stimulation may therefore not be influenced by the excitability of the cortical cells.

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