Motor potentials evoked in tibialis anterior by single and paired cervical stimuli in man.

Latencies of compound muscle action potentials evoked in lower limb muscles after transcranial stimulation shorten and amplitudes enlarge when the target muscle is contracted. After transcutaneous stimulation of the cervical cord with single electrical stimuli, the latency is not affected by muscle contraction. This is thought to be due to a lack of temporal summation at the spinal motoneurones. Results are different when paired stimuli are used. With contraction most motoneurones respond to the first cervical stimulus while in a relaxed condition they respond to the second. Temporal summation brings motoneurones to their firing threshold. Therefore, cervical double stimulation is more effective than single stimuli. It is however less powerful than cortical stimulation. Paired stimuli are most powerful at an interval of 2 ms similar to the D I-wave interval after cortical stimulation.