Peripheral and transcortical loops activated by electrical stimulation of the tibial nerve in the monkey.

Peripheral and supraspinal loops activated by electrical stimulation of the tibial nerve were studied in alert monkeys. Weak conditioning stimuli below the threshold for muscle contraction and strong conditioning stimuli which elicited a direct motor response and an H-reflex were applied to the tibial nerve, while soleus EMG was recorded. The motoneuronal excitability was measured with a test H-reflex for different intervals between conditioning and test stimuli. A facilitation of the motoneuronal pool occurred with a latency of about 50 ms for weak, as well as strong, conditioning stimuli. The facilitation was superimposed on a long-lasting inhibition which was more pronounced when strong stimuli were used. The shape of the excitability curve of the motoneurons after strong conditioning stimuli was studied before and after various lesions. The shape of the curve did not change after an ipsilateral cerebellectomy except that the facilitation was more pronounced. A few days after pyramidotomy, the facilitation diminished in size but it recovered to its initial size after 3 months. Spinal hemisection did not abolish the facilitation. We concluded from these results that a peripheral loop was activated when conditioning stimuli above the threshold for a direct motor and reflex response were applied. The facilitation might be mediated by the muscle contraction and following activation of muscle afferents. Superimposed on thus loop is a previously demonstrated transcortical loop of similar latency.