Responses of human motoneurons to high-frequency stimulation of Ia afferents

This study was designed to extend to humans the findings of classical studies on anesthetized cats, which have examined the discharge of spinal motoneurons in response to high-frequency stimulus trains delivered to Ia afferents. Experiments were conducted on the monosynaptic pathway in the flexor carpi radialis (FCR) and soleus muscles. Subjects maintained a rhythmic discharge of a single motor unit (SMU) in either the FCR or soleus while homonymous Ia afferents were stimulated with either a single- or multipulse train. An n@IPI stimulus train had n pulses (n = 2-4) and an interpulse interval (IPI) of 1-8 ms. For each condition and motor unit, surface electromyographic (EMG) activity was averaged, and peristimulus-time histograms (PSTHs) were constructed for the SMU. The magnitude of the EMG was high for IPI = 1 ms, low for IPI = 2-3 ms, and high for IPI = 4-8 ms. SMU responses showed a similar pattern, which indicated that the increased EMG response was due to the presence of multiple peaks in a PSTH. The key results indicate that: (1) a short, high-frequency stimulus train enhances the discharge probability of a motoneuron above that observed with a single pulse; and (2) the increased motoneuron responses are significantly greater for the FCR than for the soleus muscle.