title: Responses of human motoneurons to high-frequency stimulation of Ia afferents.
abstract: 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.
authors.affiliation: School of Kinesiology, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada. bawa@sfu.ca
authors.forename: Parveen
authors.initials: P
authors.lastname: Bawa
authors.forename: Gordon
authors.initials: G
authors.lastname: Chalmers
mesh_terms: D000328:Adult; D000818:Animals; D002415:Cats; D004558:Electric Stimulation; D004576:Electromyography; D019706:Excitatory Postsynaptic Potentials / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009046:Motor Neurons / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000502:physiology; D009475:Neurons, Afferent / Q000502:physiology*; D055815:Young Adult