Depression of the human nociceptive withdrawal reflex by segmental and heterosegmental intramuscular electrical stimulation

Objective To investigate the effects of intramuscular electrical conditioning in the modulation of nociceptive withdrawal reflex (NWR) and further to determine what muscle afferents are involved in the modulation of the nociceptive withdrawal reflex and the sites along the reflex pathway where the NWR modulation occurs in healthy humans. Methods The NWR elicited by a cutaneous test stimulus to the dorsal foot was modulated by a short (21 ms) intramuscular conditioning electrical stimulus at two times the pain threshold. At varying conditioning–test stimulus intervals, segmental conditioning stimulus was applied in the tibialis anterior muscle ipsilateral and contralateral to the test stimulus, and heterosegmental conditioning stimulus was applied in the contralateral trapezius muscle to modulate the NWR. Non-painful and painful intramuscular conditioning stimuli were also used to modulate the NWR and the soleus H-reflex. Results The NWR was depressed by preceding intramuscular conditioning stimuli, with a degree that depended on the conditioning–test stimulus intervals and on the conditioning site. Segmental conditioning depressed the NWR more quickly and gave a longer duration (15–1500 ms), and larger magnitude than heterosegmental conditioning, which depressed the NWR in a short temporal window (80–100 ms). No difference was seen in the magnitude of the NWR depression between the painful and non-painful intramuscular stimuli, and the soleus H-reflex was not affected. Conclusions Our results suggest that segmental and heterosegmental conditionings of NWR are mediated by myelinated muscle afferents engaging central inhibitory mechanisms rather than direct changes in the excitability of motor neurons. Significance The therapeutic effects of electrotherapy could involve these mechanisms in the treatment of muscle pain syndromes.