Modular organization of excitatory and inhibitory reflex receptive fields elicited by electrical stimulation of the foot sole in man.

OBJECTIVES: The present study aimed to investigate how the inhibitory and excitatory reflex components of the human (polysynaptic) withdrawal reflex are organized depending on the stimulation site. The reflexes were elicited during a voluntary pre-contraction (between 10 and 20% of maximum voluntary contraction) of two antagonistic muscles. METHODS: Inhibitory and excitatory reflex receptive fields to tibialis anterior (TA) and soleus (SO) were mapped in 14 healthy subjects using randomized electrical stimulation at 16 sites of the foot sole. Low, non-painful (3x perception threshold), and high, painful (1.5x pain threshold), stimulus intensities were used. RESULTS: The inhibitory reflex receptive fields were organized in a highly functional manner supporting the action of the excitatory reflex. Together the two reflexes result in an optimal withdrawal from the stimulus. Low stimulation intensity was found sufficient to elicit the inhibitory reflex. High stimulation intensity caused a reversal of the inhibition to excitation in tibialis anterior. In soleus the inhibition was facilitated for stronger intensities. CONCLUSION: In conclusion, findings in animals of a modular organization of inhibitory reflexes are reproduced in humans.