Topical anesthesia: modulation of the monosynaptic reflexes by desensitization of the skin.

Skin desensitization by topical anesthesia was studied for its effect on the motoneuron excitability of the soleus muscle. Skin areas overlying calf, tibial, quadriceps and hamstrings muscles and skin dermatomes (L2, L3, L4, L5, S1 and S2) were studied separately. Motoneuron excitabilities were measured by the H-reflex and Achilles tendon reflex (for alpha and gamma motoneurons). It was shown that anesthesia applied to all skin areas and dermatomes, except those overlying the antagonist muscles, resulted in significant facilitation of the soleus H-reflex. In these cases, the ATR showed either slight inhibition or no significant changes. Anesthesia to the skin overlying the anterior tibial antagonistic muscle produced varied and inconsistent modifications in the amplitude of the H-reflex. In these cases the ATR was either slightly facilitated or showed no significant changes. These results appear to indicate the existence of on-going excitatory and inhibitory effects from the skin on the alpha and gamma motoneuron pool probably via segmental and suprasegmental levels. A possible clinical application of these results to modulate the motoneuron pool excitability is proposed.