Habituation of the Hoffmann reflex.

The human monosynaptic H-reflex system was examined for the presence of several parametric features of habituation. Dishabituation was present, but there was no indication of an increasing rate or extent of habituation during repeated sessions, nor was there any apparent habituation of the dishabituating effect of an extraneous stimulus. Furthermore, there was only a very small effect of stochastic stimulation. There was, however, a substantial effect of voluntary activation: contraction of the soleus tended to prevent habituation, while stretch of the soleus produced sensitization during repetitive stimulation. We conclude that the H-reflex is an extremely simple response system with little or no long-term plasticity.