Excitability of human motoneurones after discharge in a conditioning reflex

A new method of exploring the H reflex excitability cycle is proposed which permits the exploration of motoneurones that discharged in response to the conditioning shock. This is made possible by using a test stimulus supramaximal for the alpha motor fibres of the nerve, which causes a collision in the alpha fibres between the orthodromic conditioning reflex volley and the antidromic motor volley. As a result of this collision, a test reflex contraction appears in the EMG, due to motoneurones that have already discharged in the conditioning response. This method permits studies in human subjects of the poststimulus refractory period of motoneurones as well as the early inhibitory phenomena related to the conditioning stimulus and response. It is shown that during the initial period of the classical H reflex excitability cycle, the test shock explores only alpha motoneurones corresponding to a fringe of cells subliminally excited by the conditioning stimulus. The difference between the populations of motoneurones tested by the two methods explains the differences between the features of the cycles obtained.