Inhibition of human motoneurones, probably of Renshaw origin, elicited by an orthodromic motor discharge

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Abstract
The pattern of variations of a test H[Hoffmann]-reflex after a conditioning H-reflex was investigated in human subjects by an experimental design in which both reflexes involved the same soleus motoneurons. This was made possible by using a method based on a collision in the motor axons between the orthodromic conditioning reflex volley and the antidromic volley elicited by a test stimulus supramaximal for the motor axons. The variations of the test reflex amplitude seen when increasing the conditioning reflex discharge were studied. This was made possible by facilitating the conditioning reflex without changing the strength of the afferent volley. This facilitation was obtained through a soleus stretch elicited by a stimulation of the plantar nerves. The amplitude of the test reflex depended only on the size of the conditioning reflex discharge. As long as the conditioning reflex was of low amplitude, all motoneurons responsible for the conditioning response could be activated by the test volley, even though these motoneurons were undergoing after-hyperpolarization. This indicated that, in man, the after-hyperpolarization of the most excitable motoneurons can be completely overcome by a large Ia afferent volley. Increasing the conditioning reflex beyond a specific value resulted in an absolute decrease in the number of motoneurons involved in the test reflex. The amount of this decrease was related only to the amplitude of the conditioning reflex. This inhibition decreased progressively as the time interval separating the test stimulus from the conditioning stimulus increased. The time course of this inhibition was studied with conditioing reflexes of different amplitudes. The duration of inhibition increased with the size of the conditioning reflex. These results strongly suggested that Renshaw cells excited by the conditioning reflex are responsible for this inhibition. The results were in agreement with observations made in animals on recurrent inhibition.