Excitability of the soleus motoneuron pool revealed by the developmental slope of the H-reflex as reflex gain.

The excitability of a motoneuron (MN) pool was evaluated by the developmental slope of H-reflexes (Hslp) evoked at a range of a stimulus intensity less than the threshold of an M-response. The Hslp has been regarded as the "reflex gain", which is the changing rate in MN excitability as a function of the increase of Ia input to an MN pool. In a comparison of two parameters used in the H-reflex technique, such as the ratio of the maximal H-reflex to the maximal M-response and the ratio of the threshold of an H-reflex to that of an M-response, the Hslp was predicted to be a reasonable parameter to evaluate motoneuronal excitability, because the Hslp is free from the effect of any collision between the H-reflex discharge and the antidromic volley drived from the occurrence of an M-response within the alpha-efferents, and the Hslp can estimate the recruitment properties of a whole MN pool. The Hslp was alleviated during dorsi-flexion and steepened during plantar-flexion, according to the inhibitory or facilitatory synaptic modifications onto soleus MNs. The developmental slope of an M-response (Mslp), which shows the recruitment property of axons of soleus MNs, was alleviated especially in plantar-flexion. In order to exclude the peripheral neuromuscular factors in evaluating substantial MN excitability, the Hslp/Mslp is proposed as a more effective parameter than the Hslp.