Different excitability of type 1 and type 2 alpha-motoneurons. The recruitment curve of H- and M-responses in slow and fast muscles of rabbits.

To evaluate the different excitability of Type 1 and Type 2 alpha-motoneurones, we analyzed comparatively the threshold and amplitude of the H-reflex and the Hmax/Mmax ratio in a slow muscle, soleus, and in a fast muscle, lateral gastrocnemius, of the rabbit. The H-reflex had almost always the same threshold in the muscles examined, but in soleus its amplitude increased much more than in lateral gastrocnemius when the stimulus intensity was increased. A clear difference in amplitude of the H-reflexes was already evident with stimuli subliminal for direct responses (M). The maximal H-reflex was always much higher in soleus than in lateral gastrocnemius. The calculation of the mean Hmax/Mmax ratio showed that it was 3 times higher in the slow than in the fast muscle. On the basis of these results, showing a clear-cut difference in the reflex excitability of slow and fast muscles and in the light of other experimental data available in the literature, we tend to conclude that Type 1 alpha-motoneurones are exclusively, or at least predominatly, depolarized by stimulation of afferent Ia fibres.