Electrophysiological observations on the behaviour of monosynaptic spinal reflexes in slow and fast muscles of rabbit

To evaluate the different excitability of type 1 and type 2 alpha motoneurons, the authors analysed comparatively threshold and amplitude of the H reflex and the Hmax/Mmax ratio in soleus and gastrocnemius muscles of the rabbit. The results show that a clear cut difference exists in the reponse of slow and fast muscles to monosynaptic reflexes. This difference appears directly dependent on the percentage of slow fibers present in muscles; in fact, the H reflex and the Hmax/Mmax ratio were much higher in soleus, which is almost exclusively composed of slow fibers and, therefore, innervated almost entirely by type 1 alpha motoneurons, than in gastrocnemius, in which fast fibers predominate. On the basis of these experiments it is concluded that only type 1 alpha motoneurons respond to stimulation of 1a fibers.