Electrophysiological observations on the behavior of monosynaptic spinal reflexes in slow and fast muscles or rabbit
Messina, C.; Cotrufo, R.; Bramanti, P.
Acta Neurologica 32(1): 52-53
1977
ISSN/ISBN: 0001-6276 PMID: 855692 Document Number: 112666
In mammalian muscles, 2 main types of fibers can be distinguished: slow or type 1 and fast or type 2. To evaluate the different excitability of type 1 and type 2 motoneurons, the threshold and amplitude of the H max ratio were analyzed in soleus and gastrocnemius muscles of the rabbit. Soleus is composed almost exclusively by slow fibers, while gastrocnemius is composed predominantly of fast fibers. The H reflex almost always had the same threshold in soleus and lateral gastrocnemius; but in the former muscle its amplitude increased much more than in the latter with increasing stimulus intensity. The M response had a similar threshold in both muscles; there was not a statistically significant difference among the maximal M responses in soleus, and in lateral gastrocnemius. There was a clear cut difference in the response of slow and fast muscles to monosynaptic reflexes. This difference appeared 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 by slow fibers and, therefore, innervated almost entirely by type 1 .gamma.-motoneurons, than in gastrocnemius, in which fast fibers predominate. Apparently only type 1 motoneurons respond to stimulation of 1a fibers.