Do identical activity patterns in fast and slow motor axons exert the same influence on the twitch time of cat skeletal muscle?

1. The fast-twitch flexor digitorum (the lateral head; equivalent to the flexor hallucis longus) and slow-twitch soleus muscles of the cat were denervated, and the two nerves immediately reunited to one or the other muscle. Contraction times of the dually reinnervated muscle were examined 9 weeks post-operatively in response to separate stimulation of its own and the foreign nerve. Over a 5 week period before the terminal experiment, a variety of artificial activity patterns was imposed on the two nerves.2. Following the dual-union operation, the flexor digitorum muscle was preferentially reinnervated by its own nerve. In contrast, the soleus muscle showed no evidence of preferential reinnervation.3. When neural activity was not artificially modified, the dually reinnervated flexor digitorum or soleus muscle showed faster contractions in response to stimulation of the flexor digitorum nerve than to stimulation of the soleus nerve.4. Following a 5 week period in which neural activity was virtually eliminated by cord transection or in which the two nerves were stimulated at the same frequency, the contraction times of the dually reinnervated soleus muscle were the same in response to stimulation of either nerve.5. In contrast, under the experimental conditions described above (cord transection or nerve stimulation), the dually reinnervated flexor digitorum muscle showed a significantly faster contraction in response to stimulation of its own nerve than to stimulation of the soleus nerve.6. It is concluded that, when neural activity is absent or identical in pattern, motoneurones normally innervating the fast- or slow-twitch muscles exert the same influence on contraction times of the soleus muscle.7. The dependence of contraction times of the dually reinnervated flexor digitorum muscle upon the type of the innervating motoneurone may be explained either by selective reinnervation of a particular group of muscle fibres or by different trophic substances emanating from the motoneurones.