The development of motor unit properties in the soleus muscle of rabbits and rats was used to study the control of innervation and elimination of synapses from mammalian skeletal muscle fibers and the differentiation of those muscle fibers into various twitch types.
1. Motor unit twitches with distinctly different time courses were found in rabbit soleus muscle at a stage in development when all muscle fibers were polyinnervated. This observation implies that (1) muscle fibers have already begun their physiological differentiation into twitch types while still polyinnervated and (2) motor neurons of a specific type preferentially polyinnervate muscle fibers of a corresponding type.
2. It has recently been claimed that synapse elimination occurs preferentially among motor neurons from the more rostral of the two spinal roots contributing to the soleus muscle of the rat. Using an assay based on measurements of motor unit twitch tens ions, it was found, contrary to the previous claim, that synapses were lost to the same extent by motor neurons passing through all contributing spinal roots to both the rabbit and rat soleus muscles.
3. Two lines of evidence indicate that rabbit soleus motor neurons redistribute their terminals at a time after wholesale polyinnervation has been lost from the muscle. (1) Between 11 and 18 days and 5 weeks of age, the frequency of histochemically defined type I fibers increases from 30% to 65% while the incidence of physiologically defined slow motor units does not obviously change. (2) Over the same time period, the ratio of average slow twitch tension to average fast twitch tension quadruples after correction for changes 1n muscle fiber cross sectional area. I hypothesize that during this 3 week time window, slow twitch motor neurons take over end plates previously occupied by fast twitch motor terminals.
4. Activity has previously been shown to play a role in the overall rate of synapse elimination. I have conducted preliminary experiments to address whether a competition on active muscle fibers between terminals of active and tetrodotoxin-inactivated motor axons results in a preferential retention of active connections. With the paradigm used, there was at most a small bias favoring the survival of active synapses.
