The establishment and the subsequent elimination of polyneuronal innervation of developing muscle: theoretical considerations

An analysis is given of the polyneuronal innervation of embryonic skeletal muscle and its subsequent elimination during development. The amount of polyneuronal innervation that has been observed is consistent with the notion that initially each motor neuron distributes its contacts at random among the available fibres of a particular muscle. The idea that the elimination of excess innervation proceeds through interactions between terminals is placed on a quantitative basis. Each motor neuron is presumed to have a finite capacity for maintaining the structure and activity of its terminals, Which is shared out among them; a survival strength can be assigned to each terminal. Survival strengths undergo a process of continual adjustment. A terminal with above average strength for its endplate is strengthened at the expense of the weaker terminals, subject to the total survival strength available to each motor neuron remaining constant. It is proved that this scheme will transform the initial pattern of innervation into one in which each muscle fibre has contact from a single axon. Interpretations of the following results are given: the decrease in the spread of motor unit size during the development of innervation of the rat soleus muscle; the time course of superinnervation; the effects of neonatal partial denervation. Various suggestions are made for future experimental approaches.