Re-innervation and recovery of rat soleus muscle and motor unit function after nerve crush

In this study we have investigated the effects of peripheral nerve crush on the contractile properties of the adult rat soleus muscle. The soleus nerve was crushed close to the muscle and functional re-innervation was assessed by the measurement of the force produced by contraction induced by electrical nerve stimulation. Whole soleus muscle and single motor unit (MU) properties were studied at increasing re-innervation times 7-56 days after crush. Results showed progressive re-innervation as 50% of the axotomized motoneurones had re-innervated their muscle 7 days after crush, 72% at 14 days and re-innervation was complete at 28 days. The force parameters recovered more slowly. Tetanic contractions faded at high frequency stimulation, which did not occur in the control muscle. This disruption in the tetanic response was more pronounced in single MUs. Our results demonstrate for the first time a process of progressive axonal re-innervation by the axotomized motoneurones and provide a functional picture of the effective restoration of the neuromuscular function.