[Motor innervation of the Trembler mouse]

The effects on muscle fibres and particularly on their motor end-plates, by axons whose Schwann cells fail to form and maintain a normal myelin sheath, were examined by morphological and biochemical techniques. In the soleus (slow type), the innervation area was more extensive along the muscle fibre; the most frequent modifications were: abnormal axonal branching pattern, swellings of the terminal branches and terminal and preterminal sprouting. Additional clusters of ACh receptors were frequent and AChE activity was unaffected. In the fast part of the gastrocnemius, neuromuscular junctions were slightly affected: sprouting was slight and all the muscle fibres had a single cluster of AChE receptors; AChE activity was modified. In trembler mouse, the alteration of axon-Schwann cell relationships interacted with the regulatory processes ensuring the maintenance and stability of muscle fibre innervation.