[An ultrastructural study of the neuromuscular junctions of the cat intrinsic laryngeal muscles. I. Denervation and reinnervation].

Morphological changes of the neuromuscular junctions induced by denervation and reinnervation have been investigated electronmicroscopically using cat intrinsic laryngeal muscles. Two experiments were performed: 1. simple transection of the unilateral recurrent laryngeal nerve at the level of the second tracheal ring; 2. neurorrhaphy immediately after the transection at the same level. 1. All the ipsilateral intrinsic laryngeal muscles except for the cricothyroid muscle were denervated 48 hours after the transection of the recurrent laryngeal nerve. In the denervated neuromuscular junctions some primary synaptic clefts were narrowed and others were flattened, while the secondary synaptic clefts were structurally preserved. 2. Some of the regenerating axons returned to the neuromuscular junctions 3 weeks after neurorrhaphy, and all the muscle fibres were reinnervated after 6 weeks. The structural features of the reinnervated neuromuscular junctions were as follows; 1) synaptic vesicles of the nerve terminals were more densely observed, 2) the original postsynaptic membranes were only partially in contact with the regenerated nerve terminals. These conditions persisted until 22 weeks after neurorrhaphy, when the appearance of remyelination was found in the vicinity of the neuromuscular junctions. Using an evoked electromyogram, the presence of neuromuscular transmission was observed 3 weeks after neurorrhaphy, indicating that the structural maturation of the neuromuscular junction was not necessary for neuromuscular transmission.