[An ultrastructural study of the neuromuscular junctions of the cat intrinsic laryngeal muscles. II. Synapse formation by autonomic nerves].

The purpose of this study is to identify the origin of the nerve terminals of unknown origin observed at the previously denervated neuromuscular junctions in the cat intrinsic laryngeal muscles. The results were as follows: 1. Until 3 weeks after the transection of the recurrent laryngeal nerve, no nerve terminals were found at the neuromuscular junctions of the intrinsic laryngeal muscles except for the cricothyroid muscle, and no nerve fibres were detected in the Schwann tubes formed by Schwann cells and perineural cells. In addition, autonomic nerves around the vessels in the muscles were markedly decreased. 2. At 6 weeks, accompanied by an increase of autonomic nerves around the vessels, nerve fibres and nerve varicosities containing a number of large granular vesicles were observed in the Schwann tubes. 3. From 9 to 30 weeks, nerve terminals containing large granular vesicles were found at the neuromuscular junctions in all cases, even though the superior laryngeal nerve or the vagal nerve was transected on the ipsilateral side. 4. A spontaneous discharge was recognized in 6/8 cases after 6 weeks, but an evoked electromyogram could not be recognized. 5. The synaptic vesicles of the nerve terminals were labelled by 5-hydroxydopamine (5-OHDA), which was used as a marker for the sympathetic nerve. From these results, it was indicated that if the transected recurrent laryngeal nerve was prevented from regenerating, the autonomic nerves around the vessels entered into the Schwann tubes and reached the denervated neuromuscular junctions, instead of the motor nerve. The effect of autonomic nerves on muscle fibres was discussed.