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The innervation pattern of the arytenoid muscle (AR) was studied from the standpoint of the reinnervation process in 32 guinea pigs with typical unpaired AR. In 26 animals the muscle was denervated by cutting the left recurrent laryngeal nerve (RLN). The superior laryngeal nerve was also transected to eliminate the possibility of reinnervation from this nerve. Both light and electron-microscopic examinations in 18 denervated animals and 4 normal animals, and a glycogen depletion experiment in 8 denervated animals and 2 normal animals were carried out on the AR and the posterior cricoarytenoid muscle (PCA). 1. Light microscopic study: Anastomosis with arytenoid branches from the contralateral RLN was observed in the belly of AR. Half of the myelinated fibers in the intramuscular nerve funiculus remained degenerated as of seven days after transection. After three weeks, degeneration of the myelinated fiber was no longer seen, and the number of myelinated fibers had been restored to within the normal range. The muscle fibers of the left PCA degenerated completely, while those of the AR remained intact. 2. Electron-microscopic study. Three days after transection, the nerve terminals of half of the neuromuscular junctions (NMJs) were absent in the AR. Seven days after transection, however, both myelinated and unmyelinated fibers were recognized in the intramuscular nerve funiculus. Half of the NMJs in the AR were normal, one fourth had degenerated and the rest had regenerated. Five weeks after transection, no degeneration or regeneration of the NMJs was seen. Unmyelinated fibers were no longer found in the intramuscular funiculus. 3. Glycogen depletion experiment: As of three weeks after transection, half of the AR fibers stained with PAS despite electrical stimulation of the contralateral (right) RLN. Five weeks after transection, none of the AR fibers stained. Thus, all of the AR fibers appeared to have been the result of reinnervation from the contralateral RLN. The results of the present study indicate that transection of the unilateral RLN facilitates collateral sprouting from the contralateral RLN in guinea pigs. This collateral sprouting occurring between 3 and 7 days following denervation reinnervated the denervated muscle fibers before degeneration. Collateral sprouting branches may easily extend into adjacent denervated Schwann's sheaths, since the arytenoid branch of the RLN runs in contact with that of the contralateral RLN in the identical intramuscular nerve funiculus. Although each muscle fiber in the AR is innervated unilaterally, this unpaired muscle as a whole receives specific motor innervation from both RLNs.

MOTOR IXXERVATION OF THE GUINEA PIG ARYTENOID MUSCLE FROM THE STANDPOINT OF THE REINNERVATION PROCESS