Testing for Doubly Innervated Fibers in the Intrinsic Laryngeal Muscles of the Dog

Abstract

Experiments were performed on the cricothyroid and thyroarytenoid muscles of anesthetized dogs with the object of searching for evidence indicating polyneuronal innervation of single muscle fibers. The cricothyroid which is supplied by two peripheral nerves was tested in three different types of experiment, viz. (1) by applying tetanic stimulation to both nerves to find out whether the tension developed on simultaneous stimulation showed a “deficit” as compared to the sum of the tensions produced on stimulating each nerve alone; (2) by applying a single stimulus to each nerve to find out whether the peak tension rose when the stimulus interval was increased from zero to intervals sufficient for elicitation of a double twitch in any fibers supplied by both nerves; (3) by applying subtetanic stimulation of identical frequencies to both nerves to find out whether the contraction curves thus obtained had the same amplitude when corresponding shocks were applied in phase and sufficiently out of phase to permit any fibers supplied by both nerves to increase their discharge frequency and their tension development. The three types of experiment failed to provide any conclusive evidence for doubly innervated fibers in the cricothyroid. In the thyroarytenoid which is supplied by only one peripheral nerve the two latter types of experiment were made possible by a special experimental arrangement; however, no definite evidence for polyneuronal innervation was obtained. The conclusion was drawn that polyneuronal innervation does not occur on a significant scale in the muscles studied even though its existence on a small scale cannot be definitely excluded in view of the possible sources of error involved.