Target of Efferent Neurons of the Superior Laryngeal Nerve

In mammals, the laryngeal muscles phylogenetically are derived from two sources: from the so-called internal sphincter which gives rise to the internal laryngeal muscle and from the external sphincter from which the anlagen of the external laryngeal muscle (cricothyroid muscle) together with the pharyngeal constrictors are derived. According to this scheme, both groups of muscles receive their motor innervation from different nerves. Thus, based upon numerous experimental studies, it is generally accepted that the external branch of the superior laryngeal nerve supplies the cricothyroid and inferior pharyngeal constrictor muscles. All other (internal) laryngeal muscles are traditionally said to receive their motor branches from the inferior laryngeal nerve. As a consequence, the internal branch of the superior laryngeal nerve is described to carry only afferent (sensory) fibers, mainly for the mucosa of the larynx and trachea. But not later than 1884 this concept has been doubted. In this year, Wilhelm EXNER, a Viennese physiologist, has published the results of his experimental studies on the laryngeal muscles in animals. He has found that only some of the internal laryngeal muscles are exclusively supplied by the inferior laryngeal nerve, whereas the remaining muscles receive motor fibers from either both laryngeal nerves or only from the superior laryngeal nerve, respectively. Thus, since that time it has been questioned whether the internal branch of the superior laryngeal nerve merely consists of sensory fibers. In 1986, Eva B. KRAMMER has performed a neuro-anatomical tracer study in laboratory rats at our institute and proved that, in fact, the internal branch of the superior laryngeal nerve also supplies efferent (motor) fibers to the larynx. The appropriate motorneurons are located at typical positions within the motor nuclei of the vagus nerve. Thus, the purpose of the present study is to clarify which muscles are innervated by these motorneurons. In deep narcosis and anaesthesia, the internal branch of the superior laryngeal nerve will be electrically stimulated. As a consequence, muscle fibers innervated by this nerve will contract and depelete their glycogen storages which, in turn, will be shown by appropriate histochemical reactions on histologic cross-sections. Therefore, the results of our study will contribute to a better knowledge of the motor innervation of the laryngeal muscles in mammals.

The basics of our anatomical knowledge of the laryngeal innervation date back as early as into the classical Roman period. Claudius Galenus, the physician in ordinary to the Roman emperor Marcus Aurelius, is still remembered with the anatomical eponym "Ansa Galeni" indicating an anastomotic nerve branch which, in turn, connects the superior and inferior laryngeal nerves. Both laryngeal nerves branch off from the vagus nerve. Via the Ansa Galeni, it has been said that, except the external laryngeal muscle (i.e., the cricothyroid muscle), all other laryngeal muscles receive their motor innervation. These motor fibres have been thought to originate from the inferior laryngeal nerve, whereas the superior laryngeal nerve is predominantly sensory in nature, supplying the mucosal membrane of the larynx cranial to the vocal folds by its internal branch. On the other hand, its small external branch is purely motor and supplies the cricothyroid muscle. These findings were traded until 1884, when Sigmund Exner, a Viennese physiologist first doubted the predominance of the inferior laryngeal nerve in the motor innervation of the larynx. From several animal experiments he came to the conclusion that also the internal branch of the superior laryngeal nerve must carry a considerable amount of motor fibers. However, it remained unclear which laryngeal muscle(s) receive(s) these efferent fibres and thus represent(s) the target of these motor neurons. In 1986, Eva B. KRAMMER and co-workers were able to localize the perikarya (cell-bodies) of these motor neurons within the nuclear complex of the vagus nerve in the lower brainstem of the albino rat. Based on their anatomic tracer-study, we were able to identify the ventrolateral part of the posterior crico-arytaenoid muscle as target of the motor neurons running within the internal branch of the superior laryngeal nerve. As a method of choice, we used the glycogen-depletion technique to identify the muscle fibre target. To further support those really revolutionary results, we identified the motor branch entering the posterior crico-arytaenoid by microdissection of the larynx followed by combined immunhistochemical techniques applied on histological cross-sections of whole mount specimens. Finally, we are currently recording needle electromyograms from the posterior crico-arytaenoid muscle following isolated stimulation of the internal branch of the superior laryngeal nerve. In conclusion we may say that, according to its (also functional) bipartite nature, the posterior crico-arytaenoid muscle receives motor branches from two sources: its dorsomedial part which, in turn, counteracts all the other laryngeal muscles closing the rima glottidis and tensing the vocal folds is innervated by the inferior laryngeal nerve. Its ventrolateral part, however, which is to be addressed to as the only true dilatator of the vocal folds, receives its motor neurons via the internal branch of the superior laryngeal nerve.