Localization of adductor and abductor motor nerve fibers to the larynx.

Knowledge of the location of motor nerve fibers to the adductor and abductor muscles of the larynx may be useful in the diagnosis and treatment of innervation disorders in this organ. Anterograde degeneration and retrograde tracer anatomical techniques have demonstrated the central and peripheral positions of these two groups of motor nerve fibers in the cat. Traditional nerve fibers degeneration methods applied following intracranial transection of the vagus nerve rootlets indicated that: 1) Most of the fibers in the recurrent laryngeal nerve (RLN) are motor; 2) Almost all of these motor fibers leave the brain stem in the most rostral rootlet(s) of the vagus nerve; and 3) Motor fibers to the larynx form a discrete bundle within the trunk of the vagus nerve before forming the RLN. A tracer (horseradish peroxidase) of retrograde axoplasmic flow in motor neurons has been employed to demonstrate: 1) Dorsoventral division of the adductor and abductor neurons in the nucleus ambiguus; and 2) Diffuse arrangement of both adductor and abductor nerve fibers in the vagus nerve but collection of these fibers into abductor and adductor halves of the RLN prior to entering the larynx. These findings dispel theories of differential cord paralysis (Semon's law) based on a vulnerable position of abductor fibers at the periphery of the RLN. Furthermore, the diffuse arrangement of these fiber groups explains the usually mixed functional results obtained following reimplantation of the RLN into a laryngeal muscle.