Recurrent branches of the vagus n

The recurrent laryngeal nn. are branches of the vagus nn. that “recur”; they reverse their course and travel back to a region they have already passed through. Both vagus nn. descend through the neck toward the thorax. The left recurrent laryngeal n. then wraps around the bottom of the aortic arch and ascends back into the neck. The right recurrent laryngeal n. wraps around the bottom of the right subclavian a. and ascends back into the neck. Both recurrent laryngeal nn. ascend to the larynx, where they innervate all of the muscles of phonation except the cricothyroid m.

The axons in the vagus n. are pre-ganglionic, which synapse with post-ganglionic neurons in the walls of the target organ. In the case of the recurrent laryngeal n., this means a single axon that starts in the brain and ends where it synapses in the larynx. Giraffes have recurrent laryngeal nn., so the length of those axons may be as long as 5 meters (about 15 feet), depending on the size of the giraffe. Sauropod dinosaurs (the great big ones with the really long necks are assumed to have had recurrent laryngeal nn. as well, and in their cases the axons would be up to 27 meters long, or about 88 feet.