Distribution of the internal branch of the human accessory nerve.

It was recently reported that the internal branch of the accessory nerve not only comprises the cranial root but also various combinations of the cranial root, spinal root, and the vagal component of the vagus nerve. The aim of this study was to demonstrate the anatomical distribution of each component of the internal branch of the human accessory nerve. Ten half-heads and necks of adult cadavers were used. The internal branch of the accessory nerve had three courses: the pharyngeal branch, the descending branch to the thorax, and the recurrent laryngeal nerve. The pharyngeal branch of the internal branch originated mainly from the vagus nerve, rather than from the cranial root of the accessory nerve. All of the components of the internal branch descended to the thorax along the vagus nerve. The recurrent laryngeal nerve comprised the internal branch and the vagus nerve in all specimens, and it was separated into bundles originating from the internal branch and vagus nerve. Both bundles gave off branches to the trachea and esophagus. The laryngeal distribution of the internal branch and vagus nerve was confirmed in the posterior cricoarytenoid, lateral cricoarytenoid, and thyroarytenoid muscles. These three laryngeal muscles were innervated by the cranial root and/or vagus nerve, but the distribution pattern was different in each specimen. Although the vagus nerve and cranial root are morphologically distinct nerves in the cranial cavity, they can be regarded functionally as the same nerve based on their distribution in the laryngeal muscles.