Motoneuronal location of external laryngeal and hyoid muscles involved in primate phonation

In seven squirrel monkeys (Saimiri sciureus), horseradish-peroxidase injections were made into a number of extralaryngeal muscles shown to be involved in vocal control. Retrogradely labeled motoneurones were found in the case of the strap muscles from a position ventrolateral to the caudal hypoglossal nucleus down into the ventral horn of C2. The thyrohoid muscle was represented most rostrally, the sternothyroid most caudally; the sternohyoid took an intermediate position. There was partial overlap between sternothyroid and sternohyoid motoneurones. The thyrohyoid motoneurones were located not only rostrally but also laterally to the sternohyoid motoneurones. Omohyoid motoneurones overlapped with those of the sternohyoid and sternothyroid in the caudal medulla; in the cervical cord, they were located laterally to them. Inferior pharyngeal constrictor motoneurones were found in the nucl. retrofacialis and rostral nucl. ambiguus. Mylohyoid and anterior digastric motoneurones were restricted to the medialmost part of the trigeminal motor nucleus, with the mylohyoid being represented ventrally to the anterior digastric.