Location of respiratory motoneurons involved in phonation. An HRP study in the squirrel monkey

In 11 squirrel monkeys (Saimiri sciureus) the motoneuronal distribution of those muscles being involved in the respiratory component of phonation, that is the Mm. obliquus externus and internus, rectus and transversus abdominis, intercostalis externus and internus, and intercartilagineus, were studied with the horseradish peroxidase retrograde tracing technique. It was found that the spinal representations of the abdominal muscles overlap to a large extent. Motoneurons of all four abdominal muscles were found in the lateral, central, medial, ventral as well as dorsal part of the ventral horn, with a clear dominance of the lateral and central positions, however. Marked differences occurred with respect to the rostro-caudal distribution. While rectus motoneurons occupied up to 16 spinal segments, obliquus internus and transversus motoneurons occupied only 9, obliquus externus 14. Furthermore, the density of motoneurons in the different segments showed quite dramatic changes in the case of the obliquus externus and transversus, was relatively constant in the rectus, and formed a single sharp peak in the obliquus internus. The motoneuronal distribution of the intercostal and intercartilaginous muscles showed less overlap than that of the abdominal muscles. Intercostalis internus motoneurons were mainly found laterally in the ventral horn, intercartilagineus motoneurons laterally and centrally, intercostalis externus motoneurons laterally and medially. The intercartilagineus muscle of a specific inter costal space was found to be innervated from 1 to 2 spinal segments, the intercostalis internus from 2, the intercostalis externus from 2 to 3. As all muscles participating in the respiratory component of phonation have a spinal representation in the lateral ventral horn of the lower thoracic cord, this area is proposed to play a crucial role in vocal production.