The origin of trochlear motoneurons in the larval sea lamprey, Petromyzon marinus L. An HRP study.

The origin of trochlear motoneurons in larval lampreys was studied by injection of HRP into the orbit. Immature motoneurons had ventricular attachments, the position of which with respect to the ventricular sulci was used to study their regions of origin. Motoneurons originate from the region between the sulcus intermedius dorsalis and the sulcus intermedius ventralis, which in other parts of the brain were identified as the visceromotor and viscerosensory columns. Both ipsilateral and contralateral immature motoneurons were found. The significance of these findings is discussed in relation to the origins of the trochlear nerve and its nucleus in vertebrates.