Is there any paper which reports how, in amphioxus, the "innervation" of notochordal muscular plates is organized in the anterior-most part of the notochord which projects beyond the neural tube (and gives the name to Cephalochordata)?

This question comes from an obvious fact that the muscular plates in this part of the notochord cannot have a direct contact with the neural tube as they have in the main (so to speak, subneural) part of the notochord.