DDn is a set of six motoneurons, with cell bodies in the ventral cord, which innervate dorsal muscles. Each cell has a short, posteriorly directed and a longer, anteriorly directed process emanating from its cell body. The anterior process has a branch, which leaves the ventral cord as a commissure and runs round to the dorsal cord. (The DD1 commissure runs to the left, the rest to the right.) The commissures often run through the hypodermal ridge when entering or leaving the nerve cords (b). The commissure splits, as it enters the dorsal cord, into an anteriorly and a posteriorly directed process, which span approximately the same longitudinal region of the body as their ventral counterparts. Both the dorsal and the ventral processes run adjacent to the basal lamina bounding the cord and run in close association with the processes of the VDn neurons. In the ventral cord they run between the more dorsal processes of the VC neurons and the more ventral processes of the VAn and VBn neurons (figure 18). In the dorsal cord they run to ventral of the other motoneuron classes (figure 19). The DDn processes in the ventral cord are exclusively postsynaptic and receive synaptic input from VAn (d), VBn (*a) and VCn (*b) as corecipients at NMJs, often with short dendritic spines intercepting the NMJs (as in VAn-a). The processes in the dorsal cord are predominantly presynaptic and have many NMJs (a), most ofwhich have only muscle as the postsynaptic partner. The role of the dorsal and ventral processes is reversed in the first larval stage and switches round to the adult configuration at about the time of the first moult (White et al. 1978). The processes of DDn and VDn do not have extended, apparently undifferentiated, distal regions, as do the other classes of motoneuron; instead, they end abruptly in close proximity to the end of a neighbouring process of the same class and there is usually a gap junction between them (d). There are also a few gap junctions to VDn neurons (c). Magnifications: (a-d) x 25500.
Web adaptation, Thomas Boulin, for Wormatlas, 2001, 2002