SMDis a set of four motoneurons, which innervate muscles in the head via NMJs in the nerve ring and also send processes posteriorly down the sub-lateral cords. The cell bodies of SMDV are situated sub-dorsally, close to the neuropile of the nerve ring (j). Processes enter the ring sub-dorsally from the cell bodies and move over to the contralateral side, running near the centre of the neuropile; they then enter the ventral cord. They leave the cord via the amphidial commissures and take up their sub-ventral positions along with the other processes of the sub-cords. The cell bodies of SMDD are situated in the ventral ganglion and send anteriorly directed processes into the nerve ring (i). These processes run round the ring, near the centre of the neuropile, and leave at contralateral sub-dorsal positions to enter the sub-cords along with the other processes of the dorsal sub-cords. The dorsal processes of SMDV and the ventral processes of SMDD are large and have an irregular shape, often wrapping around the processes of SAA (c, d), with which they are closely associated. The NMJs of SMDare situated near the ventral (SMDV) and dorsal (SMDD) mid-lines at the anterior regions of the inner surface of the nerve ring and, together with RME NMJs, form distinctive structures in these regions (e, f and figure 14). SMD neurons send unusual small processes into the regions of the NMJs of their dorso-ventral symmetric partners (e-h), which have terminal electron-dense regions that look similar to presynaptic specializations. No synaptic vesicles are seen in these regions, however, so it seems unlikely that these processes are presynaptic. Their disposition suggests that they could be dendrites receiving synaptic input from RME and the symmetrically opposite members of their own class. The main synaptic outputs of SMDare to muscles (e, f, g, h) and RIA (a). The main synaptic inputs are from RIA (b), OLL (g, h), URY(*a), RIM (*e) and RIC (*b); there is probably also some synaptic input from RIV (*d) and RME (*d). There are gap junctions to RMD, RIS, RIB, RIV (SMDV only) and itself. Magnifications: (a, b) x 25500, (c-h) x 12750.
Web adaptation, Thomas Boulin, for Wormatlas, 2001, 2002