SMB is 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 SMB are situated in the ventral ganglion and send anteriorly directed processes into the nerve ring (h, i) which initially run near the centre of the ring neuropile. When the processes of SMBV reach a sub-dorsal position they loop round and run back on the inside surface of the ring, where they have NMJs before reentering the neuropile of the ventral ganglion. They then leave the ventral cord via the amphidial commissures and take up their positions, along with four other processes, in the ventral sub-lateral cords. The SMBD pair run round to the contralateral side of the nerve ring and there move down to the inside surface of the ring, where they have their NMJs. They leave the ring sub-dorsally and enter the dorsal sub-lateral cords along with four other processes. The processes of SMB form striking structures in the neuropile of the ventral ganglion, where they enlarge, flatten and wrap round each other (c). Another striking feature in the same region is formed by the synapses of SMBD onto SAAD; vesicle-filled processes from SMBD poke into the cell bodies of SAAD, where they synapse onto them (d). The NMJs of SMB are situated sub-ventrally and sub-dorsally and are intercepted by dendrites from RME (e). There are marked differences in the synaptic contacts made by SMBD and SMBV. Both have NMJs (b, e), receive their main synaptic input from AIZ (f), and a little from AVH (*b) and DVA, and have gap junctions to AVK (*e). In addition, SMBD alone synapses onto RMED (at dyadic NMJs (e)) and SAAV (a); it receives synaptic input from ALN (*b) and RIC (*b), and has gap junctions to SAAD and RIB. SMBV alone synapses onto RMEV (at dyadic NMJs) and SAAD; it receives synaptic input from PLN (*a) and ADF (*c) and has gap junctions to SAAV (g). Magnifications: (a, b, f) x 25500, (c, e, g) x 12750, (d) x 8500.
Web adaptation, Thomas Boulin, for Wormatlas, 2001, 2002