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SMD 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 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 SMD are 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 SMD are 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.
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