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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.
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