RIM is a set of two motoneurons that innervate muscles in the
head via NMJs in the nerve ring. The cell bodies are situated in the
lateral ganglia and send processes into the ventral cord via the
amphidial commissures. These run anteriorly through the neuropile of
the ventral ganglion and then make one complete circuit round the
anterior regions of the nerve ring near the outside surface. They are
closely associated with the processes of AIB for most of the
way round the ring. RIM is the only motoneuron class in the
nerve ring that does not have axons situated on the inner surface of
the nerve ring and so is only accessible to muscle arms where they
penetrate the ring at the four muscle spurs (figure 14). Large NMJs
are formed on the anterior side of each of the muscle spurs (a) and
are usually intercepted by dendrites from RMD motoneurons (d). The
processes of RIM and RMD form characteristic structures,
which are seen just anterior to the NMJ region in each quadrant, where
the RMD process flattens into a sheet and covers the anterior
surface of a varicosity in the RIM process near the NMJ (b, c).
The main neuron classes that are postsynaptic to RIM are: RMD (d), SAA (g), SMD (e) and AVB (f). The
main synaptic inputs are from AIB (*a), AIZ (*g), SAA (*b) and RIS (*b). There are several gap junctions
to AVE (h) and some to AVA, RIS and AIY (*d) .
Magnifications: (a) x 25500, (b, c, g, h) x 12750, (d-f) x 17000.